Notes on OSPF LSAs and LSA/Topology Database

As stated in an earlier post every router in an OSPF area should learn the same topology.  This data is composed of individual LSAs (link state advertisements), which are stored in the LSDB (link state database).  Sometimes this is referred to as the OSPF topology table, although the proper term would be LSDB.  The term topology table is only used comparably to EIGRPs topology table.

The Dijkstra Algorithm is used to compute the lowest cost path for each route or reachable subnet.  The Dijkstra Algorithm is also known as the Shortest Path First Algorithm, or “SPF”.

LSA Types

• 1 – Router
• 2 – Network
• 3 – Net Summary
• 4 – ASBR Summary
• 5 – AS External
• 6 – Group Membership (currently not supported by Cisco IOS)
• 7 – NSSA External
• 8 – External Attributes (not implemented in Cisco routers)
• 9-11 – Opaque

All LSAs contain a link state ID (LSID).  Depending on the LSA type, the LSID will contain specific information.  For example, with type 1 LSAs the LSID will be the RID value of a specific router.

Types 1 – 3 are used to calculate the best routes for all routes inside the OSPF routing domain.  Types 4, 5, and 7 are used to calculate routes for external (routes redistributed in to OSPF).  Types 9-11 are used as generic LSAs to allow for easy future extension of OSPF.  For example, type 10 has been adapted for MPLS traffic engineering.

LSA Type 1 – Router

Each router creates a type 1 LSA for itself, which is defined by its RID (router ID).  The LSID (link state ID) is in fact the RID.  This LSA is flooded throughout the area.  The type 1 LSA is forwarded by neighboring routers until every router in the area has a copy of it.

In addition to RID the type 1 LSA also contains:

• Routers interfaces subnet number and mask with OSPF cost for each interface on which no DR has been elected
• The IP address of the DR and a notation that the link attaches to a transit network (meaning that a type 2 LSA exists for the network) for each interface on which a DR has been elected
• Neighbor ID of router interfaces in which no DR is present, but a neighbor is reachable

For routers that are connected to multiple areas (ABR) a type 1 LSA is created for each area, with only the appropriate LSA for an area being flooded out an interface.  In other words, LSAs for area 0 will not be flooded out area 1 from the ABR generating type 1 LSAs.  Instead, only the type 1 LSA for area 0 will be flooded out interfaces connected to area 0, and type 1 LSAs for area 1 will be flooded out interfaces connected to area 1.

The use of RID in type 1 LSA is an important reason why it is recommended that RIDs are manually configured using router-id command, or by creating a unique loopback interface with appropriate IP to be assigned as RID.

Verifying Type 1 LSA

• show ip ospf database: displays a summary of the LSAs known to the router
• show ip ospf database router <RID>: displays the detailed information of a routers type 1 LSA.  This will show neighbor IDs and interface on which neighbors can be reached.  Since routers exchange these type 1 LSAs then this router will have the same information from the other routers advertised LSAs.

LSA Type 2 – Network

These LSAs are required on multi-access networks (more than 2 routers connected to the same subnet) where the router must model the topology with nodes (the routers) and connections between nodes.  Each connection must be between a pair of nodes.  The type 2 LSA is used to handle this.

First, the routers will set a transit network (or pseudonode) in their type 1 LSA.  Then, the type 2 LSA (managed by the DR) will model this by listing references back to each type 1 router LSA connected to it.

Important to note that the elected DR in a subnet creates the type 2 LSA for that subnet.  The LSID on the type 2 LSA will be the connected interface IP address of the DR.  The type 2 LSA will also have the DRs RID listed as the “advertising” router.  See the command show ip ospf database network command below.

Routers choose wether or not to use type 2 LSAs for multi-access networks based on whether a DR (designation router) has or has not been elected on an interface.

Remember that a DR is used to 1) create and flood a type 2) network LSA for a particular subnet, and 2) aid in the detailed process of database exchange over that subnet.

Verifying Type 2 LSA

Here are some commands that can be used to verify type 2 LSA configuration.

• show ip ospf database: lists a single line for each LSA (as discussed for type 1).  The type 2 LSAs will be listed under “Net Link States” with interface IP address of the DR that creates the type 2 LSA
• show ip ospf database router <RID>: type 2 LSA are listed as “connected to: Transit Network”
• show ip ospf database network <LSID>:  the LSID here is the IP address of the interface on the DR connecting to the subnet

LSA Type 3 – Summary

By design OSPF ABRs (routers connected to more than 1 area) will not forward type 1 and type 2 LSAs across areas.  This is a strategic implementation to limit the scope of SPF and save precious CPU and RAM.  Regardless, routers need to be aware of subnets in other areas.  These are advertised using the type 3 LSA.  ABRs generate a type 3 LSA for each subnet in one area, and then advertises these LSAs to other areas.

A type 3 LSA does not contain the amount of detail found in type 1 or type 2.  Instead, they summarize.  This is not the same as route summarization.  A type 3 LSA will appear to be another subnet connected to the ABR that created and advertised the type 3 LSA.  It is a “summary” in the sense that it lacks the details types 1 and 2 LSAs have.

The ABR assigns the LSID based on the subnet number being advertised, and ads its own RID to the LSA as well.

Limiting the Scope of LSAs

The maximum number of LSAs learned from other routers can be limited by using the max-lsa <number> subcommand.  What happens when a router hits a configured limited number of LSAs to learn?

1. Issue log messages, ignore, rewarn, etc. (ignore time and number of times to ignore are configurable with max-lsa)
2. Close all neighborships, discard LSDB, and then start adding neighbors again (essentially reload OSPF)

Summary

LSA types 4, 5, and 7 will be covered within redistribution section – since these apply to routing outside of the OSPF domain.

 

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