Just a little note... The BSCN book mentions that the routers MUST agree on the cost of the link, however when performing the labs i had no problems whatsoever with OSPF-specific issues as a result of cost mismatch. And none of the OSPF labs caution you to double-check this "requirement". BTW - BSCN p.127 states... "all interfaces connected to the same link must agree on the link's cost." However, this is in reference to adjusting the cost for Cisco routers that are connected to non-Cisco routers that might use different methods to derive the metric.
My conclusion is that this might have more to do with engineering traffic for the applications running over OSPF links. Correct me if I'm wrong. Elmer ----- Original Message ----- From: "Peter van Oene" To: Sent: Monday, March 04, 2002 9:39 PM Subject: RE: basic OSPF questions [7:37142] > One thing to remember is that OSPF costs are calculated > unidirectionally. For example, A's cost to C could be very different from > C's cost to A. In general, IP traffic has to be engineered in both > directions and it for some networks asymmetry in flow might make sense. I > can't think of a reason off hand while watching a hockey game, but > experience has taught me that many odd looking designs are rooted in > rational, informed theory. > > Pete > > > At 04:05 PM 3/4/2002 -0500, Ouellette, Tim wrote: > >I have a question regarding # 2. > > > >let's say both routera and router b are connected and advertising the link > >between them to router c. The connection from routera to routerc is a 64k > >frame circuit. The link betwen routerb and routerc is a 64k ISDN (1 b). If > >routera advertises the network between itself and routerb with a cost of 10, > >and routerb advertises that same network with a cost of 100. All other > >things being equal when routerc gets the two updates, he will prefer to take > >the frame circuit towards routera to get to that network. Why would anyways > >want this? What if the circuit between routerb and routerc was a backup ISDN > >that you had to pay extra for to bring up during normal business hours or > >something like that. I guess it all comes down to what your network is > >doing. Whether two boxes advertise the same cost to a network is really only > >dependent upon which path you want to take to get there. If they both > >advertise the same, you may potentially load balance. If that's not desired, > >crank up the cost of one of those boxes so it's path is less-desirable. > > > >router a --- routerb > > \ / > > \ / > > \ / > > routerc > > > >Was I just rambling? Did that make sense. > > > >Tim > > > > > >-----Original Message----- > >From: Priscilla Oppenheimer [mailto:[EMAIL PROTECTED]] > >Sent: Monday, March 04, 2002 2:48 PM > >To: [EMAIL PROTECTED] > >Subject: Re: basic OSPF questions [7:37142] > > > > > >At 08:59 AM 3/4/02, bergenpeak wrote: > > > > > > > > > > >2) Must a link cost be the same on for all routers that share the > > >link? Is there a protocol reason for this? Some other reason? > > > >I couldn't find anything in RFC 2328 that says that two routers connected > >to a link MUST agree on the cost. The RFC writers use the term MUST > >carefully. If it were required, they would put it in the RFC. > > > >I think it would be a good idea to make them agree, though.... Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=37253&t=37142 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
