ospf lsa 3 and 4

[athambi]

Could some one  explain what is the exact difference  between lsa 3 and lsa 4  and what is the significance of each of these lsa and when  will theese lsa come into the picture

Ajai

[Nagaraja Thanthry]

Hello,

Network Summary LSA (Type 3)

– Generated by Area Border Routers

– Sent to an area to advertise routes belonging to other areas

- Also used by the ABR to advertise subnets that are part of another area to the backbone area

- Network summary LSA also contains default routes

– ABR includes cost from itself to the network it is advertising in the LSA

– When the ABR advertises the summary LSA to routers within an area, the internal routers do not run SPF again

- Internal routers just add cost from them to the ABR

ASBR Summary LSA (Type 4)

– Advertised by the ABR

– Contains information about the ASBR Router

– Always the advertised address will be a host not network

- Used in conjunction with Type 5 LSA

So, essentially Type 3 is used to advertise routes external to an area (including default routes) whereas the Type 4 is used to propagate the information about the ASBR router. When the internal routers get routes distributed via another routing process, using Type 4 LSA, they determine the router that has redistributed these routes.

Hope this helps.

Regards,

NT

[chinkevi_2]

in other words, in an internal router, when you check the lsa database, you would expect:

- type 3 lsa has a list of networks in other area

- type 4 is the rid of asbr, so the router know which rid is used to pass routes out of ospf domain.