hot/cold-potato routing

Majdi S. Abbas msa at
Tue Jul 3 00:44:53 UTC 2001

On Mon, Jul 02, 2001 at 05:30:30PM -0700, Vandy Hamidi wrote:
> My question is, How is Cold-Potato routing implemented, and how are ISPs
> like Above Net able to just switch over when a backbone connection is lost?
> Are MEDs involved?

	Yes and no.  You could consider MEDs to be regional 'hints' as to
where a prefix is located in a peer's network.  Networks that accept and
honor MEDs will listen to these hints, and use them to make their forwarding

	For example, if provider X is sending us a MED of 10 for 192.168/16
in Chicago, and a MED of 100 for that same prefix in San Francisco and
New York, we will de facto deliver the traffic to them in Chicago, keeping
it on our backbone until that point.

	The important thing to understand here is that our forwarding 
process is really a layered one; we take information from BGP, in this
case, the next hop of the Chicago session with provider X.  Then we
do an IGP lookup on that next hop to determine which direction it goes
within our network.  

	Thusly, if we lose a backbone circuit, we will simply select the
next best path available within our network.  If we lose the peering
circuit in question, BGP's normal withdraw process will select the next
best route, which will frequently have a hot-potato effect.  


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