Hello Görgen
The original topology is the following:
As you correctly stated, R3 loses connectivity to its directly connected 3.3.3.0/24 network. What happens next?
- Notice the routing table of R2. R2 has a route to 3.3.3.0/24 with a hop count of 1. R2 will send an update to R3 saying “I have a route to 3.3.3.0/24 and my hop count is 1”.
- That update will reach R3, and R3 will think “I have learned of an alternative route to the 3.3.3.0/24 network via R2. Its original metric is 1, so in order for me to reach it via R2, I will add one more hop for a metric of 2.”
Whenever a router advertises a route to a neighbor using RIP, it always adds 1 to the metric before putting it into its own routing table, simply because you’re one more hop away from the destination.
This however causes problems in the specific topology because R2 and R3 keep advertising themselves as the route to the 3.3.3.0/24 network when neither one really has a route to that network. This results in each update adding one more to the metric until the metric becomes 15 (i.e. infinite) and unreachable. Does that make sense?
I hope this has been helpful!
Laz