ospf summary lsa's

carl_townshend · ‎01-29-2009

hi all, when talking about summary lsa's, what info do they advertise, or shall I say summarise? i know this is where you would create summary routes, but does it summarise the lsdb, has anyone got an example?

Giuseppe Larosa · ‎01-31-2009

Hello Carl,

summary LSAs, LSAs type 3 carry IP prefixes information.

It is the data structure used to propagate OSPF inter-area route.

ABR of (origin area, area 0) flood it in area 0.

ABR of (other area, area 0) receive the LSA in area 0 and create a new one to be flooded in attached non totally stub area with the same content.

Despite their name LSA type 3 are generated in any case of OSPF multi area scenarios and they don't require manual summarization at area boundary (with area range command).

So summary LSAs carry IP routes that can be the result of an area based summarization or simply the export of IP subnets in origin area.

Hope to help

Giuseppe

carl_townshend · ‎01-31-2009

can you give me an example on this? is it a summary lsa of routers or networks?

can you do me an example ?

davy.timmermans · ‎02-01-2009

let's say area 51

network 10.0.0.0/25

network 10.0.0.128/25

>case no- auto-summary:

if you haven't configured area x range 10.0.0.0/24 at your ABR, these 2 networks won't be summarized

they will be advertized by two LSA 3 messages in area 0 to other ABR.

Thus other ABR will see:

LSA 3 10.0.0.0/25

LSA3 10.0.0.128/25

and the other ABR will pass these info to internal routers in a specific area (standard/stub/NSSA)

Summary address/LSA 3 means prefixes from another area

If you summarize at the ABR

area 51 range 10.0.0.0/24

Then only 1 LSA3 needs to be sent.

carl_townshend · ‎02-01-2009

hi there

so does the abr have 2 main functions, summary routes, and also it does not receive udates from other routers in the area excet 1, so if there is any change it does not have to re run the algorithm, is this right ?

davy.timmermans · ‎02-01-2009

ABR receive update from other ABR/ backbone routers and from the area it's talking with.

As explained before, the ABR won't summarize (send 2 separete LSA 3 to other ABRs, backbone routers).

This evening I've more time to explain. I've to go