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Re: Receiving route with metric 0

  • From: Crist Clark
  • Date: Tue Dec 06 17:21:24 2005

Stephen Stuart wrote:
I am a little confused here. You yourself say that a valid metric
starts from 1, then how come 0 be valid for a directly connected
route. Are you saying that seeing a RIP metric of 0 on the wire is
valid?
A metric of 0 from a host would mean that the host itself is the
endpoint for the route. See the discussion in section 2 of RFC1058.

RFC1058 says:

3.6. Compatibility

   The protocol described in this document is intended to interoperate
   with routed and other existing implementations of RIP.  However, a
   different viewpoint is adopted about when to increment the metric
   than was used in most previous implementations.  Using the previous
   perspective, the internal routing table has a metric of 0 for all
   directly-connected networks.  The cost (which is always 1) is added
   to the metric when the route is sent in an update message.  By
   contrast, in this document directly-connected networks appear in the
   internal routing table with metrics equal to their costs; the metrics
   are not necessarily 1.  In this document, the cost is added to the
   metrics when routes are received in update messages.  Metrics from
   the routing table are sent in update messages without change (unless
   modified by split horizon).

   These two viewpoints result in identical update messages being sent.
   Metrics in the routing table differ by a constant one in the two
   descriptions.  Thus, there is no difference in effect.  The change
   was made because the new description makes it easier to handle
   situations where different metrics are used on directly-attached
   networks.

   Implementations that only support network costs of one need not
   change to match the new style of presentation.  However, they must
   follow the description given in this document in all other ways.

In other words:

In pre-RFC1058 implementations the sender increments the metric, so a
directly-connected route's metric is 1 on the wire.

In post-RFC1058 implementations the receiver increments the metric, so
a directly-connected route's metric is 0 on the wire.

In both cases, the metric in a reciever's database one hop away is 1.
You appear to have it backwards. As it says in the section you quoted,

	"These two viewpoints result in identical update messages being
	sent."

Either approach results in messages with metric 1. The metrics on the
internal routing tables of the machines differ in the two cases.
--
Crist J. Clark                               crist.clark@globalstar.com
Globalstar Communications                                (408) 933-4387




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