2 default routes in a stub network

Unanswered Question
Apr 25th, 2008

Folks,

I have a stub area with ABRs generating a default route into the ospf stub area, my question is that how will the routers react when they see 2 default routes with the same cost, will they load balance?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
t814687 Fri, 04/25/2008 - 10:45

yes, by default OSPF does equal cost loadbalancing for up to six routes.

serg

NAVIN PARWAL Sat, 04/26/2008 - 07:22

Serg,

We have 2 default routes here, also I remember reading that the stub router chooses only 1 default route to go out of the stub area.

Please help anyone?

Parwal

Harold Ritter Sat, 04/26/2008 - 08:32

Parwal,

Load-balancing will take place only if the cost to both ABRs is the same. You can determine the cost to each ABR using the following command:

show ip ospf border-routers

Regards,

t814687 Sat, 04/26/2008 - 17:28

Parwal, If you do not want to see 2 default routes in your stub area, use

area <> default-cost <>

command on your ABR to inject default route with higher cost.

serg

NAVIN PARWAL Sun, 04/27/2008 - 04:24

I have never seeen a router do loadbalancing between 2 default routes, so I asked, I am still not convinced, I remember reading somewhere that the routers in the stub network only use one abr to exit no matter what, I am simple not convinced.

Thanks

Harold Ritter Sun, 04/27/2008 - 05:29

Parwal,

As I mentionned above, the router will load balance on the default received from the ABRs as long as the costs to these ABRs are the same.

r2#sh ip ospf border-routers

OSPF Process 1 internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 1.1.1.1 [10] via 192.168.12.1, Ethernet0/0, ABR, Area 1, SPF 3

i 3.3.3.3 [10] via 192.168.23.3, Ethernet1/0, ABR, Area 1, SPF 3

r2#sh ip ro 0.0.0.0

Routing entry for 0.0.0.0/0, supernet

Known via "ospf 1", distance 110, metric 11, candidate default path, type inter area

Last update from 192.168.23.3 on Ethernet1/0, 00:00:17 ago

Routing Descriptor Blocks:

* 192.168.12.1, from 1.1.1.1, 00:00:17 ago, via Ethernet0/0

Route metric is 11, traffic share count is 1

192.168.23.3, from 3.3.3.3, 00:00:17 ago, via Ethernet1/0

Route metric is 11, traffic share count is 1

r2#

Regards,

Actions

This Discussion