39Routing Information Protocol (RIP)The Routing Information Protocol (RIP) tracks distances or hop counts to nearby routers whenestablishing network connections and is based on a distance-vector algorithm.RIP is based on a distance-vector algorithm; it tracks distances or hop counts to nearby routers whenestablishing network connections.RIP protocol standards are listed in the Standards Compliance chapter.Protocol OverviewRIP is the oldest interior gateway protocol.There are two versions of RIP: RIP version 1 (RIPv1) and RIP version 2 (RIPv2). These versions aredocumented in RFCs 1058 and 2453.RIPv1RIPv1 learns where nodes in a network are located by automatically constructing a routing data table.The routing table is established after RIP sends out one or more broadcast signals to all adjacent nodes ina network. Hop counts of these signals are tracked and entered into the routing table, which defineswhere nodes in the network are located.The information that is used to update the routing table is sent as either a request or response message.In RIPv1, automatic updates to the routing table are performed as either one-time requests or periodicresponses (every 30 seconds). RIP transports its responses or requests by means of user datagramprotocol (UDP) over port 520.RIP must receive regular routing updates to maintain a correct routing table. Response messagescontaining a router’s full routing table are transmitted every 30 seconds. If a router does not send anupdate within a certain amount of time, the hop count to that route is changed to unreachable (a routehop metric of 16 hops). Another timer sets the amount of time before the unreachable routes areremoved from the routing table.This first RIP version does not support variable length subnet mask (VLSM) or classless inter-domainrouting (CIDR) and is not widely used.RIPv2RIPv2 adds support for subnet fields in the RIP routing updates, thus qualifying it as a classless routingprotocol.The RIPv2 message format includes entries for route tags, subnet masks, and next hop addresses.Another enhancement included in RIPv2 is multicasting for route updates on IP multicast address224.0.0.9.742 Routing Information Protocol (RIP)