Hi Ser,
Routing will be performed locally at each site. While the routing-engine contains the routing-table, it loads the calculated paths down to the forwarding table in each switch, allowing traffic to be routed where it ingresses the virtual-chassis.
I have done stretched VC deployments for a number of customers and here is a list of pointers:
Pros:
- one box to manage, so all VLANs, IPs, routing etc. is configured ONCE
- very fast fail-over if/when the chassis splits in two (DCs become isolated)
- no spanning-tree blocking or VRRP required if you want to stretch VLANs between DCs (the L3 interface is the "same" at both sites, but ARP response/routing is done locally)
Cons:
- Recovery from a chassis split is slow (around 45 seconds for L3 and L2 to reconverge)
- Any failures that cause RE switch-over or VC-Topology or software upgrades now affect both your DCs at once. This is getting better with ISSU, but it is something to be aware of.
Needless to say, make sure your VCs are in a RING with a diverse fibre path and you'll be right.