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Re: Receiving route with metric 0

  • From: Crist Clark
  • Date: Wed Dec 07 13:41:19 2005

Glen Kent wrote:
Am all the more confused now :)


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.

Lets say we have A -- B. A is pre-RFC1058 and B is post RFC 1058. A
sends a directly connected route as 1. B increments this by 1, and
thus stores it as 2.
Let's start over, looking at the text that was in the erlier mail from
RFC1058. A is a pre-RFC1058 and B is post-RFC1058.

Host A stores directly connected networks with metric 0,

  "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."

So host A sends RIP messages with metric 1 for directly connected
networks.

Now, for post-RFC1058, host B,

   "By
   contrast, in this document directly-connected networks appear in the
   internal routing table with metrics equal to their costs;"

So, a directly connected network, unless it has for some reason a
higher cost, host B will have a cost of 1. The value in the internal
table is 1.

  "Metrics from
   the routing table are sent in update messages without change (unless
   modified by split horizon)."

So host B will send RIP updates for directly connected networks with a
metric of 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

Hmmm .. not sure. A post 1058 implementation would send a metric 0 for
a directly connected route, assuming that the other end would
increment the value and things would work out fine.
A post-RFC1058 implementation adds the cost before putting it in its
internal routing table so a directly connected network has a cost of
greater than or equal to one.

This seems to be the point of confusion. A directly connected network
in an RFC1058-style implementation must have a cost >=1.

Why must a post-RFC1058 have a non-zero cost for a directly connected
network? Imagine A and B were both post-RFC1058 and both had zero cost
for directly connected networks. A would send updates to B with a zero
metric for one of its directly connected networks. B would then add the
cost of its link to A, which is zero, before putting it in its table.
But 0 + 0 = 0... That is, the metrics never increment. It doesn't work.
--
Crist J. Clark                               crist.clark@globalstar.com
Globalstar Communications                                (408) 933-4387

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