6500 VSS architecture with two equal cost paths to a destination

[fidessats]

I have a query on how the 6500s running in VSS mode would route the traffic over an OSPF environment where it has leant about two equal cost paths, but one via the 2nd chassis.

Proposed setup - 2 6509s running in VSS. Switch-1 in VSS has a layer 3 connection (via a LES circuit) to one of a pair of Nexus 7Ks at another office. Switch-2 in the VSS has another layer 3 connection (via LES by another provider) to the other N7K at the other office. The L3 connections would use /30 ranges and allow each Nexus to form an ospf neighbour relationship with the VSS. We want to keep both offices environments separate so although we do have L2 circuits we're using these to provide L3 connectivity between sites & exchange routing info via them using ospf.

Each Nexus will advertise all the directly connected networks it knows about to the 6509s running in VSS. Thus I couldn't figure out if for example we have users/servers behind our VSS 6509s, these would need to go via their default gateway to get to a network located off the LAN, the default gw ip in VSS setup exisits on the control plane on switch-1 (in normal operation). Then to get to a network that is located off the Nexuses at the other site it would have 2 equal cost paths to it, however one of these paths would be via the VSL link and off the switch-2 chassis. I wasn't therefore sure if we'd actually ever see any transmit traffic via the 2nd L3 connection because I have a funny feeling that I've read the VSS always choses the local chassis egress rather than going via the VSL to use another port...

Would anyone know the answer to whether we'd get roughly equal distribution of traffic across the 2 circuits between sites?

All of the above is theorectical at the moment as currently both circuits are connected to a single Nexus/6509 chassis however for improved resilence I want to move one of the circuits to be physically attached to the other Nexus & 6509 chasis at each site but I wasn't then sure how the traffic flows would be affected.

[Nathan Spitzer]

I have a query on how the 6500s running in VSS mode would route the  traffic over an OSPF environment where it has leant about two equal cost  paths, but one via the 2nd chassis.

A: As far VSS is concerned the VSS cluster is one router. So the the question can be simplified to: 

I have a query on how the 6500s running in VSS mode would route the   traffic over an OSPF environment where it has leant about two equal cost   paths.

How does ANY router handle OSPF routing with multiple equal-cost routes: It uses CEF load-balancing. Assuming you are using per-session load balancing (as opposed to per-packet) the answer depends strongly on your traffic. If you have lots of different Source IP-Destination IP pairs then yes, you should get pretty much equal distribution. However if you your traffic is dominated by a single Source-Destination pair then probably not. 

Since this doesnt take into account which chassis the links are in it might cause some traffic to traverse the VSL links. Personally I dont worry too much about minimizing traffic on the VSL links. However in some situations it might be a concern.

... I have a funny feeling that I've read the VSS always  choses the local chassis egress rather than going via the VSL to use  another port...

That was in reference to MEC and AFAIK applies only to etherchannels that terminate on the different chassis that make up the VSS clusrter . That does NOT apply to multiple L3 links.

[Joseph W. Doherty]

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VSS will try to avoid using its VSL links for passing traffic.  I.e. given two equal cost paths, VSS will prefer to use the egress path on the same chassis.

Some documentation of this can be found here: http://www.cisco.com/en/US/products/ps9336/products_tech_note09186a0080a963a9.shtml#ecmpVSS

NB: If your ingress is MEC, then the egress to dual equal cost paths should "mirror" the MEC distribution.