Path: utzoo!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!ucsdhub!ucsd!rutgers!aramis.rutgers.edu!hedrick From: hedrick@aramis.rutgers.edu (Charles Hedrick) Newsgroups: comp.dcom.lans Subject: Re: What about "brouters"? Message-ID: Date: 9 Oct 88 17:17:00 GMT References: <281@fed.FRB.GOV> <25255@bu-cs.BU.EDU> <2944@uoregon.uoregon.edu> Organization: Rutgers Univ., New Brunswick, N.J. Lines: 45 You said that you don't think the cisco HyBridge is a brouter. I don't know any of the products that you consider brouters, so I'm not in a wonderful position to make comparisons. But the HyBridge does routing in two different ways. For some protocols it acts just like any other router. For others it acts just like any bridge. As far as I know, there is no interaction between the two types of routing. You can apparently select which protocols are handled which way. When it is acting as a bridge, the HyBridge is compatible with either the proposed IEEE standard spanning tree protocol or the DEC LANbridge. (You get to choose. In theory they could support both protocols, so that a HyBridge could connect sets of bridges that use different protocols, but I don't know whether they've gone that far.) You should be able to put the HyBridge in the middle of a set of LANbridges and the routing will all still work. However LANbridges have network monitoring and control facilities in addition to the spanning tree algorithms. I have a feeling that the HyBridge does not implement that, so they aren't completely interchangable with LANbridges. There is a protential problem with having a HyBridge in parallel with a LANbridge, but it's exactly the same as having any router in parallel with a bridge. Consider the following configuration: ----------------- network 1 | | HyBridge LANbridge | | ----------------- network 2 It would work fine if the HyBridge is set up to bridge all protocols. But if the HyBridge is handling IP as a router, then you're got a router and a bridge in parallel. Things can get very exciting indeed. Suppose a host on net 2 sends something to a host on net 1 via the router. The first time the router gets such a packet, it will issue an ARP request for the host on net 1. Unfortunately, the bridge will forward the ARP request to net 2. There will be two responses, one directly from the host on net 1, and the other from the net 2 side of the router, which thinks that a host on net 2 is trying to use proxy ARP. The response from the back end of the router will probably arrive second, since it has a more complex path. Thus it will probably be believed, and we'll end up with a circular path. However this configuration will probably work if you disable proxy ARP on the router. Note that this problem has nothing to do with the HyBridge in particular. It will happen any time you have a bridge in parallel with a router that does proxy ARP.