Re: Dual Homed BGP

[Octavio Alvarez <octalnanog () alvarezp org>]

On 1/23/20 6:01 PM, Brian wrote:
> Hello all. I am having a hard time trying to articulate why a Dual Home 
> ISP should have full tables. My understanding has always been that full 
> tables when dual homed allow much more control. Especially in helping to 
> prevent Async routes.

If you don't have full tables you will certainly have a default route 
somewhere, either set statically by you or advertised by your upstream.

Accidents may happen: your ISP may blackhole a destination; sometimes 
they adjust loads, sometimes their redundancy is not properly set up or 
they may have an otherwise incorrect BGP configuration. Sometimes they 
just mess up.

If you have full tables, you will simply stop getting the advertisements 
for the bad destinations from the bad ISP and your routers will take 
care of it because they will keep getting the advertisements from the 
other exit.

If you don't have full tables and the mistake is in a destination for 
which the default route is used, your traffic will most probably be lost 
because they don't have enough information to know a different exit and 
you may get a call at midnight. It may or may not happened to me. ;-)

Octavio.