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OSPF > OSPF - Basic Operation > Flashcards

OSPF - Basic Operation Flashcards

1
Q

There is a large OSPF domain with over 10,000 LSAs in the network. What could be done with the group pacing interval to help on this network?

A

The group pacing interval could be decreased, default is 240 seconds. Means not waiting as long for group of LSAs.

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2
Q

A route is tagges as IA in the routing table.

What does this mean?

A

Its an OSPF Inter Area route

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3
Q

What is the difference between a direct Ack and a delayed Ack?

A

Direct Ack is sent immediately to ack an LSA while a delayed Ack waits up to the retransmit timer to Ack multiple LSAs in a single LSAck packet.

Direct ACk are always unicast while delayed Ack may be multicast depending on the network type.

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4
Q

Flooding is OSPF is considered reliable, what does this mean?

A

Every LSA sent must be acknowledged

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5
Q

How can the source router be identified in an OSPF packet header?

A

Header includes router ID of originating router and area ID

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6
Q

True of False?

OSPF will still use a router ID for a physical interface after a physical interface fails or is deleted?

A

True

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7
Q

_​_Why does OSPF need areas?

A
  • Areas reduce amount of routers
  • reduce flooding of LSA, so less CPU intensive
  • less DB interaction
  • SPF algorithm less intensive
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8
Q

What command is used to find a routers OSPF neighbours?

A

Show ip ospf neighbors

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9
Q

Why use Network LSAs in OSPF, would router LSAs not suffice?

A

Network LSAs are used in a multi access network and describe all routers in the multi access network in one LSA to the rest of the network . If using router LSA, would need multiple LSAs to describe the multi access network.

Not scalable, Eg. 100 routers in multi access network all would send router LSAs throughout the OSPF domain instead of just to the DR

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10
Q

What does the link state ID field signify within a link state request packet?

A

The ID depends on the LSA type which is also defined in the link state request

Eg. Type 1 LSA, ID is router ID

Eg. Type 2 LSA, ID is network

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11
Q

Based on the diagram, what path would a network attached to router A take to reach the external subnet if the external network was redistributed as a type 1 route and a type 2 route.

A

Type 1 - A -> B -> D

Type 2 - A -> C -> D

External Cost is 30 B -> D, 20 C -> D

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12
Q

What command is used to show type 4 LSA in an OSPF LSDB?

A

show ip ospf database asbr-summary

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13
Q

What does the summary mean ina network summary LSA? Does it summarize routes?

A

No it doesn’t summarize routes. It summarizes the type 1 & type 2 LSAs for a particular network to a single LSA advertisement for that network outside the area.

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14
Q

Router Decisions on receiving LSA

  1. If it already exists
  2. If doesn’t exist
A
  1. Compare seq numbers
    • if same then ignore
    • if higher add to LSDB, send ack, flood, re-run SPF
    • if lower, send LSU to source with updated LSA info
  2. Add to LSDB, send ack, flood, run SPF
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15
Q

What is the link state ID of a network LSA?

A

IP address of the interface of the DR that connects to the multi access network

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16
Q

What is the default behaviour for a Type 7 LSA when it reaches an ABR?

A

The ABR will not translate / flood these LSAs to any attched area.

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17
Q

If an OSPF paket has an AuType of 2, what does this mean and what would you expect to see in the Authentication field?

A

MD5 Authentication

Expect to see in Authentication field:

  • Key ID
  • Authentication Data Length
  • CryptoSequence Number
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18
Q

How does the master / slave know DB exchange process is complete?

A

When both routers have sent and received DD packet with M bit set to 0

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19
Q

In a broadcast network DROther sends an LS Update to DR / BDR. Does the BDR flood it?

A

No, BDR listens for updates on 224.0.0.6 like the DR but only records LS Updates. It does not reflood or ack them unless DR fails.

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20
Q

What are the two ways to acknowledge an LSA update?

A
  1. Explicit Ack - send LSAck with LSA header back to neighbour
  2. Implicit Ack - Update packet received with exact same instance of LSA - neither is more recent than the other
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21
Q

What happens when a non ABR receives a network summary LSA?

A

As it is a network summary LSA, it does not run SPF algorithm

Adds cost in LSA to cost to get to ABR

Route added to routing table along with calculated cost - distance vector behaviour (no shortest path tree for outside area)

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22
Q

In which OSPF neighbour state is a DR / BDR election?

A

Two-Way

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23
Q

Describe the OSPF neighbour Full state?

A
  • Neighbours fully adjacent
  • Adjacencies appear in router LSAs and network LSAs
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24
Q

LSA Flooding

  1. Explain the concept of LSA flooding?
  2. What types of LSA packets are flooded?
  3. Are LSAs flooded outside of the area they originate?
A
  1. Flooding means sending the packet throughout area, could be point to point links, broadcast network etc. but means router just sends out to its adjacencies and then next router sends it to theirs
  2. LS Updates and LS Acks
  3. No
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25
Q

There is a very small OSPF domain, what could be done with the group pacing interval to make the network more efficient?

A

Increase the group pacing interval, default is 240 seconds. More LSAs in less updates by incrceasing.

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26
Q

What type of LSA does the image display?

A

Type 2 Network LSA

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27
Q

What is the name of the algorithm OSPF uses to calculate the shortest path tree from LSAs in LSDB?

A

Djikstra’s Algorithm

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28
Q

What is the attached router field in a network LSA?

A

Lists Router IDs of all routers on multiaccess network that are fully adjacent with the DR

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29
Q

What is the name for a Type 7 LSA?

A

NSSA External LSA

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30
Q

Link state updates which are sent due to a retransmission. Are they potentially multicast (based on network type) like the original LSA?

A

No they are always unicast no matter what network type

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31
Q

In a broadcast network who does a DR router form adjacencies with?

A

All routers on the multi-access network

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32
Q

Is the hello interval interface specific or router specific?

A

Interface specific.

Can be configured with command ip ospf hello interval

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33
Q

In an OSPF broadcast network a DROther sends an LSA to a DR / BDR. The LSA isn’t acked within the retransmission time so the DROther needs to send to the DR again. What address does it use?

A

It sends the LSU unicast to the DR and not multicast as it did originally.

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34
Q

How are LS Updates sent for each network type?

A
  1. Point to Point - Multicast on 224.0.0.5
  2. Point to Multipoint - Unicast
  3. Virtual Link - Unicast
  4. Broadcast - sent to DR / BDR on 224.0.0.6, DR sends to DrOthers on 224.0.0.5 and DROthers send out all their other OSPF enabled interfaces
  5. NBMA - Unicas to DR / BDR, DR unicasts to all adjacent neighbours
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35
Q

What factors dictate the size of an OSPF area?

What is a good guide for the max umber of routers depending on the above factors?

A
  1. Number of Links
  2. Stability of Topology
  3. Memory & CPU of routers
  4. Use of summarization
  5. Number of LSAs entering the area.

30 - 200 routers

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36
Q

What are the duties of a DR on an OSPF broadcast network?

A
  1. Represent the multi-access network and its attached routers to the rest of the OSPF area
  2. Manage the flooding process on the multi-access network
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37
Q

For acknowleging LS Updates is implicit or explicit acks more efficient?

A

Implicit - if neighbour is already sending back an LS Update then can include that LSA to be acked in the update instead of sending an explicit LS Ack.

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38
Q

In determining the most recent LSA, why if the sequence numbers are the same are checksums compared next and can they garauntee the latest LSA?

A

Checksums offer a tiebreak, without it two LSAs could live in network til next refresh in 30 mins

They cannot garauntee the most recent LSA

They provide OSPF a consistent way to always reach a decision

Provides vendor interop

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39
Q

On a multi-access network who do routers form an adjacency with?

A

Both DR & BDR

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40
Q

Of the below LSA types, which ones would you see in a totally stubby area?

  • Type 1
  • Type 2
  • Type 3
  • Type 4
  • Type 5
  • Type 7
A
  • Type 1
  • Type 2
  • Single Type 3 for default route
  • No Type 4
  • No Type 5
  • No Type 7
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41
Q

How many bits is an area and how can it be represented?

A

An area is 32 bits and can be represented as dotted decimal or a decimal number.

Eg. Area 271 = 0.0.0.15

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42
Q

What is the command to show router LSAs in LSDB?

A

show ip ospf database router

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43
Q

What is a partitioned area?

Is it a problem?

A

An area where a link failure causes one part of the area to become isolated from the other.

Not a problem for non backbone area, treated now as two separate areas, as long as both areas still have an ABR. Traffic between these new areas will now traverse via the backbone.

Partition of backbone is a problem, creates two isolated OSPF domains. Better design required to prevent it.

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44
Q

Two routers in a stub area fail to form an adjacency, what is most likely the issue?

A

One router is configured as a stub and the other isn’t

E bit set to 0 in hello packet of stub router, if one not configured as stub then mismatch hellos

All routers within stub area must be configured as stub

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45
Q

What is the major concern if an OSPF packet is configured with AuType = 1?

A

This is clear text password

It is transferred in the authentication field of the OSPF packet

If receving router interface is configured with same password then packet is accepted

Problem: Attacker could join OSPF network with router by sniffing out password in OSPF packets and configuring router with the same details

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46
Q

What are the 8 fields in an LSA header?

A
  1. Age
  2. Options
  3. Type
  4. Link State ID
  5. Advertising Router
  6. Sequence Number
  7. Checksum
  8. Length
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47
Q

What are the 4 restrictions on creating a stub area?

A
  1. All routers in area must be stub (E bit set in hellos)
  2. Virtual Links cannot be configured or transit a stub area
  3. No router in stub area can be ASBR
  4. Stub area can have more than one ABR but they will all source default route so cannot determine optimal gateway
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48
Q

What is the TTL of an OSPF packet?

Where is this TTL defined?

A

1

In the IP header (outer encapsulation of OSPF packet)

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49
Q

What is a network summary LSA?

What command is used to see them in a LSDB?

A

Means by which ABR tells internal routers to tehir area of destinations on attached area

show ip ospf database summary

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50
Q

In an OSPF packet header, there are 5 values for Type. What does type signify and what are the possible values?

A

Type defines the type of OSPF packet

1 byte

There are 5 possible values 1 - 5

  1. Hello
  2. Database Descriptor
  3. Link State Request
  4. Link State Update
  5. Link State Ack
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51
Q

What is the OSPF MD5 authentication process?

A
  • Router creates a hash with OSPF packet data and password
  • Sends the hash in the OSPF packet
  • Receiving router knows the location of the hash based on the authentication data length, strips off the hash
  • Receiving router has same password, calculates hash again against the packet
  • if hash values are identical then packet source has been authenticated
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52
Q

How does a router inform other routers that it is an ASBR?

A

The E bit in the router LSA is set if it is an ASBR

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53
Q

What OSPF network types require a DR / BDR?

A
  1. Broadcast
  2. NBMA
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54
Q

The LSA refresh timer is every 30 minutes but an LSA isn’t flushed from an LSDB for 1 hour. Why is there such a difference in times?

A

The group pacing interval can be set up to 30 minutes on a router. So even though the refresh is every 30 minutes, if the pacing interval was set to max then a packet may not arrive on a router for close to 30 minutes after being sent by originating router

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55
Q

Once the DB sync process (all DD packets sent) has completed which state does OSPF transition to?

A
  • If entries still on link state request list then Loading
  • If no entries left on link state request list then Full
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56
Q

What is the link state ID in a router LSA?

A

Router ID of originating router

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57
Q

What is a good use case for configuring an area as a stub area?

A

An area that has a single exit via the ABR and one that has a lot of Type 5 external routes.

ABR will source a default route and no Type 5s will be required in the area

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58
Q

If an OSPF packet has an AuType of 0, what does that mean?

A

No authentication

Authentication field not examined

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59
Q

All OSPF networks fall into two types, what are they?

A
  1. Transit
  2. Stub
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60
Q

BGP routes are redistributed into OSPF, no metric is defined. What will the cost of these redistributed routes be in OSPF?

A

Cost of 1

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61
Q

Direct Ack

  1. What is a direct Ack?
  2. What situations is it used?
A

A direct ack is when an ack is sent immediately and unicast

It is used when:

  • a duplicate LSA is received from a neighbour, possibly indicating neighbour never received original ack.
  • LSAs age is Max Age, no instance of LSA in receiving routers LSDB
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62
Q

What criteria must satisfy ECMP for a destination?

A

Equal cost, equal path type

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63
Q

What is a network LSA and where is it flooded?

What command is used to see the network LSAs in the LSDB?

A
  • LSA sent by DR in multi access network
  • Lists all attached routers including DR in multi access network (treated as one virtual router) along with network and subnet mask
  • Flooded within area only
  • show ip ospf database network
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64
Q

Is there an adjacency formed between DROther routers in a broadcast / NBMA network?

A

No

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65
Q

What does the IP address in the DR / BDR field of an OSPF hello packet represent?

A

Represents the interface of the DR / BDR on the network

Not the router ID of the DR / BDR

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66
Q

What are the four OSPF path types?

A
  1. Intra Area
  2. Inter Area
  3. Type 1 External
  4. Type 2 External
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67
Q

What LSA type number does an ASBR originate is a NSSA area?

A

Type 7

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68
Q

True or False?

DR & BDR are elected, another router joins multiaccess network and has a higher priority than DR / BDR. It will become DR / BDR.

A

False

If DR / BDR already elected then no new re-election and DR / BDR continue

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69
Q

What command is used to check all details related to an OSPF enabled interface?

A

show ip ospf interface

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70
Q

If a network designer wanted packets to leave the OSPF network at the closest exit point to the external destination.

Which network path type would they choose and why?

A

External Type 2

Just takes into account the cost of external route and not internal cost to ASBR

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71
Q

An OSPF packet is only exchanged with a directly connected neighbour, how does it garauntee this in an OSPF packet?

A

TTL is 1 for all OSPF packets

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72
Q

What does a network Summary LSA advertise and to who?

A

Advertises destinations outside an area into that area

ABR learns routes from other area(s) it is attached to and advertises them into an area

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73
Q

What is the order used for OSPF to select a router ID?

A
  1. Manual Configuration
  2. Highest IP address on any loopback
  3. Highest IP address on any physical interface
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74
Q

How does OSPF track an LSA to make sure it is acknowledged?

A
  • Router sends LSA
  • LSA added to link state retransmission list for every neighbour it has been sent to
  • If no ack within retrasnmission interval (5 seconds default) then router resends LSA
  • Once LSA is acked it is removed from link state retransmission list
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75
Q

What area is an AS External LSA associated with?

A

It is associated with no area, only LSA that isn’t

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76
Q

Link State Refresh

  • What is a link state refresh?
  • What two characteristics of an LSA are altered as part of a refresh?
  • How often does it take place?
  • What does a receiving router do with the LSA?
A
  • Originating router refreshes his originating LSAs and re-floods them.
  • Sequence number incremented and age set to 0
  • Every 30 minutes
  • Replaces the old LSA and starts to age the new LSA
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77
Q

What is the age field in an LSA?

How is it incremented?

A

The time in seconds since an LSA was originated

Incremented by InfTransDelay at every router it exits (default value for Cisco 1 second)

Incremented also when in DB

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78
Q

What are the 10 pieces of information in a hello packet?

A
  1. Originating Router ID
  2. Area ID of router interface
  3. Address mask or router interface
  4. Authentication Info
  5. Hello Interval
  6. Dead Interval
  7. Router Priority
  8. DR / BDR
  9. Flags
  10. Router IDs of originating routers neighbours
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79
Q

An OSPF hello packet must match subnet mask, area ID, hello interval, dead interval, options and authentication for it to be accepted. MTU must match, why is that not considered here?

A

MTU is defined in the DD packet

If MTU mismatched here then adjacency doesn’t move past exstart

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80
Q

Router LSA

What does a router LSA describe?

Where is it flooded to?

What command do you use to see router LSAs?

A
  • Describes all of a router’s links, their outgoing cost etc. and OSPF neighbours on those links
  • Flooded within area only
  • show ip ospf database router
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81
Q

What mechanism does OSPF use to make sure that its link state DB is not corrupted?

A

It verifies the checksum every 5 minutes in the LSDB to ensure LSAs have not been corrupted.

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82
Q

Is a Network Summary LSA propogated outside of the area it is advertised into? If not, then how do other areas learn about the routes from the network summary LSA?

A

It is not advertised outside of the area it is advertised into

It is regenerated by other ABRs and propogated into other areas

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83
Q

Database Description Packet

  1. What is the purpose of a database description packet?
  2. What does it contain in its body?
A
  1. Provides a summary description of the LSDB of a router
  2. Packet contains LSA headers only, enough information for receiving router to decide whether it has the latest copy of LSAs in its own database
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84
Q

In the diagram, how will packets route from R3 to R5?

A

Via R1 (area 0) and not direct via R4

Distance vector behaviour outside area and not link state (shortest path tree)

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85
Q

How does OSPF choose the best route if multiple entries for the same route exist?

Eg. Destination = 172.16.64.205

Routes in routing table:

  1. 16.64.0/18
  2. 16.64.0/24
  3. 16.64.192/27
A

Will use longest match

Chooses 172.16.64.192/27

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86
Q

What is the one characteristic required of OSPF databases in an area?

A

show ip ospf database database-summary

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87
Q

What is a virtual link?

Why would you need a virtual link?

Is it good design practice?

Which router type is a virtual link configured between?

A
  • Provides link to backbone through non backbone area, essetially a tunnel.
  • Area doesn’t have direct access to backbone or fix issue with partitioned backbone
  • Poor design practice, should design to avoid potential partitioned backbone (redundant links) or area not directly connecting to the backbone.
  • Configured between ABRs, they are considered neighbours on virtual link.
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88
Q

Of the below LSA types, which ones would you see in a not-so-stubby area?

  • Type 1
  • Type 2
  • Type 3
  • Type 4
  • Type 5
  • Type 7
A
  • Type 1
  • Type 2
  • Single Type 3 for default route
  • Type 4
  • No Type 5
  • Type 7
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89
Q

What is a type 1 LSA?

A

Router LSA

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90
Q

What OSPF network types use multicast?

A
  1. Point to Point
  2. Broadcast
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91
Q

What is the purpose of an ABR?

A

Gateway for inter-area traffic via the backbone. Summarizes its information into the backbone. Backbone then propogates to other areas.

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92
Q

DD Master / Slave Negotiation

  1. Which OSPF state does the master / slave negotiation happen in?
  2. How is it decided which router becomes master?
  3. What happens once DD negotiation complete?
A
  1. Exstart
  2. Negotiation Process:
    1. Both neighbours claim to be master
    2. Both send empty DD packet with MS bit set
    3. DD seq number set by both neighbours independently
    4. Nieghbour with higher router ID becomes master
    5. Seq number set to masters seq number
  3. Router OSPF process moves to exchange state
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93
Q

NBMA Networks

  1. Give examples of NBMA networks?
  2. What is NBMA and how does it differ to broadcast?
  3. Is there a DR / BDR?
  4. Are packets unicast or multicast?
A
  1. Frame Relay / ATM
  2. NBMA is two or more routers but no broadcast capability
  3. Yes, DR / BDR elected
  4. All packets are unicast
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94
Q

If a network designer wanted the closest exit point from OSPF too external networks.

Which network path type would they choose and why?

A

External Type 1

Cost to external route includes internal cost to ASBR

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95
Q

In a type 4 LSA what is the subnet mask address?

A

Mask is 0

It is advertising a router (ASBR) and not a network

Uses same LSA format as Type 3 so mask needs to be set to something

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96
Q

What is the default external path type in OSPF?

A

E2

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97
Q

What is an OSPF packet set to for QoS?

A

110b (Internetwork Control)

High Precedence

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98
Q

An OSPF packet has a value of 5 in its type field. What type of OSPF packet is it?

A

Link State Ack

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99
Q

Describe the OSPF neighbour Loading state?

A
  • router sends link state requests to neighbours requesting most recent LSAs, process started in Exchange state but not yet finished so Loading
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100
Q
  1. A router needs to flush an LSA from all router databases, how does it achieve this?
  2. Can this be done by any router?
A
  1. Originating router sets the age of an LSA to Max Age (3600)
  2. A router can only prematurely age an LSA that it originates
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101
Q

What is the value of the DR / BDR field in an OSPF hello packet if the network type does not require a DR / BDR?

A

0.0.0.0

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102
Q

What does an AS Extrenal LSA advertise and who advertises them?

A

Advertises routes external to OSPF AS or default routes external to OSPF AS

Sourced by ASBR

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103
Q

What defines an OSPF internal router?

A

All OSPF interfaces belong to the same area

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104
Q

What is the command to see all LSAs in a LSDB?

Does this command show the full LSA details?

A

show ip ospf database

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105
Q

Two routes have equal cost, 1 is an E1 route and the other is an E2 route.

Will they load balance traffic between them?

A

No, ECMP will only work for equal cost, equal path-type.

If both E1 or both E2 then yes they would load balance

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106
Q

What is a type 4 LSA?

A

ASBR Summary LSA

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107
Q

Does a link state request packet support requesting multiple LSAs in one packet?

A

Yes

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108
Q

What is the purpose of an ASBR?

A

Gateway for external traffic

Inject routes into OSPF learned from other protocols

ASBR can be internal, backbone or ABR

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109
Q

How does Djikstra’s algorithm generate a shortest path for a network?

A
  • Based on cost
  • cost associated with each interface
  • route cost is sum of outgoing interfaces to destination
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110
Q

Describe the OSPF neighbour init state?

A
  • Hello packet sent
  • Two way communication not formed yet
  • router lists all its neighbour IDs in its hello packet
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111
Q

Why are DRs / BDRs required in broadcast networks?

A
  1. Too many LSAs and adjacencies required in network [n(n-1)/2) adjacencies]
  2. Two many copies of same LSA in network as receiving router forwards to all its neighbours and the same for the next and so on (n2 LSAs)
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112
Q

Why is router 2 considered a DROther when it has a higher priority than router 1?

A

Router 1 was configured 1st

Once BDR is elected, a router with higher priority cannot take over

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113
Q

In the diagram, what issue does an ASBR Summary LSA resolve?

Which router in the diagram sources the ASBR Summary LSA?

A

R4 is redistributing RIP routes into OSPF.

R2 will receive the routes as type 5 LSAs with the next hop as R4 as next hop is not altered as route distributed accross areas.

ASBR Summary LSA is sent into area 1 (to R2) to tell it to get to the ASBR to go via R5.

R5 sources the ASBR Summary LSA

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114
Q

What is a NSSA External LSA?

A

Type 7 LSA

Almost identical to Type 5

Type 7 only flooded within NSSA area it was sourced, Type 5 flooded through OSPF AS

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115
Q

Router Priority

  1. What does it do?
  2. What are the range of values?
  3. What is the default router priority for Cisco?
  4. What does router priority of 0 mean?
  5. What command is used to change the router priority?
A
  1. Each OSPF enable router interface has a router priority. Used for DR / BDR election in multi-access networks.
  2. Range: 0 - 255. Higher better
  3. Default for Cisco devices is 1
  4. 0 means interface is not eigible for DR / BDR election. Set if you don’t want router becoming DR / BDR
  5. Changed at a per interface level: ip ospf priority
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116
Q

A hello packet contains router IDs of neighbours over what time period?

A

Within the last router dead interval

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117
Q

In OSPF why is the backbone area (area 0) required?

A

All areas connect to area 0

Essentially hub and spoke with areas

Prevents route loops - distance vector protocols prone to route loops. If all routees go through area 0 then this can be avoided.

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118
Q

What is the command to configure a NSSA totally stubby area?

A

area x nssa no-summary

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119
Q

What is a not-so-stubby area and why may it be required?

A

A NSSA area allows external routes to be advertised into OSPF AS while retaining characteristics of stuf area to rest of the AS

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120
Q

A virtual link is configured but isn’t coming up. It looks like the problem may be with the transit area. What may be the issue?

A
  • Transit area may be configured as stub (area must have full routing information), this is not allowed when configuring a virtual link
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121
Q

What is a stub area?

A

An area where external LSAs are not flooded (no Type 5 LSAs)

ABR sends type 3 LSA to advertise single default route to stub area

Type 3 LSA not flooded outside area, default behaviour

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122
Q

Delayed Ack

  1. What is a delayed Ack?
  2. What is the advantage of it?
  3. How does it work on a broadcast network where multiple neighbours send LSAs to DR?
  4. WHat is the length of time an Ack can be delayed to wait for more LSAs to be acked together?
A
  1. A delayed ack delays the length of time before acknowledging so that more LSAs can be acknowledged in a single LSAck packet.
  2. Advantage is that is more efficient. Less LSAck packets being sent.
  3. LSAs from multiple neighbours can be acked in a single multicast packet in a broadcast network
  4. Can be delayed less than the retransmit time which is 5 seconds by default
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123
Q

DB Sync Process

  1. Who controls synchronisation?
  2. How many DD packets can be outstanding at a time?
  3. How does the slave ack a DD packet?
  4. What happens if master doesn’t receive ack from slave?
  5. True / False? Slave only send DD packets as responce to master?
A
  1. Master
  2. 1
  3. Slave acks by sending DD packet back but with same seq number
  4. If master doesn’t receive ack within retransmit time (5 seconds) then it resends the DD packet
  5. True
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124
Q

How is a link state update packet flooded through an OSPF area?

A

Originating router sends to its directly connected neighbours

They re-encapsulate the packet and send it to their neighbours

So on until every router in the area has the LSU

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125
Q

What is the purpose of the crypto sequence number in an OSPF Authentication field in a packet header?

A

Provides non decreasing sequence number

Prevents replay attacks when MD5 authentication is used

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126
Q

Does a stub area still have routes to networks in different OSPF?

A

Yes, stub area still allows Type 3 LSA routes, blocks routes coming from outside OSPF AS (Type 5)

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127
Q

What is the default hello interval on an OSPF broadcast network?

A

10 seconds

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128
Q

What is an OSPF network destination type?

A

Network address where packets can be routed

Destinations entered in routing table

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129
Q

How does a router inform other routers that it is the ABR?

A

The B bit in the router LSA is set if it is an ABR

130
Q

If valid hello is received what does the router do next?

A

If router ID already in neighbour table then dead timer reset.

If router ID not listed, added to router table.

131
Q
  1. What address do DROthers use to send hellos?
  2. What address do DROthers use to send LSUs and LSAck
  3. What address does DR flood updates to DROthers on?
A
  1. 224.0.0.5
  2. 224.0.0.6. Only DR / BDR listen on this address
  3. 224.0.0.5
132
Q

Describe the OSPF neighbour down state?

A
  • initial state
  • no hellos from neighbour in the last dead interval
133
Q

What is a link state update packet and what are the two scenarios when they are used?

A

Used to send LSA information

  1. Response to LSR
  2. Flood LSAs
134
Q

A router realises after receiving all DD packets from its neighbour that it require some LSAs. Where does it record the LSAs it needs?

A

Link State Request List

135
Q
  1. What value is OSPF reference bandwidth?
  2. What is a potential issue with it?
  3. How can this be resolved to improve OSPF cost calculation?
A
  1. 108
  2. Anything above 100Mbps has a cost=1 as fractions are not allowed
  3. Command: auto-cost reference-bandwidth resolves this
136
Q

If two routers have same priority in DR / BDR election, which one is elected?

A

The router with the highest router ID

137
Q

Of the below LSA types, which ones would you see in the backbone?

  • Type 1
  • Type 2
  • Type 3
  • Type 4
  • Type 5
  • Type 7
A
  • Type 1
  • Type 2
  • Type 3
  • Type 4
  • Type 5
  • No Type 7
138
Q

Is DR / BDR a property of a router interface or a router itself?

A

Property of a router interface, same router might not be DR on another of its OSPF enabled interfaces

139
Q

What does an ABR do if it has multiple routes to the same network and it wants to send a network summary LSA into its attached area?

A

ABR will only originate one network summary LSA for each network

Sends the one with the lowest cost into the area

140
Q

What two fields dow a link state update packet contain?

A
  1. Number of LSAs in a packet
  2. LSAs - complete LSAs
141
Q

What is a Type 1 External (E1) Path?

A
  • Destination outside of OSPF
  • Redistributed into OSPF
  • ASBR assigns cost of external routes
  • Type 1 is sum of external cost plus the the cost of the path to the ASBR
142
Q

What are the three authentication options available to OSPF?

A
  1. None
  2. Simple
  3. MD5
143
Q

OSPF Timers

  1. What are the default hello timer values for broadcast & non-broadcast networks. How can it be changed?
  2. What is the default value for dead interval and how can it be changed?
  3. What is the purpose of the wait timer, what is its default value?
  4. What is the purpose of the retransmit timer, what is its default value and can it be changed?
A
  1. Broadcast - 10 seconds, Non-Broadcast - 30 seconds. Can be changed with command: ip ospf hello-interval
  2. Default dead time interval is 4 x hello interval, can be changed with command: ip ospf dead-interval
  3. Wait timer is time a router will wait before starting DR / BDR election if not heard of DR / BDR in incming hello packets. Deafult value is 4 x hello interval.
  4. Period in seconds router will wait if it hasn’t had an OSPF packet acked before re-tranmit. Default 5 seconds. Can be changed with command: ip ospf retransmit-interval
144
Q

Why in OSPF is there an option for external path type, E1 or E2?

A

Provides option to decide if internal cost to ASBR is important or not

145
Q

An ASBR learns external destinations? What is the default behaviour for these external destinations in OSPF?

A

They are flooded throughout the OSPF AS, accross all areas

146
Q

If an OSPF packet has MD5 authentication configured, is the packet encrypted?

A

No there is no encryption

Packet is sent as normal with hash added to packet

Receiving router strips off the hash and performs its own hash on the packet with its key

If they match then packet source is authenticated

147
Q

What is the length of an OSPF packet header and what 7 pieces of information does it contain?

A

Length - 24 octets (24 bytes)

Contains:

  1. Version
  2. Type
  3. Packet Length
  4. Router ID
  5. Area ID
  6. Checksum
  7. AuType
148
Q

What command should be used to check the checksum, age and sequence number of an LSA?

A

show ip ospf database

149
Q

What is the difference between a type 5 and type 7 LSA?

A

They are almost identcal

Type 5 flooded through OSPF AS

Type 7 flooded only within single NSSA area

150
Q

What does the following command indicate?

summary-address 141.108.10.0 255.255.255.0 not-advertise

A
  • This is a NSSA area
  • this address in not translated from type 7 to type 5 on the ABR, therefore doesn’t leave area
151
Q

EIGRP routes redistributed into OSPF have a cost value of 100.

Is this expected and how is this value reached?

A

Default cost value for redistributed EIGRP is 20

Value of 100 is potentially due to the metric keyword in the redistribute command:

redistribute eigrp route-map _____ metric 100 subnets

152
Q

What command is used to show type 7 LSAs in the LSDB?

A

show ip ospf database nssa-external

153
Q

Why doesn’t a router use an LSAs sequence number, checksum and age to uniquely identify an LSA within a link state request packet?

A

It requests an LSA but not any instance of the LSA

If it requested a particular instance then this may be outdated by the name the request is fulfilled

154
Q

Virtual Links

  1. What are virtual links used for?
  2. What type of link is it?
  3. Are packets unicast or multicast?
A
  1. Used to connect an area to area 0 that is not directly connected to it, transit it through another area.
  2. Unnumbered point to point links
  3. Unicast packets
155
Q

What is the purpose of the authentication data length in the Authentication field of an OSPF packet?

A

for MD5 authentication

Length in octets of message digest appended to end of the packet

Authentication hash is not included in OSPF header, its at end of packet so length required to understand what it is

156
Q

Why is a BDR elected with a DR?

A

If DR fails then BDR takes over. Already has adjacency with all other routers so no new DR election, no new adjacencies required and no DB re-sync. If no BDR then while re-elction then network unavailable.

157
Q

In a multi-access network, does a DR & BDR become adjacent with each other?

A

Yes

158
Q

Checksum

  1. How many bit value?
  2. Is it calculated over all fields in the LSA? If not, which fields are exempt and why?
A
  1. 16 bit
  2. No, all fields except Age. Age is the only value in an LSA that changes on every hop. Excluding it prevents having to recalculate the checksum at every hop
159
Q

There are 4 types of OSPF router, what are they?

A
  1. Internal Router
  2. Area Border Router (ABR)
  3. Backbone Router
  4. Autonomous System Border Router (ASBR)
160
Q

Why would you potentially need this command on a NSSA area and where would you configure it?

Area 1 nssa default-information-originate

What LSA type would it be and why?

A
  • By default a NSSA area does not generate a default summary route
  • Both stub and NSSA totally stubby do but not NSSA
  • Configured on ABR
  • Uses type 7 LSA
  • Potentially to allow a default route from the external AS, that would e preferred over this one and could be advertised into the backbone
161
Q

On a router where are router entry destination types stored?

What is the command to see the router entries?

What are the destinations in this tables?

These entries are marked with i and I. What do they mean?

A

Separate table to destination entries

show ip ospf border-routers

Router ID of ABR or ASBR

i - intra-area route, I - inter-area route

162
Q

Where would you potentially see the following command used?

ip ospf flood-reduction

A

OSPF on demand links

Enhancement to OSPF to suppress hellos and LSAs so link doesn’t always have to be up

163
Q

What command is used to show network LSAs in a LSDB?

A

show ip ospf database network

164
Q

What are the 8 OSPF neighbour states transitioned through to form a full adjacency between OSPF peers?

A
  1. Down
  2. Attempt
  3. Init
  4. 2-Way
  5. Exstart
  6. Exchange
  7. Loading
  8. Full
165
Q

10 routers are in area 0 and 5 routers are in area 1, do they have the same LSDB?

A

No, same LSDB only within the an area

166
Q

What is the impact on flooding when more OSPF areas are used?

A

Less flooding as flooding is mainly limited to within an area

167
Q

What is the protocol number for OSPF?

A

89

168
Q

An LSA header is part of every single OSPF packet no matter what packet type, are there instances though when the header is part of the data field of an OSPF packet?

A

Yes

Used by DD and LSAck to identify a particular LSA

169
Q

What is the purpose of the router ID in OSPF?

A

Used to uniquely identify a router in the OSPF domain

170
Q

What is the key ID for OSPF authentication?

A

It is the secret key and the authentication algorithem (MD5)

171
Q

By default, how many destinations will OSPF load balance over for ECMP?

Can that number be changed and how?

A
  • 16
  • yes, maximum-paths command
172
Q

What are the two advantages of using a loopback interface over a physical interface for a router ID?

A
  1. Loopbacks active when router boots up
  2. Only fails if entire router fails
173
Q

What is the link state ID of a type 2 LSA?

A

Router ID of the DR

174
Q

A NSSA area has Type 7 LSAs with the P bit not set in the LSA header, what will the NSSA ABR do with this LSA?

A

It will not translate / flood the LSAs

175
Q

What 3 fields in an LSA uniquely identify it and which three fields identify the most recent instance of that LSA?

A

LSA Uniquely Identified by:

  1. Link State Type
  2. Link State ID
  3. Advertising Router

Most recent instance of LSA Identified by:

  1. Age
  2. Checksum
  3. Sequence Number
176
Q

What is the purpose of a Link State Ack packet?

How does it identify the LSA it is acknowledging in the packet?

Can an LSAck packet ack multiple LSAs?

A

Acknowlede an LSA

Includes the header of the LSA, enough info to uniquely identify it

Yes, LSA headers for each LSA

177
Q

If an OSPF packet has an AuType of 1, what does that mean?

A

Simple password is used

Password up to 64 bits stored in the Authentication field

178
Q

In terms of OSPF encapsulation, an LSA is sent as part of a link state update. What is the LSA encapsulated in?

A

Link State Update (OSPF Data) -> OSPF header -> IP packet

179
Q

There is no metric field in a network LSA, why?

A

The cost from the pseudonode (group of OSPF routers in multi access network) to any attached router is always 0. This LSA describes the attached routers in the multi access network only.

180
Q

DR / BDR Election

Describe the process of DR / BDR election.

A
  • Two way comms established with one or more neighbours
  • Hello packets examined for priority, DR / BDR fields
    • If DR / BDR exists, router accepts them
    • If no BDR, election held, router with highest priority becomes BDR
    • if no DR, BDR is promotoed to DR
    • New election for BDR
181
Q

What type of LSA is displayed in the image?

A

Type 1 Router LSA

182
Q

Describe the OSPF neighbour Exchange state?

A
  • Router sends DD packets, describing LSDB to neighbours
  • router may also send link state request packets requesting more recent LSAs
183
Q

What 3 fields does a link state request packet use to uniquely identify an LSA?

A
  1. Link State Type
  2. Link State ID
  3. Advertising Router
184
Q

What is the OSPF group pacing interval, what problem does it mitigate.

What is the range of values and the default value for this interval?

A

The group pacing interval is a timer to delay sending LSAs.

Problem - As LSAs refresh, lots of LSAs flooding on the network at different times along with LSAcks

Group pacing interval is a timer to delay sending LSAs to try group more of them into a single packet.

Default value - 240 seconds, range 10 - 1800 seconds

185
Q

A totally stubby area blocks type 3 LSAs, how is the default route sent into the area?

A

It allows one Type 3 LSA for the default route

186
Q

In a broadcast network, a DROther sends a LS Update to DR / BDR. What happens next?

A
  • DR Acks the update
  • BDR just records update
  • DR sends it to all DROthers on 224.0.0.5
  • DROthers send update out any other OSPF enabled interface
187
Q

What is the cost of a network summary LSA when it is sent by the ABR into an area?

A

It is the cost from the ABR to the router who originated the LSA

188
Q

What is a Type 2 Exterbal (E2) path?

A
  • Destination outside of OSPF
  • Redistributed into OSPF
  • ASBR assigns cost of external routes
  • Type 2 is sum of external cost only and not the addition of the cost of the path to the ASBR
  • Default
189
Q

What is the link state retransmission list?

A
  • When a router responds to a link state request with a link state update, this LSA update is added to the link state retransmission list.
  • Once an Ack is received to say it received the LSU then the LSA is removed from the retransmission list
  • If no Ack then retransmitted after the retransmission timer expires (default 5 seconds)
190
Q

Which three OSPF packets are used as part of the OSPF adjacency build process?

A
  • Database Description Packets
  • Link State Request Packets
  • Link State Update Packets
191
Q

Of the below LSA types, which ones would you see in a standard OSPF area?

  • Type 1
  • Type 2
  • Type 3
  • Type 4
  • Type 5
  • Type 7
A
  • Type 1
  • Type 2
  • Type 3
  • Type 4
  • Type 5
  • No Type 7
192
Q

Static routes are redistributed into OSPF, no metric is defined. What will the cost of these redistributed routes be in OSPF?

A

Cost of 20

193
Q

A link state update is sent which includes a router LSA. There are two type fields, the OSPF header type field and the LSA header type field. What are these values?

A

Outer OSPF header - Type 4 (LSU)

LSA Header - Type 1 (Router LSA)

194
Q

An OSPF packet has a value of 1 in its type field. What type of OSPF packet is it?

A

Hello

195
Q

What is a type 3 LSA?

Who originates it?

A

Network Summary LSA

Originated by ABR

196
Q

What is the purpose of the Link State DB?

A

To store valid LSAs from the OSPF network

197
Q

OSPF Cost

  1. What is it?
  2. What are its range of values?
  3. How is it calculated?
  4. How can it be changed?
A
  1. Each interface has a cost associated for packets transmitted from that interface. Cost accumulated to understand total cost to a route.
  2. Range 1 - 65535
  3. Calculated as: Reference BW / Interface BW. Reference bandwidth value = 108
  4. Can be changed at interface level with command: ip ospf cost
198
Q

An AS external LSA, does it just propogate through the area which is was sourced?

A

No, propogated through whole OSPF AS

199
Q

What is the command to configure a NSSA stub area?

A

area x nssa

200
Q

How are OSPF databases synchronized between routers?

A
  1. Database Description packets are sent between neighbours
  2. Router checks DD packets to see what entrie they are missing from their DB
  3. Uses Link State Request packets to request LSA entries
  4. These entries are sent in LS Update packets
  5. Updates are acknowledged with LSAcks
  6. DB is synced between OSPF peers
201
Q

Why would a NSSA area may be required?

A
  • The area is currently configured as a stub area
  • Some external type 5 routes are required
  • ASBR introduced into area
  • Area can no longer be considered a stub
  • Default route to ABR and then more specific routes via newly introduced ASBR
202
Q

What is the purpose of a backbone router?

A

A router with at least one interface in area 0.

ABR is also backbone router

Internal router in area 0 also backbone router

203
Q

How is the ABR and ASBR details populated in the border router table?

A

ABR - Type 3 LSA

ASBR - Type 5 LSA

204
Q

What are the two types of LSA acknowledgment and what is the difference?

A
  1. Implicit Ack - Include a duplicate of the LSA in an LS Update back to the originator. Useful if already sending an update packet with other LSAs.
  2. Explicit Ack - Neighbour sends LS Ack packet.
205
Q

An ABR receives multiple network summary LSAs for a network from other ABRs accross he backbone. Will it advertise them all into its area?

A

No, ABR will choose the lowest cost advertised in the LSAs and advertise just that LSA into the non backbone area

206
Q

Explain what an ASBR Summary LSA is?

A

Type 4 LSA

Advertise the presence of an ASBR

Identical to network summary LSA but destination advertised is an ASBR and not a network.

Work with Type 5 LSAs, they advertise external networks via ASBR, type 4 tells routers outside this area how to reach the ASBR

207
Q

On which OSPF network types do routers always form adjacencies if hellos parameters match?

A
  1. Point to Point
  2. Point to Multipoint
  3. Virtual Links
208
Q

Can a link state update packet contain multiple LSAs in a single packet? If so, how many?

A

Yes

Depends on the MTU of the packet, larger MTU therefore more LSAs

209
Q

OSPF is considered a quiet protocol, explain?

A

Hello packets exchange between neighbours as keepalives

LSAs retransmitted every 30 minutes

No other activity if topology is stable

210
Q

What is an OSPF transit network?

A
  • A network that carries packets that may sourced or routed on different networks than the transit network, just passing through
  • Two or more routers
211
Q

ABRs usually require more memory & CPU. Why?

A

ABRs maintain at least 2 separate LSDBs so require more memory & CPU for those.

212
Q

What is the name for a Type 5 LSA?

A

AS External LSA

213
Q

What 3 advantages does configuring an area as a stub provide?

A
  • Conserves memory on routers
  • Improves performance
  • Simplifies Design
214
Q

What is the default hello interval on an OSPF non broadcast network?

A

30 seconds

215
Q

Of the below LSA types, which ones would you see in a stub area?

  • Type 1
  • Type 2
  • Type 3
  • Type 4
  • Type 5
  • Type 7
A
  • Type 1
  • Type 2
  • Type 3
  • No Type 4
  • No Type 5
  • No Type 7
216
Q

In OSPF what must happen before any LSA are exchanged?

A

Neighbours must be formed

217
Q

An OSPF packet has a value of 4 in its type field. What type of OSPF packet is it?

A

Link State Update

218
Q

OSPF has two destination types, what are they?

A
  • Network entry
  • Router entry
219
Q

What OSPF network types use unicast?

A
  1. NBMA
  2. Point to Multipoint
  3. Virtual Links
220
Q

Which router in an OSPF network could possibly generate the followings LSA types:

  • Type 1
  • Type 2
  • Type 3
  • Type 4
  • Type 5
  • Type 7
A
  • Type 1 - All routers
  • Type 2 - DR
  • Type 3 - ABR
  • Type 4 - ABR
  • Type 5 - ASBR
  • Type 7 - ASBR
221
Q

Area 0

What is the purpose of area 0?

Can traffic for area 1 exchange packets directly with area 2?

A

Area 0 is the backbone

Responsible for summarizing each area to each other area

No, all inter-area traffic must pass through the backbone (area 0). Non backbone areas cannot exchange traffic directly.

222
Q

What are the two type of NSSA area and what is the difference?

A

NSSA and NSSA Totally Stubby

NSSA allows Type 3s while totally stubby NSSA does not

223
Q

An OSPF packet consists of multiple encapsulations.

What is the outside encapsulation and the inside encapsulation?

A

Outside - IP Header

Inside - OSPF packet

224
Q

Can a stub area have more than one ABR? If so, what is one potential drawback?

A

Yes, more than one ABR is possible

Issue: Default route from both ABRs so cannot determine which router is the optimal gateway to the ASBR

225
Q

What is a type 2 LSA and who sends them?

A

Network LSA

DR sends them, represeting multi access network

226
Q

In the command “show ip ospf database router x.x.x.x”, part of the output says “Routing bit set on this LSA”.

What does this mean?

A

Indicates that routing is valid, that destination is in the routing table.

This LSA is advertising a destination and the route to the next hop is in the routing table.

Example: Link failure then routing bit set disappears

227
Q

OSPF is link state within an area but distance vector for inter-area routing. True / False?

A

True

228
Q

An OSPF packet has a value of 3 in its type field. What type of OSPF packet is it?

A

Link State Request

229
Q
  1. What is the link state request list?
  2. How are elements added to the list and how are they removed?
A
  1. A router looks through the DD packets from the neighbour to understand if they have outdated LSAs or don’t have particular ones, each required LSA is added to the link state request list
  2. Link state request sent looking for particular LSAs, the updates are sent back as link state updates. LSA then removed from link state request list
230
Q
  1. What happens if there is only one eligible DR / BDR on a multiaccess network?
  2. What happens if there is no valid DR / BDR on a multiaccess network?
  3. Why would there be only one or two valid DR / BDRs on a multiaccess network?
A
  1. Router becomes DR, no BDR
  2. All routers stay in a two-way state with each other
  3. Some routers configured with router priority 0
231
Q

How does a router know two way communication is established between OSPF peers? Is an adjacency formed then?

A

If router’s own router ID in hello packet back from peer.

Adjacency may form but depends on network type. Eg. Adjacency for P2P but not for broadcast.

232
Q

What is the difference between an E1 and an E2 path?

A

E1 - path to ASBR added to cost of external route

E2 - path to ASBR not added, just cost of external route

233
Q

What three fields in an LSA ensure the most recent copy of the LSA?

A
  1. Sequence Number
  2. Max Age
  3. Checksum
234
Q

OSPF has learned a destination via an E1 path type and an E2 path type.

Which one will it install in the routing table and why?

A
  • It will install E1
  • OSPF prunes routes based on less preffered path type
  • Path types preffered 1 - 4
  1. Intra Area
  2. Inter Area
  3. E1
  4. E2
235
Q

What are the 5 purposes of hello packets is OSPF?

A
  1. Neighbour Discovery
  2. Advertise parameters routers must agree before becoming neighbours
  3. Act as keepalives
  4. Establish bi-directional communications
  5. User to elect DR / BDR in broadcast / NBMA networks
236
Q

What are the four phases of OSPF neighbour adjacency formation?

A
  1. Neighbour Discovery
  2. Bidirectional Communication
  3. Databse Synchronization
  4. Full Adjacency
237
Q

When an OSPF configured interface first comes alive:

  1. What is DR / BDR set to in Hellos?
  2. How long does it set wait timer?
  3. What happens if wait timer expires?
A
  1. DR / BDR field set to 0.0.0.0
  2. Wait timer is 4 times the hello which is equal to the dead timer (40 seconds in broadcast networks)
  3. Elects itself the DR
238
Q

What is an OSPF process ID?

A
  • Process ID is Cisco specific, not part of open OSPF standard
  • Allows multiple OSPF processes to run on a single router
  • It has no significance outside of the router
239
Q

Describe the OSPF neighbour Exstart state?

A
  • router abd its neighbour establish master / slave relationship
  • determin initial DD sequence number for exchange of DD packets
  • neighbour with highest router ID becomes master
240
Q

What command is used to show AS External LSAs in LSDB?

A

show ip ospf database external

241
Q

Based on a hello packet, how can you tell the router is configured as a stub?

A

The E bit within the OSPF options is set to 0, normally set to 1

242
Q

What are the 3 advantages of OSPF over distance vector routing protocols?

A
  1. Scalability
  2. Faster Convergence
  3. Less susceptible to bad routing information
243
Q
  1. After a DR / BDR election, does the other routers see each other as neighbours and if so, what state are other routers in?
  2. Who does the other routers form an adjacency with?
A
  1. Yes they are considered neighbours and they are in a DROther State
  2. They form an adjacency with both the DR & BDR
244
Q

Describe OSPF basic operation.

A
  1. OSPF routers send hellos out all OSPF enabled interfaces
  2. If two speakers on common link agree certain parameters, they form adjacency.
  3. Each router sends LSAs to describe all routers links and a routers neighbours.
  4. Router records LSAs received in LSDB
  5. Router send copy of received LSAs to all its neighbours
  6. Routers flood LSAs within area, so each router has identical LSDB.
  7. Routers use SPF algorithm to build loop free shortest path to each destination.
  8. Routing table populated from SPF tree
245
Q

What steps does a router take to determine the most recent LSA if it receives multiple?

A
  1. Compare sequence numbers - higher is more recent
  2. Compare checksums - highest unsigned is more recent
  3. Compare Age - if either is max age then it is most recent (being flushed)
  4. LSA Age differs by more than max age diff (15 mins)? LSA with lower age is more recent
  5. All previous conditions are equal - LSAs are identical
246
Q

Point to Multipoint

  1. Explain what this network type is?
  2. Is a DR / BDR required?
  3. Are packets unicast or multicast?
A
  1. Special configuration of NBMA, network treated as collection of P2P links
  2. No DR / BDR
  3. Unicast
247
Q

OSPF Point to Point Links

  1. How many routers?
  2. Will they form an adjacency?
  3. What destination address do they always use?
A
  1. Always a pair of routers
  2. They will always form an adjacency
  3. Destination Address - 224.0.0.5
248
Q

What is an OSPF stub network?

A
  • Packets always sourced or destined for a destination address belonging to this network
  • Only a single attached router
249
Q

What are the three types of area traffic?

A
  1. Intra Area
  2. Inter Area
  3. External (from another routing domain)
250
Q

A client has designed their OSPF network, it has a permanent virtual link in the design. Is this good design practise?

A

No, should never be permanent.

Add more redundant links or design merging of two OSPF networks better.

251
Q

Explain the OSPF dead time and how long it is?

A
  • if router doesn’t receive hello within a dead time then OSPF neighbour is declared down
  • default interval in 4 x Hello Interval
  • Interface Specific - can be changed with command ip ospf dead-interval
252
Q

Identify 3 possible design issues with this OSPF network, how critical are they and how to resolve each.

A
  1. Link failure between R1 & R2 will cause Area 1 to be partitioned. Will continue fine as partitioned. Have a link also between R3 & R4.
  2. Link Failure between R5 & R6 will cause a partitioned backbone. Critical as it splits the OSPF domain in half. Add more routers & links in Area 0 to enhance connectivity.
  3. Router failure of R6 will cut Area 2 off from the backbone. Critical issue as Area 2 has no access to backbone. Add another ABR to Area 2 to resolve.
253
Q

A DR in a multi access networks sends type 2 LSAs to the rest of the routers in the OSPF domain, does it also send type 1?

A

Yes, all DROthers send type 1 router LSAs to the DR, the DR then sends these to every other router in the OSPF area including the DROthers.

254
Q

OSPF Broadcast Networks

  1. How many routers?
  2. How are they connected?
  3. What is the addresses 224.0.0.5 and 224.0.0.6 used for?
A
  1. More than two routers
  2. Connects on a single broadcast network, share same subnet
  3. 224.0.0.5 is used by all routers and DR / BDR. 224.0.0.6 is used by all routers to send LSU and LSAck packets
255
Q

Age

  1. How many bit value?
  2. What is the range of values?
  3. What happens the age value as the LSA is flooded through the network?
  4. What happens the age value in a LSDB?
  5. What happens when LSA reaches max age?
A
  1. 16 bit
  2. 0 to 3600 (60 minutes)
  3. Originating LSA sets age to 0
    • every router increments the age by the InfTransDelay
    • Cisco switches default is 1
    • Can be changed - ip ospf transit delay
  4. Age incremented in LSDB
  5. LSA is set to max age, reflooded and then flushed from DB, flushed from all routers DBs when it receives LSA with max age
256
Q

Sequence Number

  1. How many bit value?
  2. What is the range of values?
  3. Why does the range begin with an 8 and the last value begin with a 7?
  4. Who sets the LSA value?
  5. What happens when LSA reaches Max Sequence number and new instance is created?
A
  1. 32 bit
  2. Starts at 0x80000001, ends at 0x7FFFFFFF
  3. Sequence numbers are signed intergers, half are negative, half are positive. Number 8 in hex, MSB is a 1 which indicates negative. Number 7 in hex, MSB is 0 which indicates positive.
  4. Router who originates the LSA
  5. Router flushes old LSA
    • sets max age on LSA
    • refloods it
    • all routers ack that LSA
    • router then floods LSA with starting sequence number again
257
Q

How does OSPF control if NSSA learned Type 7 LSAs are forwarded into other areas?

A

P-Bit (from NP bit) in LSA header Options fields

P-Bit set - NSSA ABR will translate type 7 to type 5 and flood to other areas

P-bit not set - NSSA ABR will not translate / flood the LSAs to other OSPF areas

258
Q

What 8 fields are included in a DD packet?

A
  1. Interface MTU
  2. Options
  3. I-Bit
  4. M-Bit
  5. MS-Bit
  6. Sequence Number
  7. LSA Headers
259
Q

What are the 3 fields within a link state request packet?

A
  1. Link State Type
  2. Link State ID
  3. Advertising Router
260
Q

What is a link state request packet?

A

Request LSAs based on reviewing DD packets

261
Q

What is an OSPF router destination type?

A

Destination is a router address

Route to ABR or ASBR

Inter Area Packet - Then need to know how to get to ABR

External Network Packet - Then need to know how to get to ASBR

262
Q

What is a totally stubby network?

A

Uses default route to reach all external destinations including external area and external AS.

263
Q

A NSSA area has Type 7 LSAs with the P bit set in the LSA header, what will the NSSA ABR do with this LSA?

A

Translate Type 7 LSA to Type 5 and flood to other OSPF areas in AS.

264
Q

What are the 5 OSPF network types?

A
  1. Point to Point
  2. Point to Multipoint
  3. Broadcast
  4. NBMA
  5. Virtual Link
265
Q

True or False?

Link state updates and link state ack packets can carry multiple LSAs?

A

True

266
Q

What happens in the OSPF exchange state?

A
  1. Neighboure sync link state DBs by describing all LSAs (headers) in DD packets
267
Q

What is the difference between the below commands?

  • area x nssa
  • area x nssa no-summary
  • area x nssa nssa-only
A
  • Configures an NSSA, P bit set, Type 7 to Type 5 translation on ABR, Type 3s allowed in area
  • Configures an NSSA, P bit set, Type 7 to Type 5 translation on ABR, no Type 3 allowed in area other than default route
  • nssa-only keyword meansno Type 7 to Type 5 translation on ABRs so routes don’t leave area
268
Q

What do ABRs connect an area to?

A

To another area, the backbone area (area 0)

269
Q

An OSPF packet has a value of 2 in its type field. What type of OSPF packet is it?

A

Databse Descriptor

270
Q

What 3 fields in a router LSA detail a particular link?

A
  1. Link Type
  2. Link ID
  3. Link Data

Will be multiples of these fields to descibe each OSPF enabled interface on a router.

271
Q

Which routers originate a network LSA?

A

DRs

272
Q

EIGRP routes are redistributed into OSPF, no metric is defined. What will the cost of these redistributed routes be in OSPF?

A

Cost of 20

273
Q

What do OSPF packets have in common on how do they differ?

A

All have same OSPF packet header format

OSPF Packet Data varies between packet types

274
Q

True or False?

  1. An LSAck packet can Ack multiple LSAs in a single packet?
  2. If true, how does it do this?
A
  1. True
  2. Includes only the header of the LSA to be acked so can include multiple in one Ack packet.
275
Q

Describe the OSPF neighbour 2-way state?

A
  • Router has seen its router ID in a hello packet from OSPF peer, bi-directional comms
  • for broadcast network, must be in this state for DR / BDR election
  • can also enter this state is DD packet received from neighbour while in init state
276
Q

What are the possible values for version in an OSPF packet header?

A

Version 2 or Version 3

277
Q

A router has an LSA in its LSDB for the past 35 minutes.

Is this expected behaviour and what is expected to happen to it?

A
  • No, LSA didn’t refresh after 30 minutes. Must not have receved refresh packet from originating router
  • Router will flush the LSA after 1 hour
278
Q

What command is required to redistribute static routes into OSPF with a metric value of 20?

A

redistribute static

No requirement for metric as 20 is default metric

279
Q

What is the key difference between a Type 5 LSA and a Type 7 LSA?

A

Almost identical, both originated by ASBRs

Type 5 flooded throughout OSPF AS

Type 7 only flooded within NSSA area it was originated in

280
Q

What 6 pieces of information must routers match to form OSPF adjacency? What happens if not all match?

A
  1. Area ID
  2. Authentication
  3. Network Mask
  4. Hello Interval
  5. Dead Interval
  6. Options

Packet dropped if not all of these match

281
Q

What 3 fields does a network LSA include?

A
  1. Link State ID
  2. Network Mask
  3. Attached Router
282
Q

A database description packet has three flags, what are they and what is their purpose?

A
  • I bit (Initial bit) - indicates first DD packet sent
  • M Bit (More bit) - indicates its not the last DD packet to be sent
  • MS Bit (Master / Slave bit) - set in DD packets originating from master
283
Q

When does a virtual link transition to fully functional?

What is the cost of virtual link?

A
  • Route to neighbouring ABR is in routing table
  • cost or route to he neighbour
284
Q

What 4 fields are included in a type 3 & type 4 LSA?

A
  1. Link State ID
  2. Network Mask
  3. Metric
  4. ToS
285
Q

What command would you use to see type 4 LSAs in the LSDB?

A

show ip ospf database asbr-summary

286
Q

A type 3 & type 4 LSA have an identical format, what is different within their packets?

A
  1. Type Number in LSA header
  2. Link State ID
  • Type 3 is network being advertised
  • Type 4 is router ID of ASBR
287
Q

What is the link state ID of a type 4 LSA?

A

Router ID of ASBR being advertised

288
Q

What is the link state ID of a type 3 LSA?

A

Network being advertised into area

289
Q

What command would you use to see type 3 LSAs in LSDB?

A

show ip ospf database summary

290
Q

How far through an OSPF domain are type 3 and type 4 LSAs flooded?

A

Within a single area only

291
Q

What is the network mask field set to in a Type 4 LSA?

A

0.0.0.0

Advertising router ID so no significance

292
Q

What is the network mask field set to in a type 3 LSA?

A

The network mask of the route being advertised into the area.

293
Q

If a type 3 LSA is advertising a default route, what are the link state ID and network mask fields set to?

A

They are both set to 0.0.0.0

294
Q

In a type 3 & type 4 LSA, what is the ToS field set to for Cisco devices?

A

0

295
Q

In a type 3 / type 4 LSA what is the metric field set to in the LSA?

A

Cost of the route to its destination

296
Q

What OSPF router originates a type 5 LSA and what does it advertise?

A

ASBR

External networks redistributed into OSPF

297
Q

What command is used to show type 5 LSAs in a LSDB?

A

show ip ospf database external

298
Q

Where are type 5 LSAs flooded to?

A

All areas in OSPF except stub areas

299
Q

What are the 6 fields in a type 5 LSA?

A
  1. Link State ID
  2. Network Mask
  3. E or External Metric
  4. Metric
  5. Forwarding Address
  6. External Route Tag
300
Q

What is the link state ID of a type 5 / type 7 LSA?

A

External Destination

301
Q

If a type 5 route advertises a default route, what are the link state ID and network mask fields set to?

A

Both set to 0.0.0.0

302
Q

What is the purpose of the E or External Metric in an type 5 LSA?

A

Defines if a route is an E1 or E2 route

E = 0 then E1

E = 1 then E2 (default)

303
Q

What is the difference between an E1 and E2 external route and which is default.

A

E1 - Includes internal cost

E2 - Excludes internal cost (default)

304
Q

In a type 5 LSA, what does the forwarding address of 0.0.0.0 mean?

A

Packets are forwarded to the originating ASBR

305
Q

What is the purpose of the route tag field in a type 5 LSA?

A
  • Tag that may be applied to external route
  • Not used by OSPF protocol itself
  • Used to manage routes via route-maps
306
Q

What is the difference in packet format between a type 5 and a type 7 LSA?

A

Packets are identical except forwarding address

307
Q

What command is used to show Type 7 LSAs in a LSDB?

A

show ip ospf database nssa-external

308
Q

What OSPF packets have the options field in their header?

A
  1. Hello
  2. DD
  3. All LSAs
309
Q

What are the 8 options that are part of an OSPF header?

A
  1. DN
  2. O
  3. DC
  4. EA
  5. NP
  6. MC
  7. E
  8. MT
310
Q

What is the purpose of the DN option in an OSPF header?

A

Used with L3 MPLS VPNs

Prevents Loops

311
Q

If the O option is set in an OSPF header, what does this mean?

A

Router Supports Opaque LSAs

Type 9, 10 & 11 LSAs

312
Q

What is the purpose of the DC option in an OSPF header?

A

Set if originating router is capable of supporting OSPF over demand circuits

313
Q

What is the purpose of the MC option in an OSPF packet header?

A
  • Originating router capable of forwarding Ip multicast packets
  • Used in MOSPF
314
Q

What is the purpose of the MT option in an OSPF packet header?

A
  • Originating router capacble of supporting multi topology OSPF (MT-OSPF)
  • Still only proposal, no adoption
315
Q

What is the N/P option in an OSPF header, which packet types are they used?

A
  • Used for NSSA external networks
  • Same location
  • N & P share the same option bit.
  • N if part of Hello packets
  • P if part of NSSA External LSA header
  • Use is based on which OSPF packet type is used
316
Q

What is the purpose of the N bit in an OSPF packet header and what packet type uses it?

A
  • Used in Hello Packets
  • If set, router supports NSSA external LSAs
  • Mismatch, then no adjacency formed
317
Q

What is the purpose of the P bit in an OSPF packet header and what packet type uses it?

A
  • Used by Type 7 NSSA External LSAs
  • If set, ABR will translate type 7 to type 5 and flood to other areas
  • If not set, type 7 not translated or flooded to other areas
318
Q

What type of OSPF router would originate an OSPF header with the E bit option not set?

A

Stub router

Doesn’t accept external LSAs

Set to 0 in all LSAs originated from stub area

319
Q

Where would you see an OSPF LSA with an E bit set?

A
  • All External LSAs
  • All LSAs originated in the backbone
  • All LSAs originated in non stub areas
320
Q

A router receives a hello packet with the E bit option not set in the header.

  • What does this indicate?
  • What happens if there is a mismatch in the E bit in hellos between routers?
A
  • The originating router does not support sending / receiving Type 5 External LSAs
  • No adjacency formed

OSPF (1 decks)

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