From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Tue May 11 2004 - 11:20:23 GMT-3
At 1:00 PM +0530 5/11/04, Sachin Shenoy wrote:
>Hi,
>
>I was wondering if trunking over WAN is possible ...
>What I mean by that is
>
>
> Serial Link
> SW1----R1<------------->R2--SW2
>
>Is it possible to trunk VLANs from SW1 to SW2 ???
>(So LAN IP ranges for VLANs on SW1 and SW2 will be the same !)
In certain, restricted, cases, usually requiring gigabit or faster
transmision media, yes. See such things as Private VLAN Service (more
the IETF term) and QinQ encapsulation (although the latter is not a
WAN protocol, but the distinction between LAN and WAN is less and
less useful). It's also possible using MPLS.
Why do you consider this a good idea? It has potential scalability
issues, for example, involving both absolute delay greater than
expected by LAN devices. Another scalability issue is that
uncontrolled growth at both locations may cause the safe number of
devices in a broadcast domain to be exceeded.
I've been asked to do this many times, on the basis that "layer 2 is
simpler." At a certain level of scaling, it is not. In general, in
the circumstance you describe, I'd route between the sites without
overwhelming evidence to the contrary. I can even keep VLAN numbers
consistent at both sites.
Unfortunately, studying for the CCIE lab overempasizes lots of things
that may be possible, but not a good idea in the real world.
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