2 default routes in a stub network

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Apr 25th, 2008
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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?

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t814687 Fri, 04/25/2008 - 10:45
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yes, by default OSPF does equal cost loadbalancing for up to six routes.


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


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


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


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


Harold Ritter Sun, 04/27/2008 - 05:29
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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 [10] via, Ethernet0/0, ABR, Area 1, SPF 3

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

r2#sh ip ro

Routing entry for, supernet

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

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

Routing Descriptor Blocks:

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

Route metric is 11, traffic share count is 1, from, 00:00:17 ago, via Ethernet1/0

Route metric is 11, traffic share count is 1




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