Re: BGP Multihoming 2 providers full or partial?

[William Herrin <bill () herrin us>]

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/>