The Longest Match Routing Rule
Routing to any destination is always done on a longest match basis: A router that has to decide between two different length prefixes of the same network will always follow the longer mask. Suppose, for example, that a router has the following two entries in its routing table:
198.32.1.0/24 via path 1
198.32.0.0/16 via path 2
When attempting to deliver traffic to host 192.32.1.1, the router tries to match the destination that has the longest prefix and will deliver the traffic via Path 1 in the example. Figure 3-12 illustrates the longest match routing rule. Domain C is receiving the two updates 198.32.1.0/24 and 198.32.0.0/16, and traffic toward 198.32.1.1 is following Path 1.
Figure 3-12. Following the Longest Match
If Path 1 were to become unavailable for some reason, traffic would utilize the next closest match in the routing table, which in this case is Path 2. In cases where Domain C is receiving identical routing updates with masks of equal length coming from Domain A and Domain B, Domain C would select one path or the other, or perhaps both, depending on the loadbalancing techniques offered by the specific routing implementation running in the domain. The longest match rule implies that a destination connected to multiple domains must always be explicitly announced—that is, announced in its most specific nonaggregate form—by these domains. In Figure 3-12, because Domain B does not explicitly advertise route 198.32.1.0/24, traffic from the customer toward the host must always prefer the path via the longest prefix match, through Domain A. Such a routing configuration might place an unacceptable burden on Domain A.
198.32.1.0/24 via path 1
198.32.0.0/16 via path 2
When attempting to deliver traffic to host 192.32.1.1, the router tries to match the destination that has the longest prefix and will deliver the traffic via Path 1 in the example. Figure 3-12 illustrates the longest match routing rule. Domain C is receiving the two updates 198.32.1.0/24 and 198.32.0.0/16, and traffic toward 198.32.1.1 is following Path 1.
Figure 3-12. Following the Longest Match
If Path 1 were to become unavailable for some reason, traffic would utilize the next closest match in the routing table, which in this case is Path 2. In cases where Domain C is receiving identical routing updates with masks of equal length coming from Domain A and Domain B, Domain C would select one path or the other, or perhaps both, depending on the loadbalancing techniques offered by the specific routing implementation running in the domain. The longest match rule implies that a destination connected to multiple domains must always be explicitly announced—that is, announced in its most specific nonaggregate form—by these domains. In Figure 3-12, because Domain B does not explicitly advertise route 198.32.1.0/24, traffic from the customer toward the host must always prefer the path via the longest prefix match, through Domain A. Such a routing configuration might place an unacceptable burden on Domain A.
