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OSPF Flashcards

1
Q

Link-State

A

Routing bases on the nodes in the graph
- Flood routing updates to all the routers in the network
- Idea with Link State is that all the nodes (routers) in the entire environment know about everybody else.
- Small updates are send to the routers in Link State
- Each individual node has a picture of the entire network
- IS-IS, OSPF
- Take more recourses because each device needs an entire picture of the network

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2
Q

Link-State protocol

A

Use characteristics of the route such as speed and cost as well as current congestion to determine the best path, which is typically computed by the Dijkstra algorithm (SPF).

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3
Q

Link-State Packet types

A
  • Router Links
    The router links are an indication of the state of the interfaces on a router in a certain designated area. Each router generates a router link for all of its interfaces.
  • Summary Links
    Summary links are generated by ABRs; this is how network reachability information is disseminated between areas.

Normally, all information is injected into the backbone (area 0) and in turn the backbone passes it on to other areas.

ABRs also propagate the reachability of the ASBR. This is how routers know how to get to external routes in other ASs.

  • Network Links
    Network Links are generated by a Designated Router (DR) on a segment.
  • External Links
    External Links are an indication of networks outside of the AS. These networks are injected into OSPF via redistribution. The ASBR injects these routes into an autonomous system.
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4
Q

Definition of an area

A

It defines a flooding domain

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5
Q

Flooding domain

A

All devices in the area agree on the toplogy
Changes inside the area require LSA flooding and full SPF

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6
Q

Inter-area routing

A

Similar to distance vector routing
You only know information that the neighbor is advertising to you.

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7
Q

Define Classful protocol

A

Classful routing protocols DO NOT send the subnet mask along with their updates.

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8
Q

Define Classless protocol

A

Classless routing protocols DO send the subnet mask along with their updates.

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9
Q

OSPF port

A

OSPF runs over IPv4 and IPv6, but does not use a transport protocol such as UDP or TCP. It encapsulates its data directly in IP packets with protocol number 89.

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10
Q

OSPF Control Place security

A

Yes, such as: Clear text, MD5, SHA,IPsec, etc.

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11
Q

Exensible

A

Future application support through “opaque” LSA. These are specifically written in the protocol to add new extensibility’s in it.

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12
Q

How does OSPF work in steps

A
  1. Discover OSPF neighbors & exhange topology information.
  2. Choose best path via SPF.
    We take the input which is the LSA (OSPF) database and put that in to the SPF algorithm and the output of that is the SPT (shortest path tree) and that’s gonna be used to route the traffic.
  3. Neighbor and topology table maintenance.
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13
Q

Neighbor and topology discovery

A

OSPF uses the Hello protocol to discover neighbors on OSPF enabled attached links

Hello packets are sent periodically out of each interface through IP multicast (Appendix B).

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14
Q
A
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15
Q

OSPF Multicast addresses

A

224.0.0.5
224.0.0.6

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16
Q

OSPF transport

A

Via IP protocol 89

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17
Q

LSDB

A

Link State Database

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18
Q

Distance Vector

A

Routing based on destination prefixes
- Sends routing updates to its neighbors
- Send larger update to the neighboring routers
- Each node running DV only knows about their neighbor
- Routing by Rumor
- RIP, IGRP, EIGRP, BGP

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19
Q

OSPF Adjacency States

A
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20
Q

Designated Router

A

All our OSPF routers will only form a “full” neighbor adjacency with the DR and not with all the other routers.

Rather than exchange updates with every other router on the segment, every router exchanges information with the DR and BDR.
The DR and BDR relay the information to everybody else.

Each router on the segment (which already became a neighbor) tries to establish an adjacency with the DR and BDR.

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21
Q

DR Election

A

The router with the highest OSPF priority on a segment becomes the DR for that segment. The same process is repeated for the BDR. In case of a tie, the router with the highest RID prevails.

The default for the interface OSPF priority is one. Remember that the DR and BDR concepts are per multiaccess segment.

A priority value of zero indicates an interface which is not to be elected as DR or BDR. The state of the interface with priority zero is DROTHER.
If a router is neither DR or BDR, their state will be DROTHER.

22
Q

Backup Designated Router

A

Since bad stuff can happen to our networks, we want to have a backup for our DR. If it crashes, the BDR (Backup Designated Router) will take over. All our OSPF routers will only form full neighbor adjacencies with the DR and BDR and not with all other routers.

23
Q

Hello Packet matching

A

To become neighbors the following has to match:
- Hello / Dead interval
- Area ID
- Authentication password
- Stub area flag

24
Q

When a new LSA is received it is checked against the database for changes such as?

A
  • Sequence number. Higher sequence number is gonna win.
  • Age. Periodic flooding occurs after 30 minutes. LSAs that reach MaxAge (60 minutes) are withdrawn.

When a link goes down we set the LSA to MaxAge so the route disappears.

  • Checksum. Used to avoid transmission & memory corruption. Where BGP runs on top of TCP, OSPF & EIGRP uses ‘itself’ (this checksum) to make sure everything is send and received correct.
25
LSA Flooding
Not all LSA changed require SPF to recalculate.
26
ABR
Area Border Routers Routers that belong to multiple areas, and connect these areas to the backbone area are called area border routers. ABRs must therefore maintain information that describes the backbone areas and other attached areas.
27
Internal Router
A router that has all of its interfaces within the same area is called an internal router.
28
ASBR
Routers that act as gateways (redistribution) between OSPF and other routing protocols (IGRP, EIGRP, IS-IS, RIP, BGP, Static) or other instances of the OSPF routing process are called autonomous system boundary router. Any router can be an ABR or an ASBR.
29
OSPF process-id
The OSPF process-id is a numeric value local to the router. It does not have to match process-ids on other routers. It is possible to run multiple OSPF processes on the same router, but is not recommended as it creates multiple database instances that add extra overhead to the router.
30
OSPF area-id
The area-id is the area number we want the interface to be in. The area-id can be an integer between 0 and 4294967295 or can take a form similar to an IP address A.B.C.D.O
31
OSPF authentication
By default, a router uses a Null authentication which means that routing exchanges over a network are not authenticated. Two other authentication methods exist: Simple password authentication and Message Digest authentication (MD-5).
32
The Backbone and Area 0
OSPF has special restrictions when multiple areas are involved. If more than one area is configured, one of these areas has be to be area 0. This is called the backbone. It is good network design practice to start with area 0 and then expand into other areas later on. The backbone has to be at the center of all other areas, that is, all areas have to be physically connected to the backbone. The reason is that OSPF expects all areas to inject routing information into the backbone and in turn the backbone disseminates that information into other areas.
33
intra-area routes
Routes that are generated from within an area (the destination belongs to the area) are called intra-area routes. These routes are normally represented by the letter O in the IP routing table.
34
inter-area routes
Routes that originate from other areas are called inter-area or Summary routes. The notation for these routes is O IA in the IP routing table.
35
external routes
Routes that originate from other routing protocols (or different OSPF processes) and that are injected into OSPF via redistribution are called external routes. These routes are represented by O E2 or O E1 in the IP routing table.
36
Routes type preferrance
intra-area, inter-area, external E1, external E2
37
Virtual Links
Virtual links are used for two purposes: - To an area that does not have a physical connection to the backbone - To patch the backbone in case discontinuity of area 0 occurs. The virtual link provides the disconnected area a logical path to the backbone. The virtual link has to be established between two ABRs that have a common area, with one ABR connected to the backbone.
38
Adjacencies
Adjacency is the next step after the neighbor process. Adjacent routers are routers that go beyond the simple Hello exchange and proceed into the database exchange process.
39
Adjacency build process
The adjacency build process takes effect after multiple stages have been fulfilled. Routers that become adjacent have the exact link-state database. - Down: No information has been received from anybody on the segment. - Attempt: On non-broadcast multi-access clouds such as Frame Relay and X.25, this state indicates that no recent information has been received from the neighbor. To contact the neighbor, send Hello packets at the reduced rate Poll Interval . - Init: The interface has detected a Hello packet from a neighbor but bi-directional communication has not yet been established. - Two-way: There is bi-directional communication with a neighbor. The router has seen itself in the Hello packets from a neighbor. At the end of this stage the DR and BDR election would have been done. At the end of the 2-way stage, routers decides whether to proceed in an adjacency build. The decision is based on whether one of the routers is a DR or BDR or the link is a point-to-point or a virtual link. - Exstart: Routers try to establish the initial sequence number to be used in the information exchange packets. The sequence number insures that routers always get the most recent information. One router becomes the primary and the other becomes secondary. The primary router polls the secondary for information. - Exchange: Routers describe their entire link-state database through sent database description packets. At this state, packets could be flooded to other interfaces on the router. - Load: At this state, routers finalize the information exchange. Routers have built a link-state request list and a link-state retransmission list. Any information that looks incomplete or outdated are put on the request list. Updates are put on the retransmission list until acknowledged. - Full: At this state, the adjacency is complete. The neighbor routers are fully adjacent. Adjacent routers have a similar link-state database.
40
OSPF debug commands
- show ip ospf interface This command is a quick check to determine if all of the interfaces belong to the areas they are supposed to be in. - show ip ospf neighbor The show ip ospf neighbor command shows the state of all the neighbors on a particular segment. Do not be alarmed if the Neighbor ID does not belong to the segment that you look at. The Neighbor ID is actually the RID which could be any IP address on the box.
41
Route summarization types
- Inter-area route summarization Inter-area route summarization is done on ABRs and it applies to routes from within the AS. - External route summarization External route summarization is specific to external routes that are injected into OSPF via redistribution.
42
What LSA's can OSPF summarize?
LSA type 3 and 5.
43
What is LSA type 5?
external routes
44
What is LSA type 3?
interarea routes
45
OSPF network command
advertise networks
46
OSPF area command
advertise summarization
47
OSPF redistribute connected subnets command
advertise external routes
48
OSPF passive interface
This command tells OSPF not to send hello packets on certain interfaces.
49
OSPF stub areas
- Stub Area No LSA type 5 (external routes) - Totally stub area No LSA type 5 (external routes) No LSA type 3 (inter-area routes) - NSSA (not so stubby area) No LSA type 5 (external routes) LSA type 7 (exported external routes) - Totally NSSA (totally not so stubby area) No LSA type 3 (inter-area routes) No LSA type 5 (external routes) LSA type 7 (exported external routes)
50
LSA types 1 to 8
1 Router LSA 2 Network LSA 3 Summary LSA 4 Summary ASBR LSA 5 Autonomous system external LSA 6 Multicast OSPF LSA 7 Not-so-stubby area LSA 8 External attribute LSA for BGP
51

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