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OSPF With Different area Network and redistribution with rip network

Hi

how the routers in ospf network will discover the borders routers

when i try to see the border routers with the show ip ospf border command

i get :

in Area 0 - R2 discover : all border routers in the network ( R3 - R5 - R8 - R12 )

in Area 1 - R4 discover : R3 - R5 - R12

in Area 2 - R6 discover : R5 - R8 - R12

in Area 3 - R9 discover : R8 - R12

my question is why all areas know border R12 ? how the routers discover the border router ?

  • Your areas are laid out incorrectly. All areas need to be connected to Area 0, either directly or through a virtual link. – Ron Trunk Oct 5 '16 at 14:24
  • yes i do virtual link and i have full connectivity between areas @RonTrunk – V For Vendetta Oct 5 '16 at 14:32
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There is no specific discovery for border routers. You need to understand more about OSPF and the LSAs which it advertises.

Every router in an area knows the complete topology of the area in which it resides, so every router knows by the LSAs it receives which routers in its area are border routers, and which routers are not border routers.

All OSPF routers generate Type 1 and/or Type 2 LSAs.

ABRs generate Type 3 and 4 LSAs. In OSPF, all the ABRs are connected to Area 0, so Area 0 knows about all the ABRs.

ASBRs generate Type 5 LSAs.

By looking at the received LSAs an OSPF router can tell which are border routers.

A more in-depth discussion of OSPF and the LSAs is too broad for this site. If you learn more about this, you can ask more specific questions.

  • Curse you and your lightning fingers again! ;-p – Ron Trunk Oct 5 '16 at 14:39
  • ok ... i know ittile about LSA Type ...if you have link or book to read more about Lsa and ospf ... i will really appreciate this – V For Vendetta Oct 5 '16 at 14:54
  • Unfortunately, product or resource recommendations are explicitly off-topic here. There are really a lot of books and web sites which go into more detail on this, and there is always the source: RFC 2328, which can be found by a simple search for it. – Ron Maupin Oct 5 '16 at 14:56
  • Lots of web sites and YouTube explanations can be had with a simple search. – Ron Trunk Oct 5 '16 at 14:58
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R12 is an ASBR. It's the only one flooding Type 5 LSAs.

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my question is why all areas know border R12? how the routers discover the border router ?

Answers -

  1. As per the topology R12 is a ASBR and its an Border router for the AS and hence showed as border router - Internal router are aware of this as LSA-5 is flooded through out the OSPF area.
  2. Internal router are aware of the ABR, as B-bit set to 1 in router LSA indicates the LSA originator as a ABR.

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