Why RIP uses hop count and not delay metric?

Question

Distance vector routing algorithm RIP uses hop count as a routing metric. Why it is not using delay as a metric ?

score 3 · Accepted Answer · edited Oct 07 '21 at 06:47

Because it was not designed for it.

This protocol uses fixed "metrics" to compare alternative routes. It is not appropriate for situations where routes need to be chosen based on real-time parameters such a measured delay, reliability, or load. The obvious extensions to allow metrics of this type are likely to introduce instabilities of a sort that the protocol is not designed to handle.

score 0 · Answer 2 · answered Dec 20 '14 at 23:32

Most routing protocols use statically assigned metrics — RIP, OSPF, EIGRP, BGP. In RIP, the default metric is 1, which is called the "hop count" metric. While it is possible to assign larger metrics to links in RIP, this is not recommended, since it reduces the maximum diameter of the network (RIP metrics are limited to 15).

Using dynamically computed metrics in your routing protocol is a difficult problem. Done naively, it will create a negative feedback loop, which will cause persistent routing oscillations.

We have done some experiments with dynamically computed delay-based metrics. In short, it can be made to work, but it's tricky.

Why RIP uses hop count and not delay metric?

2 Answers2