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HoldDown Timer Question
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ntmak
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I'm puzzle with the following Question, can someone enlighten me
Under which 3 situation will the hold-down time be reset ?
a) when the route received a processing task proportioned the number of link in the network
b) when another update is received indication the original route to the network has been restored
c) when the router detects faulty LSPs propagation through the internetwork
d) when another update is received indicated a new route with a better metric
e) when the router exchange update summary at area borders
f) when infinity is finally defined as some maxim number
g) when hold-down time is expire.
My answer : a,d,g
Can anyone verify the above answer
Thank You 
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 12-07-00 06:44 AM
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haubest
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12-07-00 08:02 AM
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adilux
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12-07-00 02:42 PM
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Wolverine
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12-07-00 11:07 PM
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Terje
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I am not sure about the answer here, but I would like comments to my reasoning.
quote:
Originally posted by ntmak:
I'm puzzle with the following Question, can someone enlighten me
Under which 3 situation will the hold-down time be reset ?
I guess "reset" means the timer is stopped and removed and information about the failed route is again accepted.
quote:
a) when the route received a processing task proportioned the number of link in the network
I do not understand what this is supposed to mean. What is "a processing task" in this context?
quote:
b) when another update is received indication the original route to the network has been restored
The holddown timer is running when the router should disregard updates about a failed link. The purpose is to prevent a route beeing incorrectly reinstated by outdated information sent from another router that is (was) unaware of the link failure.
This is therefore NOT the correct answer.
quote:
c) when the router detects faulty LSPs propagation through the internetwork
Faulty Link State Packets (LSPs) should be no reason to take any action (except ignoring them). BTW: LSPs are used by LS protocols, but holddown timers are used by DV protocols.
This cannot be the correct answer.
quote:
d) when another update is received indicated a new route with a better metric
According to the ICND book this is true. The ciscopress study guide says that ALL routing information regarding the failed route should be ignored untill the holddown timer expires. An old but very good document on the cisco web site agrees with the study guide. http://www.cisco.com/warp/public/103/5.html
This point deserves further study. Anyone able to clarify or point to good sources of information? Unfortunately this is difficult for me to test live as the test will require several non-production routers.
quote:
e) when the router exchange update summary at area borders
This seems unrelated to the function of the holddown timer.
quote:
f) when infinity is finally defined as some maxim number
Infinity is defined as some number as part of the protocol. Not a reasonable answer.
quote:
g) when hold-down time is expire.
Yes, of course. When the holddown timer expires, it has fullfilled its function and is no longer needed.
Comments and corrections appreciated.
Terje
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12-08-00 02:38 AM
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dmaftei
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quote:
Originally posted by Terje:
I am not sure about the answer here, but I would like comments to my reasoning.
Clear and crisp, Terje. I'm really impressed!
quote:
I guess "reset" means the timer is stopped and removed and information about the failed route is again accepted.
I always thought that "resetting a timer" means bringing the timer back to some initial value, whatever that initial value might be. But in this context "reset" has different semantics. 
quote:
a) when the route (should be router) received a processing task proportioned the number of link in the network
I do not understand what this is supposed to mean. What is "a processing task" in this context?
This comes directly from Mr. Lammle's book; it's one of the three events that cause the holddown timer to be "reset". I would be extremely interested to hear an explanation!
Speaking of Mr. Lammle's book ("CCNA Study Guide", second edition, Sybex), I've got a chance to browse it for a couple of days, and I found it quite disappointing. It has its virtues as an introduction to the concepts required for the CCNA test. On the other hand I saw a lot of mistakes, and a lot of explanations left in the air. My conclusion: whoever's interested in real, serious study, will be better off with other sources...
[This message has been edited by dmaftei (edited 12-07-2000).]
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12-08-00 08:03 AM
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firechicken
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Hey all.
quote:
b) when another update is received indication the original route to the network has been restored
My understanding is that this is true...unless the update is from the source which originally advertised the downed route.
I'll see if I can find this info in my books again.
BTW, could you please look in your book and repost answer "a"? Thanks.
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12-08-00 06:41 PM
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firechicken
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Here's an excerpt from the Cisco Comapanion Guide describing holddowns:
"During the holddown period, the route is advertised, but incoming advertisements about that network from any router other than the one that originally advertised the network's new metric are ignored."
According to two other books I have, during the holddown period, there is no mention of the router "that originally advertised the network's new metric".
Eeenee meenee minee mo...
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12-09-00 03:47 AM
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Terje
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quote:
Originally posted by firechicken:
Here's an excerpt from the Cisco Comapanion Guide describing holddowns:
"During the holddown period, the route is advertised, but incoming advertisements about that network from any router other than the one that originally advertised the network's new metric are ignored."
According to two other books I have, during the holddown period, there is no mention of the router "that originally advertised the network's new metric".
Which one makes most sense? Believe new metrics from the original source of the route poison or not?
After contemplating this for a while I have decided against the idea of trusting improved metrics from the original source. Let me illustrate my reasoning by an example:
Say router A transmits an ordinary route information packet to router B. This packet contains metrics for route X which at this time is operational. Then imidiately after transmitting the packet, router A learns that route X has gone bad. It will now send another packet (a triggered update) with route X poisoned. Normally the first packet (with route X good) will arrive at B before the second (with the poison). However this will not allways be the case. In some networks it is possible for the second packet to overtake the first. Router B will first see a message about X going bad and start its holddown timer, then shortly after that it will receive the packet with good metrics about X. If router B now trusts this second packet (despite the holddown timer running) because it came from the original source of the poison message then router B will reinstate a bad route, stop its holddown timer and propagate incorrect information into the network. That's excactly what holddown timers was designed to prevent in the first place.
Terje
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12-13-00 02:47 PM
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TheoM@gr
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The correct answers are d,f,g.
d: because when a router receives an update with beter metric marks the network as accessible and removes the hold-down timer.
f: the infinity is defined and the hold-down is no neaded any more
g: it is obvious
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12-18-00 03:53 AM
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