Counting to Infinity
Step 1
When a routing protocol is not turned on there are only directly connected networks in the routing tables of these routers and the metric is 0.
Step 2 (Run RIP)
Now we turn on RIP on these routers.
RIP sends update every 30 seconds.
After 30 sec goes by, A sends a copy of its routing table to B.
B already knew about network 2 but now B learns about network 1 as well.
Router B has now learned about network 1 from A via E0 interface so the metric now will be 1 hop.
Step 3
B also exchanges its routing table with A about network 2 and 3.
Step 4
B then passes the routing table to its other neighbor, Router C.
Step 5
C also sends its update to B and B sends it to A.
Now the network is converged.
Step 6
Now let’s assume network 4 down suddenly.
When network 4 fails, Router C detects the failure and stops routing packets out its E1 interface.
However, Routers A and B have not yet received notification of the failure.
Step 7
There will be no problem if C sends an update earlier than B and inform that network is currently down but assume B sends its update first.
C will see B has a path to network 4 with a metric of 1 so it updates its routing table, thinking that “if B can go to network 4 by 1 hop then I can go to network 4 by 2 hops” but of course this is totally wrong.
Step 8 (Counting to Infinity)
In turn, C sends an update to B and informs it can access network 4 by 2 hops.
B learns this and thinks “if C can access network 4 by 2 hops than I can access by 3 hops”.
The metric will increase to infinity so this phenomenon is called “counting to infinity”.
Solutions to prevent this phenomenon
Split horizon
Route poisoning
Poison reverse
Hold down timers
1. Split Horizon
A router never sends information about a route back in the same direction which is original information came, routers keep track of where the information about a route came from.
2. Route Poisoning
Router considers route advertised with an infinitive metric to have failed (metric=16) instead of marking it down.
First Scenario
For example, when network 4 goes down, router C starts route poisoning by advertising the metric (hop count) of this network as 16, which indicates an unreachable network. When router B receives this advertising, it continues advertising this network with a metric of 16.
3. Poison Reverse
The poison reverse rule overwrites the split-horizon rule. Notice that every router performs poison reverse when learning about a downed network.
First Scenario
For example, if router B receives a route poisoning of network 4 from router C then router B will send an update back to router C (which breaks the split-horizon rule) with the same poisoned hop count of 16. This ensures all the routers in the domain receive the poisoned route update.
4. Hold down timers
First Scenario
For example, in the above example, when B receives a route poisoning update from C, it marks network 4 as “possibly down” in its routing table and starts the hold-down timer for network 4.
In this period if it receives an update from C informing that network 4 is recovered then B will accept that information, remove the hold-down timer, and allow data to go to that network.
But if B receives an update from A informing that it can reach that network by 1 (or more) hop, that update will be ignored and the hold-down timer keeps counting.
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