Alex: While you are probably looking for "mandate" material, OSPF provides a specific example of implementation of the most specific route, in an actual routing protocol. Check out the OSPF RFC (I believe it was 1247) as well as the new one (1583). The old one talks about selecting the least ambiguous route. The new RFC I believe has changed the terminology to use "best match" or "most specific match" as opposed to "least ambiguous".

From Section 3.5 IP subnetting support (RFC1583)

"When an IP packet is forwarded, it is always forwarded to the network that is the best match for the packet's destination. Here best match is synonymous with the longest or most specific match."

From Section 1.1 Protocol Overview (RFC1583)

<bigger>"OSPF calculates separate routes for each Type of Service (TOS). When several equal-cost routes to a destination exist, traffic is distributed equally among them. The cost of a route is described by a single dimensionless metric.</bigger>" In OSPF's case there doesn't seem to be any implication of metric involvement in forwarding from the RFC. The metric is used to define the shortest path for routes to be implemented in the topological database. OSPF works well with subnetting. Each subnet carries its mask with it and these are also both present in the routing table. Donner At 03:16 PM 1/2/97 +0000, Alex.Bligh wrote:

Anyone know *which* RFC says a packets should be routed using the

most specific route in a routing table, not (for instance) the first

route in the table that matches, or, for instance, using a less specific

route that has a better metric? This is so basic I hardly

know where to find it, but I have a (for the time being anonymous)

systems vendor who really prefers the way their kernel does it ....

Oddly enough it doesn't work well with subnetting.

Alex Bligh

Xara Networks