Question

Identify and briefly explain any three key features, a distance vector protocol must incorporate in order to make it loop free? (100 words)

Answer 1

A brief explanation of three key features, a distance-vector protocol must incorporate in order to make it loop-free:

* In general, the distance vector protocol uses certain "algorithms" to make itself loop-free. Simply put, any and all packets "should not repeatedly be routed through same routers multiple times" ending up in an infinite loop or circle.

* At times, it requires distance vector protocol such as Enhanced Interior Gateway Routing Protocol (EIGRP) to suspend by forwarding to few or more destinations, at the same time, tables are in flux.

* A distance vector protocol can incorporate a "split horizon" feature making it loop-free. Split horizon stops a router from advertising of routing back onto the interface it was learned from. It is used with poison reverse technique.
A successor router knows the routing update for a route coming from its predecessor router so it would not advertise the route back to its previous router it gained information or route information from.

* Another feature is called "route poisoning", where any router connected directly to an immediate route, or subnet, detects the route is down or has failed, it advertises that route with an infinite metric, thus poisoning the route, and hence the name. Any router receiving the advertisement or the update would be aware of the route been failed and would not use it in the future.

* The feature called "hold-down timer" is used preventing any router from learning any new information about an unreachable route. As soon as any router is aware of the information about the route been down, it starts a hold-down timer on its end.
This is a hold time to refuse route updates for that route for a few minutes, say, 180 seconds, until the timer expires or after a route retraction, thus preventing forming loops.