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another RIP timer question (holddown) - cisco doc error?

I know that this question has been asked like millions of times but all of them seem to be missing something. I think there is a big difference in how we think holddown timer should work and how it actually works.

So based on cisco's documentation:

holddown

Interval (in seconds) during which routing information regarding better paths is suppressed. It should be at least three times the value of the update argument. A route enters into a holddown state when an update packet is received that indicates the route is unreachable. The route is marked inaccessible and advertised asunreachable. However, the route is still used for forwarding packets.When holddown expires, routes advertised by other sources are acceptedand the route is no longer inaccessible. The default is 180 seconds.

But based on CCIE Routing and Switching Exam Certification Guide, 3rd Edition page 192:

"After the Invalid timer expires (180 seconds) for R1’s route to 172.31.103.0/24, R1 begins a
Holddown timer for the route. Holddown starts at (default) 180 seconds, and counts down."

and based on very extensive testing of the timers demonstrated by  http://cisconinja.wordpress.com/2009/04/27/rip-timers/

" So what exactly causes holddown to occur?  Earlier when we looked atthe invalid timer, we saw the route placed into holddown when theinvalid timer expired.  As it turns out, this is the only time that the holddown timer is used."

I see the same results in lab as cisconinja blog post and the CCIE cert guide.

So my question is why does every other document on the web specify that holddown timer is started when a route of higher metric is received?

This is driving me a little crazy and I would like to get to the bottom of it.


Am I missing something?

Thank you in advance for you responses.

2 REPLIES
Hall of Fame Super Silver

Re: another RIP timer question (holddown) - cisco doc error?

Hello Tomasz,

I remember one thread created by Nicolas or Petrus some mounths ago that was about the real meaning of the holddown timer. He had done his own tests and his findings were in line with what is written in the CCIE study guide.

He was surprised that he had found this that actually was not present in previous forum threads.

I can only say as a partial justification of this lack of knowedge that RIP is not used often nowdays.

Also the implementation of Cisco RIP may be different in this aspect from that described in the original Bell-Ford algorithm.

Probably in another implementation a single holddown timer that is 6-7 times the update interval is enough instead of having flush timer, invalid timer and holddown timer. Take this is as a qualitative guess because I cannot find at the moment a comparison between Cisco implementation and other vendor implementations.

Actually, in RFC 1058 that describes RIPv1 it speaks of marking a route as invalid but it doesn't mention an invalid timer

http://www.faqs.org/rfcs/rfc1058.html

It reports:

>>

If we don't hear from G for 180 seconds, we can assume
   that either the gateway has crashed or the network connecting us to
   it has become unusable.  Thus, we mark the route as invalid.  When we
   hear from another neighbor that has a valid route to N, the valid
   route will replace the invalid one.  Note that we wait for 180
   seconds before timing out a route even though we expect to hear from


   each neighbor every 30 seconds.

if we look for invalid timer inside the RFC we don't find it.

And this is probably the root cause of this common misunderstanding.

the command reference and most CCNA courses use the standard defined description.

One should perform a test may be with the help of an offset list to create an artificial increase in metric to observe reaction of downstream router receiving the modified update.

This should show if an holddowm timer is triggered on downstream router.

Edit:

I see that the cisco ninja blog has used the offset list as a method to test holddown.

Edit2:

I've found the thread I was meaning:

https://supportforums.cisco.com/message/606498#606498

as you can see this issue has caused headaches to some other interested colleagues

Hope to help

Giuseppe

Community Member

Re: another RIP timer question (holddown) - cisco doc error?

Giuseppe

Thank you for your post. I modified the other post to continue with that thread instead of following a new one. Please check it out.

https://supportforums.cisco.com/message/606498#606498

Thank you,

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