OSPF LSAs Flashcards
LSA (Link-State Advertisement) Types
- Type 1, router: LSAs that advertise network prefixes within an area
- Type 2, network: LSAs that indicate the routers attached to broadcast segment within an area
- Type 3, summary: LSAs that advertise network prefixes that originate from a different area
- Type 4, ASBR summary: LSA used to locate the ASBR from a different area
- Type 5, AS external: LSA that advertises network prefixes that were redistributed into OSPF
- Type 7, NSSA external: LSA for external network prefixes that were redistributed in a local NSSA area
LSA Type 1: Router Link
Every OSPF router advertises a Type 1 LSA. Type 1 LSAs are the essential building blocks in the LSDB.
During the SPF tree calculation, network link types are one of the following:
- Transit: A transit network indicates that an adjacency was formed and that a DR was elected on that link.
- Point-to-point: Point-to-point links indicate that an adjacency was formed on a network type that does not use a DR. Interfaces using the OSPF point-to-point network type advertise two links. One link is the point-to-point link type that identifies the OSPF neighbor RID for that segment, and the other link is a stub network link that provides the subnet mask for that network.
- Stub: A stub network indicates that no neighbor adjacencies were established on that link. Point-to-point and transit link types that did not become adjacent with another OSPF router are classified as a stub network link type. When an OSPF adjacency forms, the link type changes to the appropriate type: point-to-point or transit.
LSA Type 1: Router Link Types Summary
LSA Type 2: Network
A Type 2 LSA represents a multi-access network segment that uses a DR. The DR always advertises the Type 2 LSA and identifies all the routers attached to that network segment. If a DR has not been elected, a Type 2 LSA is not present in the LSDB because the corresponding Type 1 transit link type LSA is a stub. Type 2 LSAs are not flooded outside the originating OSPF area in an identical fashion to Type 1 LSAs.
When the DR changes for a network segment, a new Type 2 LSA is created, causing SPF to run again within the OSPF area.
LSA Type 3: Summary
Type 3 LSAs represent networks from other areas. The role of the ABRs is to participate in multiple OSPF areas and ensure that the networks associated with Type 1 LSAs are reachable in the nonoriginating OSPF areas.
ABRs do not forward Type 1 or Type 2 LSAs into other areas. When an ABR receives a Type 1 LSA, it creates a Type 3 LSA referencing the network in the original Type 1 LSA. (The Type 2 LSA is used to determine the network mask of the multi-access network.) The ABR then advertises the Type 3 LSA into other areas. If an ABR receives a Type 3 LSA from Area 0 (backbone), it regenerates a new Type 3 LSA for the nonbackbone area and lists itself as the advertising router with the additional cost metric.
LSA Type 3: Summary
Metric calculation
The advertising router for Type 3 LSAs is the last ABR that advertises the prefix. The metric in the Type 3 LSA uses the following logic:
- If the Type 3 LSA is created from a Type 1 LSA, it is the total path metric to reach the originating router in the Type 1 LSA.
- If the Type 3 LSA is created from a Type 3 LSA from Area 0, it is the total path metric to the ABR plus the metric in the original Type 3 LSA.
R4’s perspective of the Type 3 LSA created by ABR (R5) for the 10.56.1.0/24 network. R4 knows that its metric to the ABR (R5) is 1 and that the Type 3 LSA already has a metric of 1, so its total path metric is 2 to reach the 10.56.1.0/24 network.
LSA Type 4: ASBR Summary
A Type 4 LSA locates the ASBR for a Type 5 LSA. A Type 5 LSA is flooded through the OSPF domain, and the only mechanism to identify the ASBR is the RID. Routers examine the Type 5 LSA, check to see whether the RID is in the local area, and if the ASBR is not local, they require a mechanism to locate the ASBR.
Only Type 1 or Type 2 LSAs provide a method to locate the RID within an area.
Type 4 LSAs provide a way for routers to locate the ASBR when the router is in a different area from the ASBR. A Type 4 LSA is created by the first ABR, and it provides a summary route strictly for the ASBR of a Type 5 LSA.
LSA Type 4: ASBR Summary
Metric calculation
The metric for a Type 4 LSA uses the following logic:
- When the Type 5 LSA crosses the first ABR, the ABR creates a Type 4 LSA with a metric set to the total path metric to the ASBR.
- When an ABR receives a Type 4 LSA from Area 0, the ABR creates a new Type 4 LSA with a metric set to the total path metric of the first ABR plus the metric in the original Type 4 LSA.
An ABR advertises only one Type 4 LSA for every ASBR, even if the ASBR advertises thousands of Type 5 LSAs.
LSA Type 5: External Routes
When a route is redistributed into OSPF, the router is known as an autonomous system boundary router (ASBR). The external route is flooded throughout the entire OSPF domain as a Type 5 LSA. Type 5 LSAs are not associated with a specific area and are flooded throughout the OSPF domain. Only the LSA age is modified during flooding for Type 2 external OSPF routes.
The link ID is the external network number, and the advertising router is the RID for the router originating the Type 5 LSA. The Type 5 LSA is not associated with a specific OSPF area. This is because Type 5 LSAs are flooded throughout the OSPF routing domain by default.
LSA Type 7: NSSA External Summary
A Type 7 LSA exists only in NSSAs where route redistribution is occurring.
An ASBR injects external routes as Type 7 LSAs in an NSSA. The ABR does not advertise Type 7 LSAs outside the originating NSSA, but it converts the Type 7 LSA into a Type 5 LSA for the other OSPF areas. If the Type 5 LSA crosses Area 0, the second ABR creates a Type 4 LSA for the Type 5 LSA.
LSA Type 7: NSSA External Summary
Example
Type 7 LSAs are present only in the OSPF NSSA where redistribution is occurring.
Notice that the Type 7 LSA is not present on R4; R4 contains a Type 5 LSA that was created by R5 and the Type 4 LSA (created by R4 for Area 1234).
- Link ID: External network number
- Network Mask: Subnet mask for the external network.
- Advertising Router: RID of the ASBR
- Metric Type: Type 1 O N1 or Type 2 O N2
- Metric: Metric upon redistribution
LSA Type Summary
Type 2 LSAs are present only on the broadcast network segments that have established adjacencies with other routers.
