Symantec™ Security Gateways Reference Guide - Sawmill

36 Security gateway fundamentalsRoutesRouting Information Protocol Version 2 (RIP-2)As defined in RFC 2453, RIP-2 is a UDP-based protocol based on the Bellman-Ford (distance vector)algorithm and is an enhancement to the RIP protocol discussed in RFC 1058. The term distance vectormeans that messages sent by RIP-2 contain a vector of distances (hop counts). The cost assigned to a routebetween two networks is calculated by counting the number of hops between the two networks. If there aremultiple routes to the same destination, RIP-2 chooses the route with the smallest hop count, and ignoresthe other paths.Figure 3-3RIP-2 packetCommandVersionRouting domainAddress family identifierRoute tag32-bit IP address32-bit subnet mask32-bit next-hop IP addressMetric (1-16)As shown in Figure 3-3, the structure of a RIP-2 packet consists of a command (1 byte), version (1 byte),routing domain (2 bytes), and one or more (up to 25) 20-byte groupings that include an address familyidentifier, route tag, 32-bit IP address, 32-bit subnet mask, 32-bit next hop IP address, and a metric. Table3-2 describes the information found in each of the fields in a RIP-2 packet.Table 3-2FieldCommandExplanation of the fields in a RIP-2 packetDescriptionTypically set to one (1) or two (2). A value of 1 indicates that this packet is a request for theresponding system to send all or part of its routing table. A value of 2 means that this packetis a response to a RIP-2 request and contains all or part of the sender’s routing table.Version Defines the version of the RIP protocol being used. For RIP-2 packets, this field is set to 2.Routing domainAddress family identifierRoute tag32-bit IP address32-bit subnet mask32-bit next hop addressMetricAn identifier of the routing daemon to which this packet belongs. RIP-2 supports runningmultiple instances of the routing daemon, with each instance assigned to its own domain.The address family identifier value indicates the type of address in the grouping. Forexample, an IP address equates to a field value of two (2).This system’s Autonomous System (AS) number. This is usually only required when thisrouter is a border router using an exterior gateway protocol, such as EGP or BGP, forcommunication between two ASes.The IP address of the host or network.The mask of the 32-bit IP address. This field was added to RIP-2 to overcome one of thelimitations of original RIP packets.The immediate next hop IP address to which to route packets. Generally used when not allrouters in a system use RIP-2.A number between 1 and 15 that denotes the cost to get to the declared IP address. If thisfield is set to 16, it means the destination is unreachable.