Unicast IP Routing
Unicast IP Routing |
|
RIP version 1 (v1) is defined in RFC 1058 and is widely deployed in small to medium-sized intranets.
RIP messages are encapsulated in a User Datagram Protocol (UDP) datagram sent from the router interface IP address and UDP port 520 to the subnet broadcast IP address. The RIP v1 message consists of a 4-byte RIP header and up to 25 RIP routes. The maximum size of the RIP message is 504 bytes. With the 8-byte UDP header, the maximum size of the RIP message is a 512-byte IP payload. Figure 3.8 illustrates the RIP v1 message format.
Figure 3.8 RIP Version 1 Message Format
Command A 1-byte field containing either 0x01 or 0x02. 0x01 indicates a RIP request for all (a General RIP Request) or part of the routing tables of neighboring routers. 0x02 indicates a RIP response consisting of all or part of a neighboring router's routing table. A RIP response can be sent in response to a RIP request or as the periodic or triggered update message.
Version A 1-byte field set to the value of 0x01 for RIP v1.
Family Identifier A 2-byte field identifying the protocol family. This is set to the value of 0x00-02 to indicate the IP protocol family.
IP Address A 4-byte field set to the IP network ID which can be a class-based network ID, a subnetted network ID (advertised only within the subnetted network), an IP address (for a host route), or 0.0.0.0 (for the default route). For a General RIP Request, the IP Address is set to 0.0.0.0.
Metric A 4-byte field for the number of hops to the IP network that must be a value from 1 to 16. The metric is set to 16 in a General RIP Request or to indicate that the network is unreachable in a RIP response (announcement).
RIP v1 was designed in 1988 to suit the dynamic routing needs of LAN technology–based IP internetworks. Shared access LAN technologies like Ethernet and Token Ring support Media Access Control (MAC)–level broadcasting where a single packet can be received and processed by multiple network nodes. However, in modern internetworks, the use of MAC-level broadcasts is undesirable because all nodes must process all broadcasts. RIP v1 was also designed in a time when the Internet was still using network IDs based on the Internet address classes. Today, however, the use of Classless Inter-Domain Routing (CIDR) and variable length subnetting is almost required to conserve IP addresses.
All RIP v1 route announcements are addressed to the IP subnet (all host bits are set to 1) and MAC-level broadcast. Non-RIP hosts also receive RIP announcements. For large or very large RIP internetworks, the amount of broadcast traffic on each subnet can become significant.
While producing additional broadcast traffic, the broadcast nature of RIP v1 also permits the use of Silent RIP. A Silent RIP computer processes RIP announcements but does not announce its own routes. Silent RIP could be enabled on non-router hosts to produce a routing table with as much detail as the RIP routers. With more detailed routes in the routing table, a Silent RIP host can make better routing decisions.
RIP v1 was designed for class-based IP internetworks where the network ID can be determined from the values of the first 3 bits of the IP address in the RIP route. Because the subnet mask is not included with the route, the RIP router must determine the network ID based on a limited set of information. For each route in a RIP v1 message, the RIP v1 router performs the following process:
As a result of the assumptions listed previously, supernetted routes might be interpreted as a single network ID rather than the range of network IDs that they are designed to represent and subnet routes advertised outside of the network ID being subnetted might be interpreted as host routes.
As a mechanism for supporting subnetted environments, RIP v1 routers do not advertise the subnets of a subnetted class-based network ID outside the subnetted region of the IP internetwork. However, because only the class-based network ID is being advertised outside the subnetted environment, subnets of a network ID in a RIP v1 environment must be contiguous. If subnets of an IP network ID are noncontiguous, known as disjointed subnets, the class-based network ID is announced by separate RIP v1 routers in different parts of the internetwork. As a result, IP traffic can be forwarded to the wrong network.
RIP v1 does not provide any protection from a rogue RIP router starting up on a network and announcing false or inaccurate routes. RIP v1 announcements are processed regardless of their source. A malicious user could use this lack of protection to overwhelm RIP routers with hundreds or thousands of false or inaccurate routes.
