How does RIP knows if a link is down?

[RachelG]

How does RIP knows if a link is down? Is it the same method as when a neighbor is down with invaild timer and flush timer?

Why does RIP find a link that is down faster than a neighbor that is down?

Thanks for your help!

[Flavio Miranda]

Hello @RachelG

Lets see if I can help you. RIP is a distance vector protocol and use hop count as metric. You might read this already many times during your studies. But this actually says a lot about the protocol.

 The same way when we say that OSPF is link state.

 I take the risk to say that RIP don't care about link but neighbor and only the directed connected neighbor. So it is easy for RIP to know when neighbor is down as they have physical connection.

 Those timers you mention is how RIP sanitize its routing table in order to avoid loop.

 RIP sends the complete routing table every 30 seconds as broadcast and  the more sanitized the routing table the less probable loops occur.

 There are many good material about this protocol out there. Let me know if they give better or different perception about the protocol. 

-If I helped you somehow, please, rate it as useful.-

[Joseph W. Doherty]

When a link goes physically down, the hosting device's hardware/OS is designed to notify routing protocols. This is to "speed up" link down detection. For example, hardware detection of link down might take 50 milliseconds or so, while detecting link lost via lost of neighbor adjacency might take a minute or more. Regardless of how link down is detected, the RIP notification and reconvergence process is begun.

Another difference with a link down, I recall RIP will send out a triggered updated. This to short cut waiting on the next normal route update to neighbors.