Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ukma!xanth!ames!sgi!decwrl!mogul From: mogul@decwrl.dec.com (Jeffrey Mogul) Newsgroups: comp.protocols.tcp-ip Subject: Re: Naive questions about subnets & domains Message-ID: <166@jove.dec.com> Date: 16 Aug 89 20:35:23 GMT References: <1072@adobe.UUCP>Organization: DEC Western Research Lines: 57 In article lear@NET.BIO.NET (Eliot Lear) writes: >Suppose you have a network that consists of two or more locations, >that looks something like the following: > > > Site A T 1 Site B > Highspeed Internet <---> Backup Link > Link > >Well, what happens if the T1 goes down? If each site has a different >net number, then with the blessing of appropriate routing gods, one >might even route through the Internet to get around the break >(forgetting policy issues, for the moment). If you use the same >network, then Site A continues to advertise it as before, and the >chances are that Site B will most likely be screwed, depending on what >routing protocol is in use. [...] >One way to handle such a break would be to transmit a subnet mask with >the route. Yes, this would increase routing traffic, but one would >only do such a thing when attempting to correct a situation like the >one I described. If I understand your suggestion, what you propose is that routing protocol servers on the boundaries between a subnetted network and the rest of the Internet would transmit subnet information about the subnetted network to "external" routers. I perceive an intention here to allow routers outside the "broken" network to send packets via the right boundary gateway. [I'm not quite sure how sending a "subnet mask" would accomplish this; knowing only the mask for a network doesn't allow one to infer what gateway is the optimal entry point for a given destination.] The IP subnetting model does not allow this. The whole point of subnetting is to hide a certain amount of detail, in order to simplify protocols and administration, and to keep the routing tables small. Doing this only when the subnetted network is broken doesn't help; If a large network with oodles of subnets partitions, the other routers on the Internet would be hit with tremendous increase in routing table complexity ... like a dam which bursts and floods the villages downstream. Subnetting reduces complexity at the expense of functionality. You can't compute "optimal" routes if you ignore certain kinds of information, but that may be the price we must pay to be able to compute routes at all. There are better solutions to the "partitioned subnetted network with multiple external gateways problem" (PSNWMEGP?). For example, one could do the responsible thing and provide multiple internal paths between Site A and Site B. Or, one could do something topologically equivalent, which is to set up a "tunnel" between Site A and Site B that acts like a real link between the sites, but is actually implemented by source-routing packets through the Internet. (There are policy issues to worry about here.) The most cost-effective solution might be to set up a temporary SLIP link, over a dialup line, between the two sites for the duration of the partition.