routes and ospf

Unanswered Question
Oct 1st, 2008

Hi all, If I have 2 links between 2 sites, 1 a 100 meg connection, the other a 2 mega connection, and I use ospf between them, How does ospf handle the backup route? how would I check to see the backup router is ready to take over the connection, I see with eigrp it has feasible successors....what does ospf do

cheers

Carl

I have this problem too.
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Giuseppe Larosa Wed, 10/01/2008 - 08:52

Hello Carl,

to verify backup link is healthy you just need to use the usual sh commands:

sh ip ospf neigh

sh ip ospf interface sex/y

I like to use

sh ip ospf neighbor detail

that tells since the two routers are adjacent

if they are full adjacent you know that the backup route will be used if the primary fails because they exhcanged the DB also on the backup link

(this is true till both links are in the same OSPF area or at least the same OSPF area type)

Hope to help

Giuseppe

carl_townshend Thu, 10/02/2008 - 14:00

Hi there

is there anyway I can see through these commands the the backup routes that will be used or what network the other neighbour will route

Giuseppe Larosa Fri, 10/03/2008 - 05:37

Hello Carl,

let's review the OSPF neighbor state machine:

to reach the FULL state the two OSPF neighbors have to exchange the whole Area DB also on the backup link.

So if the two have reached the OSPF FULL state and you can check this with the commands I posted in my first post you indirectly know that the backup link will be used in case of primary link failure:

the SPF is executed again after primary link failure detection and all possible and known subnets will be reached via the backup link.

So you cannot find a backup route like in EIGRP topology database but the link state nature of the protocol assures that the backup link is a usable link by SPF for path recalculations.

This is safe to say if both the primary link and the backup link are in the same OSPF area.

If they are in different areas but of the same type the behaviour should be still correct.

Hope to help

Giuseppe

carl_townshend Fri, 10/03/2008 - 07:28

Hi there

what does this info tell me ?

OSPF Router with ID (10.112.1.1) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count

10.112.1.1 10.112.1.1 433 0x80000099 0x006FA0 3

192.168.1.2 192.168.1.2 2691 0x8000006C 0x00F252 4

192.168.254.245 192.168.254.245 434 0x80000003 0x00C09A 3

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

192.168.1.2 192.168.254.245 439 0x80000001 0x00DFB7

192.168.2.1 192.168.254.245 434 0x80000001 0x00DEB8

Giuseppe Larosa Fri, 10/03/2008 - 09:29

Hello Carl,

there are three routers in area 0 you can see there 3 router LSAs in the database.

In this network in area 0 there are two LAN segments and on both links the DR is the router with the highest OSPF Router-id as could be expected:

the two lan segments are in ip subnets 192.168.1.0 and 192.168.2.0

the local router has RID 10.112.1.1

the first router to speak OSPF and build its router LSA was the router with RID 192.168.1.2 because the age of its router LSA is 2691 ( a little two much it should be lesser then 1800 seconds and then refreshed)

Some event happened 434 - 439 seconds between local router and DR because both have LSAs with age 434-439 seconds

the link count tells how many links are in area 0 in each router LSA:

two routers have 3 links and one 4 links probably two lan interfaces and a loopback on the first two (or three lan segments)

As you see the topology info is detailed and we have seen just the LSA headers not their contents.

I would suggest you to go on examining each LSA using the options of the show ip ospf database

like sh ip ospf data adv-router detail

Hope to help

Giuseppe

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