Advanced OSPF - Link-State Announcements

7 important questions on Advanced OSPF - Link-State Announcements

What command is used to show a summary of all LSAs that are received?

Show ip ospf database

What is an Autonomous System Border Router (ASBR)?

An Autonomous System Border Router, or ASBR, is an OSPF router that is redistributing routes into OSPF. Which means it is propagating routes it learned from other routing protocols, either static or dynamic.

What challenge is fixed by an LSA Type 4?

Routers on one area would not know how to reach an ASBR on another Area. The ABR uses Type 4 LSAs to propagate the ASBR networks to another area propagating that he is the gateway for those networks.
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For each of the 6 LSA Types, what routers are sending the specific LSAs?

Type 1 LSA: Each participating OSPF router
Type 2 LSA: Designated Router for the network
Type 3 LSA: Area Border Router (ABR)
Type 4 LSA: Area Border Router (ABR)
Type 5 LSA: Autonomous System Border Router (ASBR)
Type 7 LSA: Autonomous System Border Router (ASBR) within a Not So Stubby Area.

What happens with LSA Type 7 LSAs at an Area Border Router?

The Area Border Router converses these into LSA Type 5 LSAs. In these it will configure itself as the source router of these type 5 LSAs.

What are LSA Sequences used for?

A router increments the 32-bit Sequence number for the LSAs it sends out. A router only processes the LSA when it has a higher sequence number than it has in its LSDB.

How is the LSA Age used by the OSPF Routers?

LSA Age starts at 0 and is incremented by 1 each second as wel as a minimal calculated delay value when it is forwarded.
When the LSA Age of the originating router reaches 1800 it will send out a new LSA.
When the LSA Age reaches 3600 the LSA is deemed invalid and purged from the LSDB.

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