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 decision. 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. --msa