Transit LAN vs. Individual LANs

Ejay Hire ejay.hire at
Mon Feb 27 16:35:32 UTC 2006

>From my perspective...
...a physical mesh requires too many ports to be economical.

...a logical mesh has a couple of things against it.  It
requires a lot of configuration, and each router will be
connected with a trunk interface, (on the antique switches
I've worked with) every trunk will carry all the traffic in
the switch, your maximum bandwidth across the whole switch
is 1gbps, instead of the next option which gives you more
bandwidth across the switch.

...two connections (non-trunked!) from each router to
seperate switches, with each switch having a separate /29 or
/28 for connected devices and a fast-responding IGP running
between the 5 routers gives you the most bang-for-the-buck
in terms of throughput and failure responsiveness.  With
non-trunked interfaces, the switches can actually switch,
and you can squeeze more than 1gbps of bandwidth through it.
Even if you don't have that much traffic, you still have
less latency (ok, it's an infestimally tiny amount, but
every little bit helps.)  If a switch completely fails and
the ethernet ports of the connected routers go down/down,
your IGP triggers and you have a fast failover.  If a switch
fails and the ethernets stay up/up, you have a slow
failover, based on the timers of your IGP.  

Ejay Hire

> -----Original Message-----
> From: owner-nanog at [mailto:owner-nanog at]
> Behalf Of Scott Weeks
> Sent: Friday, February 24, 2006 8:03 PM
> To: nanog at
> Subject: Transit LAN vs. Individual LANs
> I have 2 core routers (CR) and 3 access routers (AR)
> currently connected point-to-point where each AR connects
> each CR for a total of 6 ckts.  Now someone has decided to
> connect them with Gig-E.  I was wondering about the
> 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
> aren't doing any logical network architecting (is that a
> real word?).
> Anyone got any suggestions, comments or helpful hints?
> Thanks,
> scott

More information about the NANOG mailing list