routing loop in ospf broadcast netowrk

shyam.timilsina · ‎06-24-2011

Dear all,

I have 30 branch all over the country.

There we have Cisco 3825 Series router at HO, and 892/k9m 1841 and 1811 routers in BO.

My branches are being connected to HO via dual link which has been linked with two ISPs,

both are Layer 2 link provided by the service provider,

ISP1 subnet: 172.19.0.0/24

ISP2 subnet: 172.20.0.0/24

usually i have the route pointing to HO ip from each branch routers.

but sometimes we experience the following problem.

Gateway of last resort is 172.20.0.24 to network 0.0.0.0

172.19.0.0/24 is subnetted, 1 subnets

C 172.19.0.0 is directly connected, FastEthernet0

172.20.0.0/24 is subnetted, 1 subnets

C 172.20.0.0 is directly connected, FastEthernet1

10.0.0.0/24 is subnetted, 2 subnets

C 10.11.11.0 is directly connected, Loopback0

C 10.168.113.0 is directly connected, Vlan2

C 192.168.113.0/24 is directly connected, Vlan1

O*E2 0.0.0.0/0 [110/1] via 172.20.0.24, 00:00:09, FastEthernet1

[110/1] via 172.20.0.22, 00:00:09, FastEthernet1

[110/1] via 172.20.0.21, 00:00:09, FastEthernet1

[110/1] via 172.20.0.11, 00:00:09, FastEthernet1

Where, there are four branches acting as the gateway for the branch router 172.20.0.13. What cause the problem,

and how can I solve this issue permanently? Please specify what can i implement additionally for fixing this

problem,

Any help will be highly appreciated.

Thanks & Regards,

Hari

Kelvin Willacey · ‎06-24-2011

Are any of the four next hop addresses your HQ router? Are any locations besides HQ advertising default routes?

lgijssel · ‎06-24-2011

The metric of 1 indicates the routes are most likely static.

You probably are redistributing the [static] default route from other sites into ospf.

Check the configs for routers 172.0.11, 21 and 22 to see how they are handling their default route.

It is obvious they should not redistribute it back into ospf but it looks as if this is what's currently happening.

regards,

Leo