Re: Failover for a multihomed site with unreachable ISP

[Christian Schild <ipng@uni-muenster.de>]

Hi Iljitsch,

Am Mittwoch, 26. März 2003 23:09 schrieb Iljitsch van Beijnum:
> 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,

Well, rare is not zero :). We just want to cover that case. 

>     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.

Hm, if just one POP disappears, the ISP (and the customer) is still rachable,
we don't have a loss of connectivity and no action has to be taken.

> 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.

I see. But in our approach 2, the longer prefix is already present in the 
routing table. Only if the shorter prefix disappears, it will be added to the 
forwarding table. It is some kind of 'automatic aggregation'. Do you think it 
will be that extremely costly? Moreover I think, systems 'that could just 
about cope' should be replaced, and they shouldn't be a concern here.

So long,
     Christian