[180375] in North American Network Operators' Group
Re: BGP Multihoming 2 providers full or partial?
daemon@ATHENA.MIT.EDU (William Herrin)
Mon Jun 1 17:19:36 2015
X-Original-To: nanog@nanog.org
X-Really-To: <nanog@nanog.org>
In-Reply-To: <556CC933.8080309@ispn.net>
From: William Herrin <bill@herrin.us>
Date: Mon, 1 Jun 2015 17:19:05 -0400
To: Blake Hudson <blake@ispn.net>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
On Mon, Jun 1, 2015 at 5:05 PM, Blake Hudson <blake@ispn.net> wrote:
> After studying failure modes and attempting to optimize BGP using partial
> routing tables, I am of the opinion that BGP with a full routing table to
> directly connected devices is by far the best way to gain the availability
> benefits of BGP. Many attempts to cost save through multi-hop BGP or traffic
> engineering end up breaking down when a fault occurs. Some faults, like link
> state, are easy to detect and work around. Other faults, like where a peer
> is up, but has no outside connectivity, are harder to detect if you're
> taking anything less than full routes.
Hi Blake,
Yes, it's better to take full routes. But taking a default from two
ISPs still has a reliability advantage over using a single ISP. And of
course if you have two connections to the same ISP there's limited in
taking full routes.
Between default routes and full routes there is a range of options
with increasing reliability. For example, years ago I had routers with
a 256k TCAM as the BGP table approached 256k. The organization I
worked for was US-centric. We needed world connectivity, but high
reliability to Asia or Europe was not essential. And a large cash
expenditure that year would have been bad. By slaving the APNIC /8's
to a single accepted BGP route, backed by static routes for those /8's
should the master BGP route fail, I maintained full connectivity while
suppressing the route count to what the hardware could handle. And of
course maintained maximum reliability to the destination region I most
cared about.
Moral of the story: if you can afford it, always take full routes. If
you can't afford it, try to. If you really can't afford it, there's
some trickery that can last you a year or two until you can afford it,
but make sure new hardware makes it into your budget.
Regards,
Bill Herrin
--
William Herrin ................ herrin@dirtside.com bill@herrin.us
Owner, Dirtside Systems ......... Web: <http://www.dirtside.com/>
