Transit LAN vs. Individual LANs

Thus spake "Patrick W. Gilmore" <patrickianai.net>
> On Feb 24, 2006, at 9:03 PM, Scott Weeks wrote:
>> I have 2 core routers (CR) and 3 access routers
(AR)
>> currently connected point-to-point where each AR
connects to
>> each CR for a total of 6 ckts.  Now someone has
decided to
>> connect them with Gig-E.  I was wondering about the
benefits
>> or disadvantages of keeping the ckts each in their
own
>> individual LANs or tying them all into one VLAN for
a
>> "Transit LAN" as those folks that
decided on going to Gig-E
>> aren't doing any logical network architecting (is
that a
>> real word?).
>
> Personally, I like the to KISS, so one big 'transit
LAN'.

ITYM two big transit LANs -- one must be prepared for a
switch to fail.

> An argument could be made for individual VLANs to keep
things like b- cast 
> storms isolated.  But I think the additional complexity
will  cause more 
> problems than it will solve.

If you have broadcast storms on a subnet with five routers
and nothing else 
on it, you've got bigger problems than config complexity.

> Or maybe I'm just too dumb to keep up with the
additional complexity. 

One must keep in mind that human error is the dominant cause
of outages, and 
since there's not likely to be backhoes running around in a
data center, 
IMHO the goal should be to remove as many ways as possible
that your 
coworkers can muck things up.

I'd go with two plain GigE switches, as dumb as I could
find them, barely 
configured or possibly not even managed at all, and one /28
(and one /64) on 
each to allow for adding more ARs later.

There are a few advantages to going with PTP VLANs, such as
eliminating 
DR/BDR elections needed on shared ones, but you'd need 10
of them to get a 
full mesh, and 15 if you add one more router.  That's just
too much 
complexity for virtually no gain, and as Owen notes, it is
generally bad for 
your logical topology to not match the physical one.

S

Stephen Sprunk        "Stupid people surround
themselves with smart
CCIE #3723           people.  Smart people surround
themselves with
K5SSS         smart people who disagree with them." 
--Aaron Sorkin