OSPF load balancing

[v.nastase]

Hi

Does anybody know how to deploy load balancing in OSPF area 0 ?

Any suggestion for documentation would be apreciated.

Thanks in advance for your help

[Aaron Harrison · Accepted solution]

Hi

OSPF only does equal-cost load balancing, so you must have two routes with equal costs to the destination.

Also this command must be set to more than one:

maximum-paths

under router ospf configuration.

I think you also need ip cef enabled.

See this link for more info:

http://www.cisco.com/warp/public/105/loadbal_cef.html#beforecef

Aaron

Hope this helps - don't forget to rate posts :-)

[Richard Burts]

I have 2 clarifications to this post.

The post says that maximum paths must be set to more than one which implies that you need to do something in your config. Maximum paths has a default value of 4. So while you can specify a value for maximum-paths it is not required to configure this parameter to load share.

The post also thinks that you need ip cef enabled. While enabling ip cef is generally a good thing and on general principles I would think that you should enable ip cef, it is not required to do load sharing.

So the first line of the post is really the answer: OSPF does equal cost load balancing and if you have two routes with equal cost to the destination then OSPF will load share.

HTH

Rick

[Kevin Dorrell]

Could I add just one bit of extra information to what Rick says, and mention something that might trip you up if you are not aware of it.

If you are running CEF, then the load-sharing by default is per-flow (so called "per-destination", but actually per flow). That is, all packets for a particular pair of IP addresses will take the same path, so their order of arrival is guaranteed. If you are process-switching, then the load sharing is per-packet - that is, packets from a single session can take different paths, so they could arrive out of order, which could upset some applications.

You can configure CEF to do per-packet, but it is not usually advisable.

And I cannot remember off the top of my head how fast-cache switching behaves, but I think it is also per-destination - so there is not any danger of packets arriving out of order.

Kevin Dorrell

Luxembourg