excellent explanation, John. to plug a source of study materials, Howard Berkowitz has an excellent paper on how routers work, including the route determination process on certification zone ( www.certificationzone.com )
per RFC 1812, all routers forward based on the longest match. Chuck ""John Neiberger"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > There are two separate processes involved here and it's > important to make a distinction between them. There is the > process by which the router builds the routing table and then > there is the route selection process. > > When building the routing table, the router looks at the > various routes available to it. For any given route, if it's > learned from multiple protocols the Administrative Distance > wins. It's important to remember that AD comes into play when > comparing identical prefixes. > > For example, let's say you learn 205.243.23.0/24 via EIGRP and > via OSPF. The router will install the EIGRP route into the > routing table because its AD is lower. > > However, if the router learns 205.243.23.0/24 via EIGRP and > 205.243.23.0/25 via OSPF, both routes will be installed because > the prefix length (subnet mask) is different. > > The router does this comparison--if necessary--with all the > routes it has learned from all available sources and then > compiles a final routing table. > > Now, a packet arrives that needs to be forwarded. It is here > that the longest match rule really applies. The route that > most closely matches the destination for the packet is the > route that will be used. Using our previous example, let's say > a packet is destined for 205.243.23.42. In this case, the /25 > route learned via OSPF will be chosen. > > If the destination were 205.243.23.150, though, then the /24 > route would be the closest match. > > HTH, > John > > ---- On Tue, 16 Apr 2002, Sean Wolfe > ([EMAIL PROTECTED]) wrote: > > > Quick question, hope it's not too trivial: > > > > When a router decides to forward a packet based on the > longest match > > principle, does this supersede other factors? > > > > For example, if there is a route to network A via EIGRP, but > a more specific > > route available via OSPF, does it choose OSPF because of > longest match, or > > EIGRP because of lower administrative distance (90 vs. 110)? > > > > Thanks folks, fun reading your posts as always. Wish me luck > as I take BCSN > > this week. -Sean. > [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=41704&t=41692 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
