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OSPF ECMP Routing issues

Devinder Sharma
Occasional Visitor

OSPF ECMP Routing issues

I have an issue of ospf not installing the equal cost routes in the routing table. Please refer to the attached network diagram and the problem description on it. I will appreciate any help and advice.

Thanks

Devinder Sharma
6 REPLIES
Mohieddin Kharnoub
Honored Contributor

Re: OSPF ECMP Routing issues

Hi

Its very obvious that you have an ECMP network, but i think you have to check all paths from the spanning tree point of view.

So on the 3500s , check both paths to the 5400s and how is the spanning tree behavior, also check the following link it could help:

http://www.hp.com/rnd/support/faqs/5300xl.htm#question105

Also, there was some issues with older firmwares, so if you can update your switches to latests version

Good Luck !!!
Science for Everyone
Devinder Sharma
Occasional Visitor

Re: OSPF ECMP Routing issues

Thanks for your prompt response.

Yes, all switches have the latest image.

There is no spanning tree loop (show span on 5400s and show span instance 1 & 2 on 3500s) and show cpu confirms that and spanning tree is disabled on all uplinks between 3500s and 5400s. Clearly there are two equal cost paths (show ip ospf ext at any 3500 lists LSAs containing the same network from both 5400s but it should move to the routing table as well).

Anything else that I need to verify.

Thanks

Devinder Sharma
Case Van Horsen
Frequent Advisor

Re: OSPF ECMP Routing issues

In the current (K.12.xx) releases of firmware, only a single route from a set of equal cost choices is installed in the routing table. Traffic is balanced by choosing a different route for each individual destination network.

This implies that traffic is balanced on per-destination network basis instead of a per-packet basis.

See page 5-110 in the routing manual.

casevh
Devinder Sharma
Occasional Visitor

Re: OSPF ECMP Routing issues

Thanks. So based on this, if for each subnet, there is only one route installed, then in case of failure of any link affecting that route, there will some disruption as convergence happens and alternate route is installed. If both routes were installed, then there will practically be no disruption.

Please advise if this is expected behaviour from OSPF ECMP standards based implementation or it is just how this is implemented in procurve.

Case Van Horsen
Frequent Advisor

Re: OSPF ECMP Routing issues

I do not know if this is the standard beahvior of ECMP or just a ProCurve behavior.

In regards to failover time, I'm using a different configuration for our core. One change I made to improve OSPF convergence time was to change the hello-interval to 1 second and the dead-interval to 4 seconds. It is working fine in a mixed Cisco/HP environment. (I also had to uniformily specify OSPF costs.)

casevh
Devinder Sharma
Occasional Visitor

Re: OSPF ECMP Routing issues

Yes, I had set hello to be 1 sec and dead interval to 2 sec on each vlan interface to improve this. I am from cisco background and I do not see load balancing to be an issue there and it installs two routes in the cisco routers / switches and I can set load sharing to be per packet or per destination, whereas in procurve it is only per destination network. Anyway I think I have satisfied myself that there is nothing more I can do and that my design is good. As a thank you to both of you, I am awarding some points as appreciation for your prompt and relevant responses.