RIP timers and reality, #2 (HOLDDOWN)
Let’s continue our discovery of the RIP timers in a slightly different topology, and discover the odd Holddown timer.
Second configuration
Normal situation: R101 advertises R100 for NETWORK with a RIP metric of 1. R100 has one route for NETWORK in his routing table via R101 with metric 1.
R102 does not yet advertise NETWORK to R100.
Breakdown: Instead of putting down R101, an distribute ACL for outgoing RIP advertisements is enabled on R101 so that R100 won’t receive RIP advs anymore, but can continue to ping R101 F0/0.
After some time, admin triggers R102 to start advertising NETWORK to R100.
Test 1
Timeline
– X min: network up and working
0 min: Action: NETWORK is not advertised anymore by R101, but R100 can’t see that directly, since it’s behind SW1. INVALID Timer starts
1 min and 30 sec: Action: R102 starts advertising NETWORK to R100 ; Reaction: in R100, two routes to NETWORK now exist in the routing table, via R102 with a counter under 30sec and via R101 via a counter at around 2min.
3 min: Reaction: in R100, instead of being marked “possibly down”, route via R101 just disappears, leaving route via R102
Conclusion
The router does not bother keeping a “possible down” route in its routing table when another working route exists
Test 2
Timeline
– X min: network up and working
0 min: Action: NETWORK is not advertised anymore by R101
3 min: “possibly down” for NETWORK on R100
3 min and 30 secs: Action: R102 starts advertising NETWORK to R100 ; Reaction: R100 directly adds route via R102 into routing table. No HOLDDOWN timer is used and the “possibly down” route via R101 disappears (like in test 1).
(6 min : end of HOLDDOWN timer)
Conclusion
It seems that the HOLDDOWN timer does not work for “possibly down” routes. This is strange. Maybe it just works for routes advertised with metric 16 by R101 or because R102 advertises the same metric as R101 (thus there would be a routing loop in the topology, as if R102 was going through R101 to reach NETWORK). This 2 cases will be studied in further tests.