RIP冷靜計時器(Hold-down Timer) 說明

IOS中RIP的路由資訊交換軟體, 針對每一個目的地網段的路由資訊, 一共有四組計時器:

1. 更新週期計時器 (Update Timer)
2. 宣告失效計時器 (Invalid Timer)
3. 冷靜計時器 (Hold-down Timer)
4. 清除計時器 (Flush Timer)
   

所以針對每一個目的地網段的路由資訊, 大致上會有以下的生命狀態循環:

1. 正常情況下, 收到更新通知時, 該目的地網段的最短距離和方向, 都被重新更新一次.
2. 無論何時一收到更新, "宣告失效計時器", 和 "清除計時器" 都歸零, 重新計時.
   
3. 當超過宣告失效計時器的設定時間, 都沒有收到任何更新時, 會被宣告為失效. 但是, 該路由資訊仍可以殘存一段時間.
4. 當超過 "清除計時器" 的設定時間, 還是沒有收到更新, 該路由資訊就會被清除.
5. 任何時間一收到立即失效的更新通知(Poisoning)時, 就啟動冷靜計時器. 在冷靜期間, "宣告失效計時器", 和 "清除計時器", 都繼續計時; 一收到非Poisoning的更好的路由資訊, 在收到的同時結束冷靜期. 如果 "清除計時器" 先到期, 那就會被清除; 反之如果 "冷靜計時器" 先到期,  就回到(1)接受更新通知的正常狀態.
   

[Cisco.com參考資訊]

update	Rate in seconds at which updates are sent. This is the fundamental timing parameter of the routing protocol. The default is 30 seconds.
invalid	Interval of time (in seconds) after which a route is declared invalid. The interval should be at least three times the value of update time. The interval is measured from the last update received for the route. The route becomes invalid when there is an absence of updates during the invalid time that refresh the route. The route is marked inaccessible and advertised as unreachable. However, the route still forwards packets until the flush interval expires. The default is 180 seconds.
holddown	Interval (in seconds) during which routing information regarding better paths is suppressed. The interval should be at least three times the value of update time. 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 as unreachable. However, the route continues to forward packets until an update is received with a better metric or until the holddown time expires. When the holddown expires, routes advertised by other sources are accepted and the route is no longer inaccessible. The default is 180 seconds.
flush	Amount of time (in seconds) that must pass before the route is removed from the routing table. The interval is measured from the last update received for the route. The interval should be longer than the larger of the invalid and holddown values. If the interval is less than the sum of the update and holddown values, the proper holddown interval cannot elapse, which results in a new route being accepted before the holddown interval expires. The default is 240 seconds.

http://www.cisco.com/en/US/docs/ios/12_0/np1/command/reference/1rrip.html#wp1018019