Lesson BGP & OSPF path selection in VSS routing environment

[lpassmore]

Hi, I would like a lesson on how traffic is passed in the following environment:

One 3945 router with interfaces connected to a pair of 4500X switches configured as VSS pair. One link into each of the 4500 running as routed interfaces using separate IP subnets meaning there are two equal cost paths between the router and the 4500X.

We are running a single OSPF area and iBGP between the devices. 

I would like to find out, in normal circumstances where both equal cost links are operating normally, how the 4500 selects the path to send a packet to the router.  We would be trying to avoid traffic passing through the VSL but want to know if the system is smart enough to do that.

Is there somebody out there who can tell me if the VSS process will select the path directly to the router or if it cannot be guaranteed to do so.

I also would like to get opinions on whether it is best to create two iBGP neighbour relationships on the link addresses or one relationship between the loopback addresses.

Thanks 

LP

[Reza Sharifi]

Hi,

The OSPF traffic would not pass through the VSL link.  The path would directly go from each 4500 to the 3945 (Equal cost load balancing). I think, the 3900 series supports Etherchannel, if this is the case you can also create a L-3 Portchannel between the VSS and 3945 router.  This way you use one /30 instead of 2 and you still have redundancy.  For BGP, I would do one peering with Loopbacks.

HTH