few question about RIP

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Mar 6th, 2009
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Hi every body!

I have few questions about RIP.


Hold down timer kicks in once router receive a route with metric=16,

I am just wondering how long router waits for an update before removing the route from the routing table?


thanks a lot!

Correct Answer by Edison Ortiz about 8 years 2 months ago

Sarah,


A route in hold-down state won't be seen in the routing table.


You need to enable debug ip rip and you will see the incoming/outgoing RIP routes. In your case, 199.199.199.0/24 will be received by Router A with a metric = 16 during a hold-down timer.


Try it :)


__


Edison.

Correct Answer by badalam_nt about 8 years 2 months ago

Sarah, for me this holddown process is not clear. I will post as well about it with the results of my tests in GNS3.


But to answer to your other question regarding flush timer:

Suppose we have LAN1-R1-R2-R3-LAN2

LAN1-R1: 1.0.0.0/8

R1-R2: 2.0.0.0/8

R2-R3: 3.0.0.0/8

R3-LAN: 4.0.0.0/8

- R2 received regularly (each 30s by default) an update from R3 for 4.0.0.0

- at the moment the router received an update for 4.0.0.0 it started for that route both the invalid timer and the flush timer

- suppose no more updates are received for 4.0.0.0 from R3.

- during the period the invalid timer runs (default 180s), R2 will keep in its routing table the 4.0.0.0 with metric 1 and as well it will advertise towards R1 the 4.0.0.0 with metric 2.

- from the moment the invalid timer expired until the flush timer expires the route 4.0.0.0 is still kept in the routing table but it is marked as possibly down:

R 4.0.0.0/8 is possibly down, routing via 3.3.3.2, FastEthernet0/1

Also during this period (by default 60s = 240s - 180s) the R2 advertises the 4.0.0.0 route to both its neighbours with metric 16.

- when the flush timer expires R2 removes the 4.0.0.0 route from its routing table and stops advertising it.


I think (need to check) that PacketTracer has a bug and it keeps the route another 240s after invalid timer expires, instead of only 60s=240s-180s.

Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hello Vishwamurti,

we are all referring to the same source/book!


And Jon is right the holddown timer is triggered by a change in route metric (increase).


Hope to help

Giuseppe


Correct Answer by viyuan700 about 8 years 2 months ago

Hi sarah,


My question is when flush timer starts? does it start after the route is declared being invalid? in that case the route stays in routing table for 180+ 240 sec( 180 for invalid timer,240 flush timer).


Route stays for 60secs more after invalid timer. i.e total 240sec (180 invlaid+60).


"The Flush timer expires after a total 240 seconds, or 60 seconds past the Invalid timer. As a

result, R1 flushes the route to 172.31.103.0/24 from its routing table, which also removes the

Holddown timer for the route.

"

check this link for more


http://books.google.com/books?id=dKzpj4r7KCwC&pg=PA211&lpg=PA211&dq=The+Flush+timer+expires+after+a+total+240+seconds,+or+60+seconds+past+the+Invalid+timer.+As+a&source=bl&ots=qMrTV3TS8P&sig=OAO5DQ0GTgeapOgAuYv0aBZlHOg&hl=en&ei=616ySYCyDtKgtwePvYnDBw&sa=X&oi=book_result&resnum=1&ct=result

Correct Answer by Jon Marshall about 8 years 2 months ago

Sarah


The holddown timer kicks in whenever a router receives an advertisement for a route with an increased metric ie. the metric doesn't have to be 16.


Edit - please ignore the above as it dosn't make much sense (even to me!) - been a long day :-)


Jon

Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hello Sarah,

two other per route timers can be involved:


invalid timer: 180 seconds (also holddown is 180 seconds)


flush (garbage) timer: 240 seconds


so it can take up to 240 seconds to remove a route from the routing table.


To be noted that network convergence is dependent on the topology so to reach other devices 30 seconds/hop ( default update interval) could be needed in addition.


Convergence is not a strong point of RIP including RIPv2.


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Fri, 03/06/2009 - 15:12
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Hello Sarah,

two other per route timers can be involved:


invalid timer: 180 seconds (also holddown is 180 seconds)


flush (garbage) timer: 240 seconds


so it can take up to 240 seconds to remove a route from the routing table.


To be noted that network convergence is dependent on the topology so to reach other devices 30 seconds/hop ( default update interval) could be needed in addition.


Convergence is not a strong point of RIP including RIPv2.


Hope to help

Giuseppe


mattkaya56 Fri, 03/06/2009 - 15:28
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1) Does this mean that very time there is a route update - all metric 16's routes are marked for deletion and Invalid, Hold down & Flush start off and upon flush timer expiry the route is no longer in the table??


2) If there is a new update that now advertises the same route with a lower metric - what happens???


Thanks in advance


viyuan700 Sat, 03/07/2009 - 04:10
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Hi Mathew,

"If there is a new update that now advertises the same route with a lower metric - what happens??? "



If an update arrives from a neighboring router with a better metric than originally recorded for the network, the router marks the network as accessible and removes the hold-down timer.


If at any time before the hold-down timer expires, an update is received from a different neighboring router with a poorer metric, the update is ignored. Ignoring an update with a higher metric when a holddown is in effect enables more time for the knowledge of the change

to propagate through the entire network.





sarahr202 Sat, 03/07/2009 - 07:40
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Hi Viyuan !

Here is what my cisco book says:

" When router receives a route with metric=16, it starts the hold down timer, during this time, update for the route with better value is ignored,"

thanks a lot!

viyuan700 Fri, 03/13/2009 - 00:37
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When router receives a route with metric=16, it starts the hold down timer, during this time, update for the route with better value is ignored


I have 2-3 books such as ICND2 Feb 2008, Sybex CCNA 2007 which says and the same thing mentioned 2 times in ICND2


If an update arrives from a neighboring router with a better metric than originally recorded for the network, the router marks the network as accessible and removes the hold-down timer.



If we try to get the message behind above sentence whatever your book says is right (I think that book made your job easier).


we know RIP is working on hop count if there are more routes between A & B the route with least hop is selected.


So if there are 2 routes between A & B i.e 2 hops and 3 Hops . Route with 2 hops is working and if thats fails the route with 3 hops is ignored till the holddown timer expires or flush. Since 3 hop metric was not better it was ignored.


So my book is implicitly conveying the same message as your book. Since least hop count is always used by RIP so updates are ignored during holddown timers as other routes dont have better metric thats what your book says.


I dont have a lab right now but if you have can try one siutation. I can be wrong but this what is my understanding.


1. Lets you have A & B 2 hops away


2.you have one one more connection between A & B which is 1 hop but shutdown this interface. otherwise RIP will use this route instead of 2 hop.


3. Simulate a link failure with 2 hop link and bring up the 1 hop interface


4. Also change timers to 30 update, 60 invlaid, 180 holddown, flush timer 240.


5. I am reducing the invalid timer as holddown timer starts after invalid timer and there is sufficient time to see whether the update from the 1 hop route is ignored or not during holddown timer.



sarahr202 Sat, 03/07/2009 - 00:56
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Thanks Giuseppe!

Let me first understand following:

Invalid timer:

The time the router waits before declaring a route invalid.


Flush time:

specify the time the route stays in routing table before being removed.


My question is when flush timer starts? does it start after the route is declared being invalid? in that case the route stays in routing table for 180+ 240 sec( 180 for invalid timer,240 flush timer).


Hold down timer:

This times starts when router receives a route with infinate metric( 16 in rip).


Correct Answer
viyuan700 Sat, 03/07/2009 - 03:50
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Hi sarah,


My question is when flush timer starts? does it start after the route is declared being invalid? in that case the route stays in routing table for 180+ 240 sec( 180 for invalid timer,240 flush timer).


Route stays for 60secs more after invalid timer. i.e total 240sec (180 invlaid+60).


"The Flush timer expires after a total 240 seconds, or 60 seconds past the Invalid timer. As a

result, R1 flushes the route to 172.31.103.0/24 from its routing table, which also removes the

Holddown timer for the route.

"

check this link for more


http://books.google.com/books?id=dKzpj4r7KCwC&pg=PA211&lpg=PA211&dq=The+Flush+timer+expires+after+a+total+240+seconds,+or+60+seconds+past+the+Invalid+timer.+As+a&source=bl&ots=qMrTV3TS8P&sig=OAO5DQ0GTgeapOgAuYv0aBZlHOg&hl=en&ei=616ySYCyDtKgtwePvYnDBw&sa=X&oi=book_result&resnum=1&ct=result

Correct Answer
Jon Marshall Fri, 03/06/2009 - 15:19
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Sarah


The holddown timer kicks in whenever a router receives an advertisement for a route with an increased metric ie. the metric doesn't have to be 16.


Edit - please ignore the above as it dosn't make much sense (even to me!) - been a long day :-)


Jon

viyuan700 Sat, 03/07/2009 - 04:00
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Hi Jon,

You are right:) even after a long day.


This is from the CCIE R & S book link posted in earlier message


"A per-route timer (default 180 seconds) that begins when a route's metric changes to a larger value. The router does not add an alternative route for this subnet to its routing table until the Holddown timer for that route expires."

Correct Answer
Giuseppe Larosa Sat, 03/07/2009 - 04:32
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Hello Vishwamurti,

we are all referring to the same source/book!


And Jon is right the holddown timer is triggered by a change in route metric (increase).


Hope to help

Giuseppe


Correct Answer
badalam_nt Sat, 03/07/2009 - 04:49
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Sarah, for me this holddown process is not clear. I will post as well about it with the results of my tests in GNS3.


But to answer to your other question regarding flush timer:

Suppose we have LAN1-R1-R2-R3-LAN2

LAN1-R1: 1.0.0.0/8

R1-R2: 2.0.0.0/8

R2-R3: 3.0.0.0/8

R3-LAN: 4.0.0.0/8

- R2 received regularly (each 30s by default) an update from R3 for 4.0.0.0

- at the moment the router received an update for 4.0.0.0 it started for that route both the invalid timer and the flush timer

- suppose no more updates are received for 4.0.0.0 from R3.

- during the period the invalid timer runs (default 180s), R2 will keep in its routing table the 4.0.0.0 with metric 1 and as well it will advertise towards R1 the 4.0.0.0 with metric 2.

- from the moment the invalid timer expired until the flush timer expires the route 4.0.0.0 is still kept in the routing table but it is marked as possibly down:

R 4.0.0.0/8 is possibly down, routing via 3.3.3.2, FastEthernet0/1

Also during this period (by default 60s = 240s - 180s) the R2 advertises the 4.0.0.0 route to both its neighbours with metric 16.

- when the flush timer expires R2 removes the 4.0.0.0 route from its routing table and stops advertising it.


I think (need to check) that PacketTracer has a bug and it keeps the route another 240s after invalid timer expires, instead of only 60s=240s-180s.

sarahr202 Sat, 03/07/2009 - 09:29
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HI every body!

I performed a lab to demonstrate that route indeed stays in routing table for 240 seconds

Three routers, rb,rd,ra are connected in traingular fashion. Rd e0 has 199.199.199.1/24, rd is connected to ra via s0( 200.200.200.4)

The routing table on ra: ( please focus on " R 199.199.199.0 via 200.200.200.4 s1" entry)


Router A# show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area

* - candidate default, U - per-user static route, o - ODR

P - periodic downloaded static route


Gateway of last resort is not set


C 200.200.200.0/24 is directly connected, Serial1

R 201.201.201.0/24 [120/1] via 202.202.202.2, 00:00:20, Serial0

[120/1] via 200.200.200.4, 00:00:25, Serial1

C 202.202.202.0/24 is directly connected, Serial0

R 199.199.199.0/24 [120/1] via 200.200.200.4, 00:00:25, Serial1

RouterA#


Now i removed the cable connected to e0 of rd to simulate link failure.

I ran the command right after i removed the link from e0 on rd. Oddly enough the entree 199.199.199.0 was removed right away .



Router A #show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area

* - candidate default, U - per-user static route, o - ODR

P - periodic downloaded static route


Gateway of last resort is not set


C 200.200.200.0/24 is directly connected, Serial1

R 201.201.201.0/24 [120/1] via 202.202.202.2, 00:00:00, Serial0

[120/1] via 200.200.200.4, 00:00:00, Serial1

C 202.202.show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area

* - candidate default, U - per-user static route, o - ODR

P - periodic downloaded static route


Gateway of last resort is not set


C 200.200.200.0/24 is directly connected, Serial1

R 201.201.201.0/24 [120/1] via 202.202.202.2, 00:00:00, Serial0

[120/1] via 200.200.200.4, 00:00:00, Serial1

C 202.202.202.0 is directly connected serial 0


Does any body knows why the route !99.199.199.0 is removed right away ? according to my cisco book, it should be there for 240 seconds.

thanks a lot!

Correct Answer
Edison Ortiz Sat, 03/07/2009 - 10:22
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Sarah,


A route in hold-down state won't be seen in the routing table.


You need to enable debug ip rip and you will see the incoming/outgoing RIP routes. In your case, 199.199.199.0/24 will be received by Router A with a metric = 16 during a hold-down timer.


Try it :)


__


Edison.

sarahr202 Sat, 03/07/2009 - 13:51
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Thanks a lot Edison ! The route was indeed seen in debug ip rip outputs with metric=16.

thanks a lot have a nice weekend!

badalam_nt Sun, 03/08/2009 - 09:52
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Edison, the route will be received by Router A with metric =16 NOT during the hold-down timer period, but only during 60 seconds. Do you know why only 60seconds???

After that the route is not advertised anymore. Very strange this behavior.

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