Re: Failover for a multihomed site with unreachable ISP

[Iljitsch van Beijnum <iljitsch@muada.com>]

Christian,

On woensdag, maa 26, 2003, at 13:39 Europe/Amsterdam, Christian Schild 
wrote:

> First, we consider a failure a seldom and abnormal event. Only if a 
> direct
> connect fails, the network (or the ISP) has to take some failover 
> action.
> This means that - if you think of the size of the global routing table 
> -
> in default behaviour the table is small (only /32 prefixes) and only in
> case of a failure a more specific prefix (/48 or shorter) is 
> neccessary.
I see two problems with this:

1. An ISP's aggregate route (the /32) disappearing from the global 
routing table
   is an extremely rare event. Last mile problems are infinitely more 
common,
   and after that there is the class of partial ISP failures (for 
instance,
   a single POP is partitioned from the network) where the aggregate 
remains
   visible.

2. Conditional announcement makes the global routing system less 
predictable,
   which is dangerous. The sudden appearance of thousands of more 
specific
   /48s could easily break systems that could just about cope with the 
regular
   aggregated situation.