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=41697&t=41692 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
