From: Avi Freedman <freedman@netaxs.com> Subject: Re: sell shell accounts? To: vansax@atmnet.net (Jim Van Baalen) Date: Fri, 19 Jul 1996 17:17:19 -0400 (EDT) Cc: richards@netrex.com, agislist@interstice.com, nanog@merit.edu [...] Nope, remember - there is no magic. Any mesh of PVCs that one makes over a switched network must reflect the toplogy of that network, and one can set up a matching set of active routing sessions and route weights which will cause traffic to flow the same way. [...]

If I understand what you are asserting, I probably don't agree with it. I assume that we are talking about using wide-area ATM networks, (or more specifically, wide-area ATM services provided by, for example, carriers). Consider the following configuration ________ _______________________ ________ | Router | loop A | | loop C | Router | | A |=========| Wide-Area ATM Service |========| C | |________| |_______________________| |________| | | local loop B | ________ | Router | | B | |________| The "long" path in this diagram between Router A and Router C is through Router B, with a VC between (A,B) and a VC between (B,C). The "shortcut" path in this diagram is a VC between (A,C). The biggest disadvantage of routing packets between A and C via B is that every packet (needlessly) traverses local loop B twice. Furthermore, this local loop is perhaps the most expensive component in the system. The desire for a full mesh of VCs between routers becomes more compelling if you have more than three routers. The diagram is left as an exercise to the reader; I rather dislike having to draw in ASCII. Note that the two paths between A and C, (directly versus via B), seem to take distinctly different paths. (Perhaps, I missed your point). It is not clear to me that being able to ping router B along the path between A and C buys you much. It sounds a bit (well, maybe only a bit) like wanting to cut a T1 in half to install a router in the middle so that you can tell which half of the T1 went down. (Again, I am assuming that the network in question is using ATM services, so the carrier is responsible for keeping the VCs up.) -tjs