I did some experimenting to make sure my understanding of the holddown timer is
correct, and evidently it isn't... I thought it's purpose was to prevent a loop
from forming when a route isn't updated for 180 seconds (by default) by not
accepting updates about the route that have a worse metric.
However, I found that when a route goes into holddown, its metric goes to
4294967295, so how could there be a worse route?
So I set up a small topology to test:
Lo1 10.0.3.3
Lo2 10.0.3.6
R3-------------------------R4-----------------------------R5---------------------------R6
192.168.34.0 192.268.45.0 192.168.56.0
All router are running RIPv2 in all interfaces, timers are default, autosummary
disabled, but on R6, Lo2 is shut down. R4 installs a route to 10.0.3.0/24 with
metric 1 and next-hop of R3:
R4#sh ip route
...
C 192.168.45.0/24 is directly connected, Ethernet1/0
R 192.168.56.0/24 [120/1] via 192.168.45.5, 00:00:02, Ethernet1/0
10.0.0.0/24 is subnetted, 4 subnets
R 10.0.3.0 [120/1] via 192.168.34.3, 00:00:00, Ethernet0/0
R 10.0.6.0 [120/2] via 192.168.45.5, 00:00:02, Ethernet1/0
C 10.0.4.0 is directly connected, Loopback1
R 10.0.5.0 [120/1] via 192.168.45.5, 00:00:02, Ethernet1/0
C 192.168.34.0/24 is directly connected, Ethernet0/0
Now, I enable "passive-interface default" on R3, and wait for the route to go
into holddown on R4, which it does in 180 seconds:
*Mar 2 02:50:55.437: RT: delete route to 10.0.3.0 via 192.168.34.3, rip metric
[120/1]
*Mar 2 02:50:55.441: RT: SET_LAST_RDB for 10.0.3.0/24
OLD rdb: via 11.13.11.13
*Mar 2 02:50:55.441: RT: no routes to 10.0.3.0, entering holddown
*Mar 2 02:50:55.441: RT: NET-RED 10.0.3.0/24
R4#sh ip route 10.0.3.0
Routing entry for 10.0.3.0/24
Known via "rip", distance 120, metric 4294967295 (inaccessible)
Redistributing via rip
Last update from 192.168.34.3 on Ethernet0/0, 00:03:13 ago
Hold down timer expires in 169 secs
Now I bring up Lo2 (10.0.3.6) on R6, which advertises 10.0.3.0.24 to R5. When
R5 advertises this route to R4, it does so with a metric of 1, which is worse
than the original metric that R3 was advertising.. However, R4 still installs
the route, even though it is in holddown:
*Mar 2 02:51:45.449: RIP: Update sent via Ethernet1/0
R4#sh ip route 10.0.3.0
Routing entry for 10.0.3.0/24
Known via "rip", distance 120, metric 4294967295 (inaccessible)
Redistributing via rip
Last update from 192.168.34.3 on Ethernet0/0, 00:03:59 ago
Hold down timer expires in 124 secs
R4#
*Mar 2 02:51:55.453: RT: delete subnet route to 10.0.3.0/24
*Mar 2 02:51:55.453: RT: NET-RED 10.0.3.0/24
*Mar 2 02:51:55.853: RIP: received v2 update from 192.168.45.5 on Ethernet1/0
*Mar 2 02:51:55.853: RT: SET_LAST_RDB for 10.0.3.0/24
NEW rdb: via 192.168.45.5
*Mar 2 02:51:55.857: RT: add 10.0.3.0/24 via 192.168.45.5, rip metric [120/2]
*Mar 2 02:51:55.857: RT: NET-RED 10.0.3.0/24
Routing entry for 10.0.3.0/24
Known via "rip", distance 120, metric 2
Redistributing via rip
Last update from 192.168.45.5 on Ethernet1/0, 00:00:08 ago
Routing Descriptor Blocks:
* 192.168.45.5, from 192.168.45.5, 00:00:08 ago, via Ethernet1/0
Route metric is 2, traffic share count is 1
What am I missing here? It seems the holddown didn't do anything...
Doug
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