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ag-CCNA COPY PLUS > 8 - OSPF and Routing Protocols > Flashcards

8 - OSPF and Routing Protocols Flashcards

1
Q

What are routing protocols designed to exchange routes between routers in different Autonomous Systems called?

A

EGP (Exterior Gateway Protocols)

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2
Q

What is an ASN?

A

An Autonomous System number

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3
Q

What is the only EGP in use today?

A

BGP (Border Gateway Protocol)

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4
Q

What are the three main branches of routing protocol algorithms for IGP protocols?

A
  1. Distance vector
  2. Advanced distance vector (hybrid)
  3. Link-state
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5
Q

What type of routing protocol is RIP?

A

Distance vector

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6
Q

What type of routing protocol is OSPF and IS-IS?

A

Link state

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7
Q

What type of routing protocol is EIGRP?

A

Advanced distance vector / hybrid

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8
Q

What metric does RIP use?

A

Hop count (between router and destination subnet)

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9
Q

What metric does OSPF use?

A

Cost (sum of all interface cost settings for all links in route)

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10
Q

What metric does EIGRP use?

A

Calculation based on bandwidth and delay.

Calculated based on route’s slowest link and cumulative delay associated with each interface in the route

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11
Q

What command is used to change the perceived bandwidth for an interface?

A

The bandwidth command

cost = reference bandwidth / interface bandwidth

RP/0/RSP0/CPU0:router# configure
RP/0/RSP0/CPU0:router(config)# router ospf 1
RP/0/RSP0/CPU0:router(config-ospf)# auto-cost reference-bandwidth 1000

To use this command, you must be in a user group associated with a task group that includes appropriate task IDs. If the user group assignment is preventing you from using a command, contact your AAA administrator for assistance.

By default OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface.

The OSPF metric is calculated as the mbps value divided by bandwidth, with mbps equal to 108 by default.

If you have multiple links with high bandwidth (such as OC-192), you might want to use a larger number to differentiate the cost on those links. That is, the metric calculated using the default mbps value is the same for all high-bandwidth links.

Recommended usage of cost configuration for OSPF interfaces with high bandwidth is to be consistent: Either explicitly configure (by using the cost command) or choose the default (by using the auto-cost command).

The value set by the cost command overrides the cost resulting from the auto-cost command.

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12
Q

What routing protocol sent routing protocol messages as IP broadcast addresses?

A

RIPv1

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13
Q

What is route redistribution?

A

When two routers or AS are operating different routing protocols, and routers can take routes learned by one protocol and redistribute them into the other routing protocol and vice versa

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14
Q

How does IOS choose between routes learned using different routing protocols?

A

Administrative distance. Lowest is best

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15
Q

What is the default administrative distance for RIP?

A

120

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16
Q

What is the default administrative distance for OSPF?

A

110

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17
Q

What is the default administrative distance for EIGRP?

A

90

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18
Q

What is the default administrative distance for a connected route?

A

0

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19
Q

What is the default administrative distance for a static route?

A

1

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20
Q

How do you change the administrative distance of a route directly?

A

Using the IP route command:

ip route 192.168.1.0 255.255.255.0 192.168.1.1 <210>

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21
Q

What is an LSA?

A

A Link State Advertisement used in OSPF to be flooded / collectively advertise almost every detail about the network to all other routers to ensure every router in the network has the exact same information

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22
Q

What is an LSDB?

A

Link State Database. A collection of all LSAs known to the router

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23
Q

How does the OSPF flooding process work?

A

Every router floods an LSA containing its information to other routes. All routers receiving LSAs can learn the content and forward it if necessary.

To prevent LSAs flooding in circles routers communicate to neighboring routers asking if they already have the LSA

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24
Q

What is the LSA aging timer?

A

Determines how often an LSA is re-flooded. Default is 30 minutes

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25
Q

True/False: The LSDB states each router’s best route to a destination

A

False. Routers must do math (SPF algorithm) using the information in the LSDB

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26
Q

What are the three main phases of OSPF?

A
  1. Becoming neighbors
  2. Exchanging databases
  3. Calculating/adding best routes
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27
Q

What command shows OSPF neighbors?

A

show ip ospf neighbor

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28
Q

How does the OSPF neighbor model allow new routers to be dynamically discovered and added without requiring reconfiguration of routers?

A

OSPF routers listen for Hello messages from new routers. Upon receiving them they attempt to become neighbors and exchange LSDBs

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29
Q

What OSPF process allows new neighbor relationships to be formed?

A

OSPF Hello process

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30
Q

What is the RID in OSPF?

A

Router ID. The routers unique name/identifier for OSPF. It is included in the OSPF Hello message

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31
Q

How many bits is there in a routers OSPF RID?

A

32

32
Q

True/False: OSPF must have an area 0, and all other areas should connect to it

A

True

33
Q

What does IOS choose the RID from by default?

A

The highest IP of all configured loopback interfaces. Otherwise the highest IP of all active physical interfaces

34
Q

Where are OSPF Hello messages addressed to when being sent out?

A

Multicast 224.0.0.5

35
Q

How often are Hellos sent?

A

10 seconds (default Hello timer)

36
Q

What is the dead interval for?

A

Determines how long to wait without receiving Hellos from a neighbor before it is declared dead

37
Q

What is the recommended value for the dead interval in ?

A

4 x the Hello timer

38
Q

What is true of routers at the 2-Way state in OSPF?

A
  1. Router has received a Hello from a neighbor, with it’s own RID listed as being seen by that neighbor
  2. Router has checked all parameters in Hello from neighbor and no problems
  3. Both routers reaching this state are ready to exchange their LSDB with each other
39
Q

How do routers determine which information to share when exchanging LSDB information so they don’t have to obtain entries they already have?

A

They provide a list of known LSAs in their respective databases. Then each router can check which ones it has known and only request the other for ones it doesn’t know yet

40
Q

What are LSUs and what are they used for?

A

Link State Updates. These LSU packets are what are actually used to send the LSA information. They hold LSA data structures (not packets)

41
Q

What is the full state in OSPF?

A

The state reached after routers have finished exchanging LSDB information (aka LSUs containing LSA data structures)

42
Q

How do routers react to topology changes in OSPF?

A

One or more routers change LSAs, then they must flood the changed LSAs to each neighbor so they can in turn update their LSDB

43
Q

How do routers in OSPF actually calculate routes?

A

Using SPF

44
Q

What do routers need to do with LSAs periodically even during a stable network?

A

Reflood each LSA that a router created every 30 minutes by default

45
Q

What are the three maintenance tasks done by routers in OSPF?

A
  1. Maintain neighbor state (Hello and Dead intervals)
  2. Flood any changed LSAs to each neighbor
  3. Periodically reflood unchanged LSAs (30 mins by default)
46
Q

What type of networks does OSPF use a network type of broadcast on?

A

Ethernet links

47
Q

How is LSDB exchange in OSPF different on Ethernet/Broadcast network types compared to simple point to point?

A

OSPF routers elect a designated router (DR) and a backup designated router (BDR). Database exchange now takes place between the DR and each router, not between every pair of routers

48
Q

What is the Designated Routers job in OSPF?

A

To make sure all other OSPF routers in the subnet get a copy of each LSA

49
Q

What does the BDR do?

A

Watches the status of the DR and takes over if it fail

50
Q

What is the Multicast IP 224.0.0.5 reserved for?

A

All OSPF routers (for hellos)

51
Q

What is the Multicast address for All SPF DRs?

A

224.0.0.6

52
Q

How does the use of DR/BDR along with multicast IP addresses make the exchange of OSPF LSDBs more efficient on networks with more than two routers on the same link?

A

DRs can send packets to all routers in the subnet using the Multicast 224.0.0.5 and all routers can send packets to DRs with 224.0.0.6

53
Q

Why is the 2-Way state normal for OSPF routers in Ethernet/Broadcast networks that utilize DR/BDRs?

A

Because only the DR and BDR routers do a full LSDB exchange with all other routers in the subnet. Thus the others never reach the full state because they don’t exchange LSDB’s directly with each other

54
Q

What is the difference between neighbors and adjacent neighbors/fully adjacent neighbors?

A
  1. Neighbors are routers on the same link that have reached the 2-Way state
  2. Those neighbors that have continued and reached the Full state are called adjacent neighbors
55
Q

True/False: A router can just copy information from the LSDB into a route in the routing table

A

False. It must do some SPF math

56
Q

How does SPF work?

A

SPF calculates all routes to the destination subnet, then compares metrics to select the one with the lowest (best) metric to add to the routing table

57
Q

What is the SPF/OSPF metric for a given route actually calculated from?

A

The Metric is the sum of OSPF interface costs for all outgoing interfaces in the route

58
Q

True/False: OSPF considers costs of outgoing and incoming interfaces when calculating a route metric

A

False. It only considers the outgoing interface costs

59
Q

What is the downside of OSPF single area design in large networks?

A
  • Routers take time to calculate SPF, causing slow convergence times
  • Larger topology database consumes more memory on each router
  • Single interface change anywhere in network forces every router to run SPF again
60
Q

What is an ABR and what does it do with regards to summary information?

A

Area Border Router. Is a router that has interfaces in more than one area (backbone and non backbone area)

Creates summary information about each subnet in one area to advertise into other areas

61
Q

What is an OSPF area?

A

A set of routers and links that share the same detailed LSDB information

62
Q

How do OSPF areas reduce SPF calculation time?

A

Smaller topologies in each LSDB mean there are less entries to process, because a given LSDB only maintains entries for that area

63
Q

What is a Summary LSA used for?

A

Because routers still need to know about subnets in other areas to be able to create IPv4 routes for those, so summary LSAs contain information such as SubnetID and mask for some subnets in other areas

64
Q

What are the pros of using multiple areas in larger OSPFnetworks?

A
  • Faster SPF calculation, hence faster convergence
  • Smaller LSDB per area using memory
  • Network changes allow faster convergence
  • Less information advertised
65
Q

What is the LSA type number and purpose of a Router LSA?

A

Type 1

Describe the router

  • RID
  • Interfaces
  • IP/mask
  • Current interface state
66
Q

What is the LSA type number and purpose of a Network LSA?

A

Type 2

Lets everybody know who the DR is in the network

  • Generated by DR
  • DR/BDR IP address
  • Subnet ID and mask
67
Q

What is the LSA type number and purpose of a Summary LSA?

A

Type 3

Describe a subnet in another area

  • ABR Summary route
  • Subnet ID, mask, RID of ABR that advertises the LSA
68
Q

What LSA type describes the location of an ASBR?

A

Type 4 Summary LSA

69
Q

What LSA type describes an ASBR Summary Route?

A

Type 5 External LSA

70
Q

What is the default administrative distance for eBGP?

A

20

71
Q

What is the default administrative distance for IGRP?

A

100

72
Q

What is the default administrative distance for IS-IS?

A

115

73
Q

What is the default administrative distance for External EIGRP?

A

170

74
Q

What is the default administrative distance for iBGP?

A

200

75
Q

What is the default administrative distance for a EIGRP summary route?

A

5

ag-CCNA COPY PLUS (34 decks)

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