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  #1  
Old 01-02-2012, 08:12 AM
wim4 wim4 is offline
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Join Date: Sep 2011
Posts: 1
Default When does the Route Flush Timer start?

Hello,

It is not totally clear when the (RIP) Route Flush Timer starts.
Is it after the route becomes invalid (so after the Route Invalid Timer times out)?
Or is it when a route update is received (so when also the Route Invalid Timer starts)?

In my book (CCNA, version 5) it seems to say that it starts when the route becomes invalid, but it also says that the value of the Route Invalid Timer must be less than that of the Route Flush Timer.
That seems to contradict each other.

Thanks in advance for your answer.

Regards, Wim.
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  #2  
Old 01-02-2012, 06:18 PM
ciscodaze ciscodaze is offline
 
Join Date: Dec 2010
Location: Left Coast, California
Posts: 306
Default

Quote:
Originally Posted by wim4 View Post
Hello,

It is not totally clear when the (RIP) Route Flush Timer starts.
Is it after the route becomes invalid (so after the Route Invalid Timer times out)?
Or is it when a route update is received (so when also the Route Invalid Timer starts)?

In my book (CCNA, version 5) it seems to say that it starts when the route becomes invalid, but it also says that the value of the Route Invalid Timer must be less than that of the Route Flush Timer.
That seems to contradict each other.

Thanks in advance for your answer.

Regards, Wim.
The flush time deletes the dead route from the routing table at 240 secs (default), which is 60 seconds longer than the invalid timer.

After the invalid time expires, rip needs time to advertise the down route -- thats why they say not to make the flush time LESS than the 180 sec invalid time -- otherwise there would be no time for rip to advertise the down route.

Timers are tweakable (not recommended!!) so the point is, leave time for the router to advertise the down route before it flushes it.
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Last edited by ciscodaze; 01-02-2012 at 06:20 PM.
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  #3  
Old 01-02-2012, 07:05 PM
ciscodaze ciscodaze is offline
 
Join Date: Dec 2010
Location: Left Coast, California
Posts: 306
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William, I should also add you are walking on some spooky ground here, as the holddown timer is not part of the RFC specification for rip, so searching for info on it in RFC wont yeild results (I did find one RFC about extensions that mentions holdtime, but not much help) . This has lead to a lot of confusion between the cisco implementation of the holddown time, and the invalid timer.

I would explain it in 2 easy sentences if I could, but I can't. Maybe someone else can clarify.

For the exam, just memorize whats in whatever book you are using and you will be fine. Otherwise, do some googling around, it will be a good learning experience.

Then come back and tell us what you learned.
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Kevin NET+SEC+A+CCNA
'All that is not eternal is eternally out of date' ~ C.S. Lewis

Last edited by ciscodaze; 01-02-2012 at 10:28 PM. Reason: \
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  #4  
Old 01-02-2012, 11:17 PM
ciscodaze ciscodaze is offline
 
Join Date: Dec 2010
Location: Left Coast, California
Posts: 306
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William, heres a good link to check out at the learning center, regarding rip timers.

If 3 heavy-hitter CCIE dudes need to hash out how the hell rip timers work, dont feel bad that you dont get it all right now!

https://learningnetwork.cisco.com/thread/20422
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'All that is not eternal is eternally out of date' ~ C.S. Lewis

Last edited by ciscodaze; 01-02-2012 at 11:21 PM.
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