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tekjansen101
Beginner

OSPF and multiple static routes to same destination

Heyall..

So we all know that static routing presumably does "equal cost" load balancing for traffic (since static routes are only up until the next hop this equal cost label is somewhat moot). What I'd like to know is how redistributing static routes to the same destination get injected into OSPF...because if what I'm going by is any indication, it would appear only one route seems to get installed in the routing table irrespective of the number of static routes configured going to a single destination.

For instance, lets assume we have network 10.0.0.0/24. We have two static routes going to said network, one via interface Serial1 and the other via interface Serial2 like so:

!

ip route 10.0.0.0 255.255.255.0 Serial1

ip route 10.0.0.0 255.255.255.0 Serial2

!

redistribute static

!

When you run "show ip route | inc 10.0.0.0" from some point on the network, you'd presumably get :

rtrA#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

U - per-user static route, o - ODR

.

.

E2 10.0.0.0 via .... (whatever the Forward Path next hop is)

.

.

So basically the route gets installed once...or so it would appear. Sorry not at work at the moment so can't copy/paste the exact output but its something to that effect.

Is this behavior normal ? What do the powers that be say over the same ?

5 REPLIES 5
dan_track
Beginner

Hey,

Have you tried "redistribute static subnets"?

Dan

hey dan

i think we actually do have redistribute static subnets. in our actual config we aren't using Serial interfaces rather SVIs, and in turn the SVIs have been associated to the routed ports.

Giuseppe Larosa
Hall of Fame Master

Hello Omran,

>> it would appear only one route seems to get installed in the routing table irrespective of the number of static routes configured going to a single destination.

This is correct OSPF generates a data structure of type 5 that is flooded in the OSPF domain (unless stub areas)

When traffic arrives at originating ASBR node it will use the multiple paths to destination.

There is no need to increase OSPF database size with an information that is useful only on the local node.

should both serial interfaces be turned down the LSA will be removed from OSPF domain

Hope to help

Giuseppe

Thanks for your reply Guiseppe.

Here's the thing though. I know the ASBR is using the multiple paths, however for everything else connected to the network, the one path thats being advertised into OSPF and installed in the routing table seems to be the only one used.

I say this because if I ping a node in the destination network from the ASBR after bringing one of the two interfaces down, I get a 50% packet loss. Please note in this test the static route is not removed from the routing table.

However if I try to ping the same node from anywhere outside the ASBR, I don't get any packet loss and everything seems to work fine.

Hello Omran,

>>I say this because if I ping a node in the destination network from the ASBR after bringing one of the two interfaces down, I get a 50% packet loss. Please note in this test the static route is not removed from the routing table.

try to use an extended ping and specify a source ip address on the ASBR that is not on one of the two links.

What happens in this case?

Hope to help

Giuseppe