In routed environments, routers communicate with one another to track available routes. Routers can dynamically learn about available routes using the RIP. The switch software implements standard RIP to exchange IP route information with other routers.
RIP uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. Each router advertises routing information by sending a routing information update every 30 seconds (one interval). If RIP does not receive information about a network for 180 seconds, the metric associated with the network rises to infinity (U); that is, the metric resets to 16, which means the network becomes unreachable. If RIP does not receive information about a network for 120 seconds, it removes the network from the routing table.
RIP is a distance vector protocol. The vector is the network number and next hop, and the distance is the cost associated with the network number. RIP identifies network reachability based on cost, and cost is defined as hop count. One hop is the distance from one router to the next. This cost or hop count is the metric (see the following figure).
RIP version 1 (RIPv1) advertises default class addresses without subnet masking. RIP version 2 (RIPv2) advertises class addresses explicitly, based on the subnet mask.
The switch supports RIPv2, which supports variable length subnet masks (VLSM) and triggered router updates. RIPv2 sends mask information. If RIP does not receive information about a network for 90 seconds, the metric associated with the network rises to infinity (U); that is, the metric resets to 16, which means the network becomes unreachable. If RIP does not receive information about a network for 180 seconds (six update intervals), it removes the network from the routing table. You can change the default timers by configuring the RIP interface timeout timer and the holddown timer.
A directly connected network has a metric of zero. An unreachable network has a metric of 16. Therefore, the highest metric between two networks can be 15 hops or 15 routers.