[c-nsp] GRE tunnel to do span vlan across two datacenters?

[Jason Gurtz]

A firm has proposed creating a GRE tunnel between two datacenters (using a
3750X stack at each) to create the spanned vlans needed for VMWare
failover application.

Clearly there is tunnel overhead but I sense there are other failure modes
here that aren't so clear to me--I am familiar in concept with GRE tunnels
but don't have a heck of a lot of opex. Can anyone share more insight on
the merit (or lack of) with this proposed design? I am aware (via this
list, thanks!) of several shortcomings surrounding 3750 based stacks, but
cisco alternatives seem pricier still or too big. There is dark fiber
available, what about VPLS w/ LDP or L2TP solution?

Current network is L3 at the access layer w/ OSPF (4507-sup6 access, 4900M
cores):

     A1
     /\
   /    \
 C1------C2
   \    /
     \/
     A2

Maybe it is better to just overlay stp back on to the network w/root and
alt-root at C1/C2 (V1 and V2 are the proposed 3750X stacks)? Scary to me,
but an an argument can be made for less complexity -vs.- tunnling/vpn
based approach.

     A1     .V1
     /\ . ' /
   /. ' \ /
 C1------C2
   \` . / \
     \/ ' . \
     A2     'V2

OTOH, by the time this actually gets done maybe TRILL will be out ;)
Hopefully this enterprisy topic is not too OT!

~JasonG