OSPF path choose

Unanswered Question
Sep 25th, 2009

hi Expert,

I would like to ask

If there are two way (via area 0 with metric 100, via area 10 with metric 10)

to learn the ip address 1.1.1.1 (In area 0) from Router A where Router A is ABR (area0 and area 10).

The Router A will choose via route with area 0 with metric 100 or area 10 with metric 10

I have this problem too.
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Richard Burts Fri, 09/25/2009 - 09:34

Jack

There are some aspects of your question that I do not understand. But it is clear that if the destination is in area 0 then RouterA will choose the route through area 0 and not the route through area 1. OSPF always prefers an intra-area route over an inter-area route.

HTH

Rick

acbenny Fri, 09/25/2009 - 19:55

Hi Rick,

Maybe I attach the diagram so that you can easily understand.

According to the diagram

R27,R28,R5,R6 loopback address is in Area 0

Sw27,Sw28 loopback address is Area 20

Sw5,Sw6 loopback address is Area 10

My question is normally R28 's routing table route to 1.1.1.1 will be via 10.10.30.29. However once 10.10.30.28 link down. R28 go 1.1.1.1 will via R5 (larger metric) or Sw28 and Sw27 ( less metric) As I know both

AD of O and O IA also 110

Attachment: 
Giuseppe Larosa Fri, 09/25/2009 - 22:54

Hello Jack,

Rick's answer is absolutely correct and provides the answer you are looking for:

until an intra-area path exists it is used regardless of metric.

So if the link between RTR27_1 and RTR28_1 fails and if prefix 1.1.1.1 is in area 0, a new path within area 0 topology is used if available.

OSPF has a detailed hierarchy of preferences:

O > O IA > O E1 > O E2

all these routes have the same AD, but AD is used by the process that mantains the ip routing table not by OSPF itself.

OSPF process doesn't use AD at all, but it compares the LSA types.

If only one OSPF process is involved, it makes its own choices before passing the proposed prefix to the ip routing table manager process.

If multiple OSPF processes are involved each of them makes its own decisions and proposes its best path for a given prefix. ("ships in the night").

But this is not your scenario.

Hope to help

Giuseppe

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