[j-nsp] EX4200 resilience (VC vs 10GB cross-connect)

[Joe Hughes]

Hi

Take the following scenario;

Several racks - each with a pair of EX3200 switches (cross-connected) - each
with separate L3 uplinks back to a pair of aggregation layer EX4200s (so, 1
from each rack switch), all running OSPF. I'm trying to understand if there
are any drawbacks in making the two aggregation layer EX4200s a VC - or
whether a simple L3 cross-connect using a LAG or the 10Gbps ports makes more
sense, factoring in things like resilience, ease of upgrades etc.

If you go on the basis the two EX4200s are two distinct switches with a L3
path between them, it is easy for me to visualise how the network will
behave and (hopefully) understand how failover scenarios will play out. The
obvious disadvantage of this is you are using up vital 10Gbps ports which
could be used elsewhere, and it does seem non-sensical given the higher
speed VC ports (+ the other VC advantages).

I've read the VC Best practice guide and in all their examples, they have
two sets of 2-switch VCs at the aggregation layer - which is making me
wonder if a single VC (of two switches) and treated as one switch poses more
of a risk than simply two distinct switches. I'm guessing if you have two
links back from each rack - each to a different member of the VC, then the
'risks' are pretty much the same as having two separate switches? In terms
of software upgrades - is it possible to upgrade one member at a time (as
you would in a non-VC setup) so as to not interrupt connectivity from the
access\rack switches? Are there any other situations\operations on a VC that
would take both switches offline - further making it more sense to either
have two switches, or two sets of 2 members VCs?

Cheers

Joe