OSPF routing Loop?

Somehow I must be going nuts ...

I'm trying to set up a backup link. Currently, all traffic on the backbone is routed via OSPF-learned routes, which works fine. I have now added a backup line, which is also OSPF enabled. "show ip ospf nei" correctly shows all OSPF neighbors that should be there, none that shouldn't. Network looks something like this (simplified):

R1----------R2 | | | | | | R3----------R4

R1-R2 and R3-R4 are 100M LAN links, R1-R3 is a POS, R2-R4 a 2M link.

When R1-R3 goes down, it takes about three seconds for R1 to announce the loss of OSPF neighbor to R2. R2 then adjusts its routing to use R2-R4 for the link to the other side, and announces the routes. R1 accepts the routing changes - then, R2 switches back to using R1 as gateway to the outside ... (whereas R1 still uses R2 as gateway to the outside).

WTF is going on here? R1 is getting the route from R2, so why isn't R2 using what it's announcing instead of switching back to R1???

Hints appreciated!

This looks interesting. Can you please post 'show ip ospf database' from R1, R2 and R3 when POS link is down?

Slightly longer dumps --- what excerpts would be the most interesting?

How many areas do you have? Where do you get your "outside" routes? Do you redistribute them? From where?

Just area 0.0.0.0 . Even tried adding IBGP on both routers with default announced from the other side. Still can't seem to get the one router to use the OSPF/BGP announced default route, but still sticks to the other route ... the crazy thing I can't understand is that the router right when the primary link goes down will use its backup connection, announces it to the local OSPF network, then seems to get it back from the other router and switches it over to the second even though its default is back to him ...

What am I missing here?

-garry

1. do a trace, to ident where the loop is.
2. connect to this router and verify the OSPF settings and statics etc.

could be that some router are not right in OSPF, hence the packets hits the

HTH Martin

Hi Martin,

done a trace and it displays something along the lines of...

1. 192.168.1.2
2. 192.168.2.3
3. 192.168.1.2
4. 192.168.2.3

there was more lines after 4 but displayed the same thing.

The only routes inside the router are the OSPF learned routes and the directly connected routes.

I th> > Hi folks,

ok, so this IP routes back to the previous one. eigther #1 forwards to wrong hext hop, or #2 does - This is what i means. There are several commands that you need to get familiar with inorder to determine the choise of route in the RTM. first is "show ip route" This will show you the complete route table. Use this to find the match for your destination packet. Then you have if its from static or OSPF, and from which neigbour its been seen/learned. Drill down this road and you should find the answer. If everythin looks fine, then the posibility of simulator software errors/"Works as designed"-features exsits.

HTH Martin

hi Martin,

thanks for your thoughts. It does seem as though the software is at fault, 1 router only shows 2 other routers in it's routing table, 1 router shows 1 router and the another router shows all routes in it's routing table. I'll contact Boson and see what they say about it.

Does this mean that a routing loop shouldn't have occurred here?

thanks aga> > Hi Martin,

Hi,

It is kinda hard to say, without any config's posted. if you could post the running config for all routers, aswell as the output of sho ip route for all router.

Regards Martin

its miight be alil advanced and im not sure if the sim will do it but configure one of the links from roter 2 or 3 to router 1 or 4 as a floating static, meaning set the prioiry of ospf link higher than that of the primary and it will only use the one link but if it goes down the network will re converge with the other link automatically