[6672] in North American Network Operators' Group
Re: Which RFC?
daemon@ATHENA.MIT.EDU (Paul G. Donner)
Thu Jan 2 12:20:00 1997
Date: Thu, 02 Jan 1997 12:10:49 -0800
To: "Alex.Bligh" <amb@xara.net>, nanog@merit.edu
From: "Paul G. Donner" <pdonner@cisco.com>
Cc: amb@xara.net
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
>
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