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13639 Views 15 Replies Latest reply: May 20, 2012 11:40 PM by Joel Alfredo RSS 1 2 Previous Next

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RIP Holddown Timer

Aug 28, 2010 9:52 AM

Paul Stewart  -  CCIE Security, CCSI 6,959 posts since
Jul 18, 2008

I thought I fully understood this, but I am having a couple of doubts.  Additionally this should be a simple topic, but I see all sort of contradictions in the documentation I have read.

 

Some publications seem to say that a rip route goes into holddown when a downstream router advertises it as unaccessible (metric 16 or poison route).  Other documentation states that if a downstream router advertises it with a worse (higher) metric.  From what I can tell a router goes into holddown when it is has been advertised as inaccessible by the source of the route in the route table.  Can someone confirm that is the only trigger to put the route into holddown?

 

The next clarification involves the behavior of a route in holddown.  Some documenation states that it ignores all routes to the same destination.  Other documents state that the router ignores any routes that are equal to or inferior to the route before it was placed into holddown.  From what I am seeing, I believe that the router ignores all routes to the destination when the route is in holddown.  The exception would be if the original router started advertising the route as reachable again.

 

My third (and last) question is in regards to RIP Holddown timers on current code.  For example does it still work the same on 12.4T.  I have a router running this code and when it receives a poisoned route, it is immediately flushed from the table.  So there seems to be no concept of holddown in this version of code.  Could be a bug or something.  "show ip protocols" still shows a holddown timer.  Has anyone seen this behavior?

  • Anthony Sequeira, CCIE,VCP 1,011 posts since
    Nov 9, 2008
    Currently Being Moderated
    Re: RIP Holddown Timer

    I know our own Keith Barker was playing with these pesky timers recently on 12.4 T code and he wrote up his findings here:

     

    http://blog.ine.com/2010/04/15/how-basic-are-rip-timers-test-your-knowledge-now/

     

    I hope you find it helpful.

  • Anthony Sequeira, CCIE,VCP 1,011 posts since
    Nov 9, 2008
    Currently Being Moderated
    Re: RIP Holddown Timer

    Actually I do not think so. Without running my own tests - I think we can take the Command Reference at face value here - it states:

     

    "A route enters into a holddown state when an update packet is received that indicates the route is unreachable."

     

    I think the texts we read about the Holddown Timer are speaking generically about Holddown Timers - not the Cisco holddown timer addition to RIP (as I am sure you know - it is not in the RIP RFC).

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    Re: RIP Holddown Timer

    I thought I fully understood this, but I am having a couple of doubts.  Additionally this should be a simple topic, but I see all sort of contradictions in the documentation I have read.

     

    Some publications seem to say that a rip route goes into holddown when a downstream router advertises it as unaccessible (metric 16 or poison route).  Other documentation states that if a downstream router advertises it with a worse (higher) metric.  From what I can tell a router goes into holddown when it is has been advertised as inaccessible by the source of the route in the route table.  Can someone confirm that is the only trigger to put the route into holddown?

     

    The next clarification involves the behavior of a route in holddown.  Some documenation states that it ignores all routes to the same destination.  Other documents state that the router ignores any routes that are equal to or inferior to the route before it was placed into holddown.  From what I am seeing, I believe that the router ignores all routes to the destination when the route is in holddown.  The exception would be if the original router started advertising the route as reachable again.

     

    My third (and last) question is in regards to RIP Holddown timers on current code.  For example does it still work the same on 12.4T.  I have a router running this code and when it receives a poisoned route, it is immediately flushed from the table.  So there seems to be no concept of holddown in this version of code.  Could be a bug or something.  "show ip protocols" still shows a holddown timer.  Has anyone seen this behavior?

     

    Hello Paul -

    I labbed this up using this topology:

    3 routers CLN-3.png

    With the assistance of syslog, and some patience, here is what I learned today.

     

    Holddown timer begins when a route learned from a downstream router exceeds the upstream routers invalid timer.

     

    When hold down  starts on the upstream router it won't believe ANYTHING new about that route, better metric or worse from the original source, until the holddown timer expires.

     

    I had started with R1 sending 1.1.1.1 with a metric of 10.   After disabling the Fa0/0 on R1 and waiting for R2s invalid timer to expire, and then during holdown time on R2, enabled the Fa0/0 again on R1, and sent the 1.1.1.1 with a metric of 5, but R2 wouldn't budge, during it's holddown.

     

    If a triggered update is sent from a downstream router, (R1 to R2, regarding 1.1.1.1, R2 will delete the route immediately (along with poison reverse, and pass on the bad news to R3).

     

    If R1 simply stops talking, and R2 hits the invalid timer for that route, it will begin the holdown, and send a poisoned route to R3.  Because R3 got the poisoned route from its downstream router (R2), R3 will delete the route, and not go into hold down.

     

    I learned a ton today.

     

    Let me know if you have other questions.

     

    Best wishes,

     

    Keith

  • Catalyst52 197 posts since
    Dec 22, 2010
    Currently Being Moderated
    Re: RIP Holddown Timer

    What method did you use to have R1 send metrics of 10 and 5?

  • Joel Alfredo 46 posts since
    Oct 28, 2010
    Currently Being Moderated
    Re: RIP Holddown Timer

    Thank you very much, I´m studding to pass ICND2 with Wendell Odom book, and trying to test the hold down timer in a lab with GNS3. Nothing had sense to me when I was testing in the lab, what was explained in the book, because I saw how all the switches just deleted the routes when they received a trigger update with a poison route, without passing the hold down timer. Now I´ve found this discussion and everything is getting sense. But, as I told you, I want to pass the exam, and that’s why I ask you taking into consideration your experience in Cisco certifications exams. Should I answer a possible question in the exam with what is implemented really in actual Cisco IOS, or should I stay on line with the official certification study book (Wendell Odom 2nd Edition)? This is even a “Key Topic” in this book when it says:

     

    “After hearing a poisoned route, start a holddown timer for that one route.

    Until the timer expires, do not believe any other routing information about the

    failed route, because believing that information may cause a routing loop.

    However, information learned from the neighbor that originally advertised the

    working route can be believed before the holddown timer expires.”

     

    Thank you very much for your time,

     

  • Alkuin Melvin 127 posts since
    Jul 8, 2009
    Currently Being Moderated
    Re: RIP Holddown Timer

    Hi, what if the R3 have new connection with a new router, for ex R4. I have labbed this and why the holddown timer only occured on R2?

     

    Thanks

  • Joel Alfredo 46 posts since
    Oct 28, 2010
    Currently Being Moderated
    11. May 20, 2012 4:44 AM (in response to Alkuin Melvin)
    Re: RIP Holddown Timer

    Hello, we had another thiscussion related with this one here:

     

    https://learningnetwork.cisco.com/thread/23076?start=0&tstart=0

     

    Maybe it could help you.

  • Alkuin Melvin 127 posts since
    Jul 8, 2009
    Currently Being Moderated
    12. May 20, 2012 10:54 AM (in response to Joel Alfredo)
    Re: RIP Holddown Timer

    Hello Joel, thanks for the link. I have read the discussions in the thread, but my question still can't be answered clearly. Can you give me some information if it is true the holddown timer only occur on R2, even R1 have RIP connectivity to another router ?

  • Joel Alfredo 46 posts since
    Oct 28, 2010
    Currently Being Moderated
    13. May 20, 2012 11:44 AM (in response to Alkuin Melvin)
    Re: RIP Holddown Timer

    Hello Alkuin, any router would start the hold down timer if it does not receive any update (by default during 180 seconds) to a route from its next hop (for that route). I mean, if R1 has any other router connected, and this router stops to listen updates to 1.1.1.1 from R1, it would start the hold down timer also, after 180 seconds.

  • Alkuin Melvin 127 posts since
    Jul 8, 2009
    Currently Being Moderated
    14. May 20, 2012 8:06 PM (in response to Joel Alfredo)
    Re: RIP Holddown Timer

    Hello Joel, so if for ex link between R1 and R2 down. After invalid timer, the hold down timer on R2 will start, because R2 still sending the route of 1.1.1.1 to R3, hold down timer on R3 will not be activated. So, in this case only R2 activate the hold down timer. CMIIW. One more thing, once R2 activate the hold down timer, it will trigger a poisoned route to other router ("If R1 simply stops talking, and R2 hits the invalid timer for that route, it will begin the holdown, and send a poisoned route to R3.") . CMIIW

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