NOTE
RIPv1 can perform equal-cost load balancing.
The routing table in Figure 5.5 contains RIP-derived routes, which are recognized from the key to the left
of each entry. Of significance in these entries are the bracketed tuples; as discussed in Chapter 3, "Static
Routing," the first number is the administrative distance, and the second number is the metric. It is readily
seen that RIP has an administrative distance of 120, and as already stated, the metric for RIP is hop count.
Therefore, network 10.8.0.0 is 2 hops away, via either E0 or S1. If more than one route exists to the same
destination with equal hop counts, equal-cost load balancing will be performed. The routing table of
Figure 5.5 contains several multiple, equal-cost routes.
Figure 5.5. This routing table contains subnets of networks 10.0.0.0 and 172.25.0.0. All networks not
directly connected were derived by RIP.
When a packet enters a RIP-speaking router and a route table lookup is performed, the various choices in
the table are pruned until a single path remains. First, the network portion of the destination address is
read and the routing table is consulted for a match. It is this first step of reading the major class A, B, or C network number that defines a classful routing table lookup. If there is no match for the major network,
the packet is dropped and an ICMP Destination Unreachable message is sent to the packet's source. If
there is a match for the network portion, the subnets listed for that network are examined. If a match can
be found, the packet is routed. If a match cannot be made, the packet is dropped and a Destination
Unreachable message is sent.
NOTE
Definition of a classful route lookup