Communications Server for z/OS V1R7 TCP/IP Implementation, Volume 1: Base Functions, Connectivity, and Routing

Chapter 5. Routing 149

5.1.3 Routing Information Protocol (RIP)

This section provides an overview of the RIP protocol. RIP is designed to manage relatively

small networks.

RIP uses a hop count (distance vector) to determine the best possible route to a network or

host. The hop count is also known as the routing metric, or the cost of the route. A router is

defined as being zero hops away from its directly connected networks, one hop away from

networks that can be reached through one gateway, and so on. The fewer hops, the better.

The route that has the fewest hops will be the preferred path to a destination. A hop count of

16 means infinity, or that the destination cannot be reached. Thus, very large networks with

more than 15 hops between potential partners cannot make use of RIP. The information is

kept in a distance vector table, which is periodically advertised to each neighboring router.

The router also receives updates from neighboring gateways and uses these to update its

routing tables. If an update is not received for three minutes, a gateway is assumed to be

down, and all routes through that gateway are set to a metric of 16 (infinity).

Basic distance vector algorithm

The following procedure is carried out by every entity that participates in the RIP routing

protocol. This must include all of the gateways in the system. Hosts that are not gateways

may participate as well.

򐂰 Keep a table with an entry for every possible destination in the system. The entry contains

the distance D to the destination, and the first gateway G on the route to the network.

򐂰 Periodically, send a routing update to every neighbor. The update is a set of messages

that contains all the information from the routing table. It contains an entry for each

destination, with the distance shown to that destination.

򐂰 When a routing update arrives from the neighbor G', add the metric associated with the

network that is shared with G'. Call the resulting distance D'. Compare the resulting

distance with the current routing table entries. If the new distance D' for N is smaller than

the existing value D, adopt the new route. That is, change the table entry for N to have

metric D' and gateway G'. If G' is the gateway from which the existing route came, G' = G,

then use the new metric, even if it is larger than the old one.

RIP version 1

RIP is a protocol that manages IP routing table entries dynamically. The gateways using RIP

exchange their routing information in order to allow the neighbors to learn of topology

changes. The RIP server updates the local routing tables dynamically, resulting in current and

accurate routing tables. The protocol is based on the exchange of protocol data units (PDUs)

between RIP servers (OMPROUTE). There are various types of PDUs, but the two most

important PDUs are:

REQUEST PDU Sent from an OMPROUTE server as a request to other OMPROUTE

servers to transmit their routing tables immediately.

RESPONSE PDU Sent from an OMPROUTE server to other OMPROUTE servers either

as a response to a REQUEST PDU or as a result of expiration of the

broadcast timer (every 30 seconds).

RIP V1 limitations

Since RIP is designed for a specific network environment, it has some limitations. These

should be considered before implementing RIP in your network.

򐂰 RIP V1 declares a route invalid if it passes through 16 or more gateways. Therefore, RIP

V1 places a limitation of 15 hops on the size of a large network.