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? On 12/5/05, Tony Varriale <tvarriale@comcast.net> wrote:

RIP metric of 0 means it's a directly connected route. Valid metrics are 1 - 15, with 16 used as "dead".

TV ----- Original Message ----- From: "Glen Kent" <glen.kent@gmail.com> To: "NANOG list" <nanog@merit.edu> Sent: Monday, December 05, 2005 4:09 AM Subject: Receiving route with metric 0

Nanogers,

We are running RIP on one of our small cutomer routers and we are receiving routes with RIP metric 0. Is this valid? I thought each RIP router sends a metric of atleast 1, which is also what the RIP RFC seems to suggest.

Has anyone ever come across such a scenario, i.e seeing RIP routes with metric 0?

Thanks, Kent

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

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 The information contained in this e-mail message is confidential, intended only for the use of the individual or entity named above. If the reader of this e-mail is not the intended recipient, or the employee or agent responsible to deliver it to the intended recipient, you are hereby notified that any review, dissemination, distribution or copying of this communication is strictly prohibited. If you have received this e-mail in error, please contact postmaster@globalstar.com