RIP v1

Background

The original specification of RIP, defined in RFC 1058, was published in 1988.

When starting up, and every 30 seconds thereafter, a router with RIPv1 implementation broadcasts to 255.255.255.255 a request message through every RIPv1 enabled interface. Neighbouring routers receiving the request message respond with a RIPv1 segment, containing their routing table. The requesting router updates its own routing table, with the reachable IP network address, hop count and next hop, that is the router interface IP address from which the RIPv1 response was sent. As the requesting router receives updates from different neighbouring routers it will only update the reachable networks in its routing table, if it receives information about a reachable network it has not yet in its routing table or information that a network it has in its routing table is reachable with a lower hop count. Therefore, a RIPv1 router will in most cases only have one entry for a reachable network, the one with the lowest hop count. If a router receives information from two different neighbouring router that the same network is reachable with the same hop count but via two different routes, the network will be entered into the routing table two times with different next hop routers. The RIPv1 enabled router will then perform what is known as equal-cost load balancing for IP packets.

RIPv1 enabled routers not only request the routing tables of other routers every 30 seconds, they also listen to incoming requests from neighbouring routers and send their own routing table in turn. RIPv1 routing tables are therefore updated every 25 to 35 seconds. The RIPv1 protocol adds a small random time variable to the update time, to avoid routing tables synchronizing across a LAN. It was thought, as a result of random initialization, the routing updates would spread out in time, but this was not true in practice.

Network Mask

RIP 1 derives the network mask of received networks and hosts from the network mask of the interface from which the packet was received.

If a received network or host is on the same natural network as the interface over which it was received, and that network is subnetted (the specified mask is more specific than the natural network mask), then the subnet mask is applied to the destination.

If bits outside the mask are set, it is assumed to be a host. Otherwise, it is assumed to be a subnet.

Auto Summarization

The Check Point implementation of RIP 1 supports auto summarization.

This allows the router to aggregate and redistribute nonclassful routes in RIP 1.