Transit LAN vs. Individual LANs

Stephen Sprunk stephen at
Sat Feb 25 19:56:37 UTC 2006

Thus spake "Patrick W. Gilmore" <patrick at>
> 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.


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 

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