Hop count and route updates
A RIP router learns destinations from its neighbors and normally adds one hop to each received metric before comparing paths. It prefers the smallest metric. Metrics through 15 describe reachable routes, and 16 means unreachable. A path through fewer routers wins even if it uses slower links.
RIP sends its routing information to neighbors about every 30 seconds and can also send a triggered update when a metric changes. Split horizon limits which learned routes are advertised back toward their source. With poisoned reverse, the return advertisement carries metric 16.
RIPv1 broadcast and RIPv2 multicast
RIPv1 broadcasts updates and omits the subnet mask from each entry. The receiver infers the mask from the address and its interface context, which limits how a subnetted network can be advertised. RIPv2 carries an explicit mask, so it can describe prefixes of different lengths.
RIPv2 normally sends unsolicited updates to 224.0.0.9 and can include authentication, a next hop and a route tag. Both versions use UDP port 520. Configuring both sides for the same version avoids losing mask information at a version boundary.
Try it in a lab
- Connect three IPv4 routers in a chain with RIPv2 enabled on the transit links. Advertise a loopback with a /27 prefix at the far end.
- On the first router, confirm the learned route retains /27. Capture an update on a transit link and identify the destination 224.0.0.9, UDP port 520, route mask and metric.
- Withdraw the far loopback and observe the triggered update or subsequent route expiration. Check that the first router eventually removes the learned route.