OSPF loadbalancing

Unanswered Question
Feb 3rd, 2010

hi

is it possible to load balance between tow paths between two routers with OSPF running on them?

Case 1 :If cost of each link is different ?

Case  2: If cost of links are same ? yes by default.

Case 3 : If cost of each link is manually adjusted to be same ?

Please share the experience

Thanks

Subodh

I have this problem too.
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Giuseppe Larosa Wed, 02/03/2010 - 12:29

Hello Subodh,

1) no unless in an MPLS traffic engineering context using Traffic engineering OSPF extensions

2) yes

3) not recommended unless the true speeds of the two links are very close.

A possible approach when there is a 2:1 ratio in speed or 3:1 ratio in speed between the two links is to logically divide the faster link in 2 or 3 logical links (using Vlan subinterfaces or FR PVCs and subinterfaces, L2 dependent).

After having done this the 2+1 links or 3+1 links can have OSPF metric adjusted to build equal cost paths.

This approach can be possible or not depending on the L2 technology in use.

Hope to help

Giuseppe

Ganesh Hariharan Sun, 02/07/2010 - 07:16

hi

is it possible to load balance between tow paths between two routers with OSPF running on them?

Case 1 :If cost of each link is different ?

Case  2: If cost of links are same ? yes by default.

Case 3 : If cost of each link is manually adjusted to be same ?

Please share the experience

Thanks

Subodh

Hi Subodh,

Yes, OSPF supports equal cost load balancing.If the cost of the links are the same, OSPF will insert both routes into the routing table. Also you can force the cost of the links (even if the links are not equal cost) so that both routes are accepted as equal cost routes.The route that will be selected for sending a packet will ultimately depend on the switching type.You can do per packet or per destination load balancing, i think by default on a router fast switching is enabled which means load balancing is done per destination.  If you want per packet you can turn off fast switching on the interface in question using "no ip route-cache".

Hope to help

Ganesh.H

bapatsubodh Sun, 02/07/2010 - 08:06

Hi Ganesh.Iyer

Ya, I did adjust the costs of WAN links so that those appear as same cost to ospf. It is installing two routes in routing table. ( As recomended in Giusplar post there is not much difference in two links bandwidth ).

I will try with changing switching method.

I was wondering if we can add these two links in one logical multilink interface at both end of routers and run OSPF. Then i think load balancing will be taken care by multilink.

But that may or may not be the case, as I may require one of WAN link to use for some another activity.

I will try with changing switching method.

Thanks

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