Counting to Infinity

I'm reading Routers and Routing Basics (CCNA 2 Companion Guide) and have a question about RIP route poisoning. If you have two connected routers (R1 and R2) and one of R2's interfaces fails, it will advertise this network to R1 with a metric of 16.

The book says that after this route with metric 16 gets installed in R1's route table, R1 will forward packets to this network to R2. Is this correct? I thought that with an infinite metric, R1 would drop the packet. No?

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Bob Simon
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Yes. Infinite metric routes will I believe result in dropped packets.

I cannot comment about the book you are looking at however in my experience many CCNx books are perhaps rushed out due to market forces and contain more than a few errors.

While if what you need is to brush up for an exam and want focussed material these books have their place however there are truely good books about.

For IP routing:- Routing TCP/IP by Jeff Doyle seems to be the one. (Second Edition now)

I have read much of it and it very nice with clear (*and complete*) explanations and simply no significant errors that I have noticed.

The RIP RFC is a nice enough read too. RFC1058

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The book I'm using is by Wendell Odom and Rick McDonald published by Cisco as part of their Networking Academy Program. It's actually one of the best books I've read covering routing basics and it's written in an easy to understand style and with lots of examples. I like this book and am thinking about getting the next one in the series.

While I typically find lots of errors in networking books, this one has had remarkably few that I've caught. That's why I was surprised to find something that really didn't look right. Since you agree that routers should not forward packets to destinations with infinite metric in the route table, I think I'll write to the authors and see what they have to say about this.

Reply to
Bob Simon



the more general point is that with Internet standards - go back to the source. RIP v2 RFC 2453 has a lot of background explanation of why things are done - so check there

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have a quick hunt for "poison" in the file - there is some stuff about this under "split horizon with poison reverse"

More generally RFCs are usually written to explain things clearly, and if you have problems with a text book check the original RFCs.

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Minor point - perhaps even nit-picking, but a 'down' route is never placed in the route table. One will never see a RIP metric of 16 in the route table.

If the route has been placed in "hold down," (route table shows "possibly down") then yes, packets are still forwarded towards the destination until either the timer expires or the route is re-discovered with a better metric (at the time the route was available) from any source, or same or better from the original source that was in the FIB.

What will actually happen in a real operating environment can vary due to various implementations such as - flash updates, split horizon, enabled/disabled and timers.


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