On 2 Jul 1997, Sean M. Doran wrote: ==>> Layer 2 heals itself, without the knowledge of layer 3. Layer 3 ==>> experiences a latency increase between certain paths. ==>> ==>> Sorry, had to do it. ==>> *duck* I've seen a "latency increase" on the order of 12 seconds round-trip from San Jose to Reston, VA before. Most layer 3 protocols don't like 6 second one-way trip time for a packet (almost 100x greater than normal). Granted, this was a rare case, and the particular design at the time didn't have a large negative effect (other than the reston-sanjose link); had it been a hub-and-spoke-based network and reston were a hub, I would rather let the L2 backups do their jobs than re-route me through Bangladesh and provide > 2s round trip to a lot of my leaf sites. ==>(To augment one of my own questions, when you have two ==>routers in a full mesh (FR, ATM, whatever using VCs) and ==>the VC between N-1 and N-2 goes out of service, how do N-1 ==>and N-2 decide what path(s) to use to talk to each other? ==>Feel free to consider a network using any commonly ==>available IGP and the iBGP hack.) Depends on the engineering... =) I've purposely built configurations which consisted of 20 subinterfaces in order to make a fully meshed network appear as a large number of point-to-point links in order to combat some of the problem mentioned by Sean in the "anecdotes" list. This was a re-engineer from a multipoint network due to a provider whose switched network was a bit green yet (no, I won't mention who it was, so don't ask). I've long been a fan of keeping layer 2 as the transport and avoiding "routing issues" in favor of letting layer 3 control them. Layer 2 should provide the pipes, and layer 3 can do the transport. I can accept that costs make up a big portion of the decision to use L2 switching in a network. Particularly, the difference between using a chunk of a carrier's fiber in the form of a switched service vs dedicated services plays a large part in the decisions made for connectivity. /cah