Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!asuvax!ncar!csn!boulder!recnews From: Watt-Alan@mickey.ycc.yale.edu Newsgroups: comp.dcom.sys.cisco Subject: Re: backplane backbone, routing or bridging? Message-ID: <9106210211.AA09464@mickey.ycc.yale.edu> Date: 21 Jun 91 02:11:40 GMT Sender: news@colorado.edu Organization: University of Colorado, Boulder Lines: 68 Return-Path: >Date: Thu, 20 Jun 1991 15:45:02 -0700 (PDT) >From: Steve Hubert >Sender: Steve Hubert >Subject: backplane backbone, routing or bridging? >To: cisco@spot.colorado.edu > >We are about to connect all N (N ~ 20) of our routers to a single AGS+ in >order to use the AGS+ backplane as our "backbone subnet". There will be point >to point ethernet between each router and the backbone router. We carry only >IP traffic over the backbone. It occurs to us that, instead of assigning a >subnet number to each of these point to point links, we could use the AGS+ as >a bridge and use only one subnet number instead of N. I don't think this has >anything to do with the normal bridge-vs-router debate. This would be an N- >port bridge with a single router attached to each port. Does anyone have any >opinions on advantages/disadvantages of the two approaches? For example, is >one way faster than the other? Thanks. > >Steve Hubert >Networks and Distributed Computing, Univ. of Wash., Seattle >hubert@cac.washington.edu > > You may not be able to do this. You cannot both route and bridge the same protocol in a cisco chassis, even if you only route on some interfaces and only bridge on others. In other words, if you want to route IP in either the AGS+ chassis, you cannot bridge IP in it. I wouldn't do it anyway, as you lose the chance of gathering all that per-protocol information in the AGS+ (such as IP accounting). If you want to bridge, look into getting a genuine multiport bridge (A.K.A. fast packet switch). This would undoubtedly save you money over the AGS+ if that's all you plan to do with it. However you do not have to assign a whole subnet to each point-to-point ethernet. You can have a single subnet which is shared by all the point-to-point ethernets on both ends. Assuming you subnet a class B on an 8-bit boundary, and the "point-to-point cloud" is subnet 100, you could define the subnet mask 255.255.255.252 to allow 6 bits of "sub-subnet" and 2 bits of "host" (0 is reserved, 3 is broadcast, 1 and 2 are the two endpoints. AGS+ intf address Brcast Router intf address ---- ------- ------ ------ ------ ------- ether0 XX.YY.100.1 XX.YY.100.3 AGS 1 ether0 XX.YY.100.2 ether1 XX.YY.100.5 XX.YY.100.7 AGS 2 ether0 XX.YY.100.6 ether2 XX.YY.100.9 XX.YY.100.11 AGS 3 ether0 XX.YY.100.10 ... ether18 XX.YY.100.77 XX.YY.100.79 AGS 20 ether0 XX.YY.100.78 ether19 XX.YY.100.81 XX.YY.100.83 AGS 20 ether0 XX.YY.100.83 There you have your 20 ethernet point to point links between the AGS+ and the 20 other routers, and plenty of address space left in subnet 100 for more. - Alan S. Watt High Speed Networking, Yale University Computing and Information Systems Box 2112 Yale Station New Haven, CT 06520-2112 (203) 432-6600 X394 Watt-Alan@Yale.Edu Disclaimer: "Make Love, Not War -- Be Prepared For Both" - Edelman's Sporting Goods [and Marital Aids?]