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!
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, 126.96.36.199/24 will be received by Router A with a metric = 16 during a hold-down timer.
Try it :)
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
- R2 received regularly (each 30s by default) an update from R3 for 188.8.131.52
- at the moment the router received an update for 184.108.40.206 it started for that route both the invalid timer and the flush timer
- suppose no more updates are received for 220.127.116.11 from R3.
- during the period the invalid timer runs (default 180s), R2 will keep in its routing table the 18.104.22.168 with metric 1 and as well it will advertise towards R1 the 22.214.171.124 with metric 2.
- from the moment the invalid timer expired until the flush timer expires the route 126.96.36.199 is still kept in the routing table but it is marked as possibly down:
R 188.8.131.52/8 is possibly down, routing via 184.108.40.206, FastEthernet0/1
Also during this period (by default 60s = 240s - 180s) the R2 advertises the 220.127.116.11 route to both its neighbours with metric 16.
- when the flush timer expires R2 removes the 18.104.22.168 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.
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
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
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 :-)
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