3Com CoreBuilder 9000 User Manual page 343

3 If no match is found among its UNI neighbors, the CoreBuilder 9000 ATM
Enterprise Switch compares the network prefix of the ATM destination
address with the network prefix of the ATM address of each of its NNI
neighbors. An Exact Match, Longest Best Match, Longest Partial Match,
and Default Port are the routing decisions that the CoreBuilder 9000
calculates when planning a route. The list is shown in the order that the
algorithm is run.


An exact match means that the neighbor's network is an exact match
of the destination address. For example, A* has neighbors B*, B1, and
B2. The destination address is B2. The neighbor B2 is an exact match.
If no full match is found, the longest best match is sought.


A longest best match means that the neighbor's network prefix fits
entirely into the destination address. For example, A* has neighbors
B*, B1, and B2. The destination address is B11*. According to the
definition, both B* and B1 are best matches because they fit entirely
into the destination address. However, B1* is a longer best match
than B*.


A longest partial match means that the neighbor's network prefix
matches at least the first nibble of the destination address. If there is
no best match, but multiple partial matches, E-IISP uses the longest
partial match that has the shortest network prefix. For example, A*
has neighbors B*, B1, and B2.1. The destination address is C3.1*. A*
notices that there are no best matches. Now it looks to see if any of
the neighbor's addresses match the first nibble of the destination
address. B*, B1, and B21 all start with B and the destination address
starts with B, so these are all partial matches. Now it looks at the
second nibble and notices that there are no matches. B*, B1, and B2.1
are all 1 nibble matches. A* routes to B* because its network prefix is
the shorter of all the partial matches found. If neither a best match or
partial match is found, the default port is used.


A default port is used when there are no possible matches. It uses the
neighbor with the shortest network prefix as long as it is not longer
than the network prefix of the switch making the decision. For
example, A* has neighbors B*, B1, and B2.1. The destination address
is C.2.1. A* looks at its neighbors and notices that none of them
begin with the C* prefix. The only option is to use the default port.
Since B* has the shortest network prefix that is not longer than A*, it
is used as the default port. If no default port is found, a RELEASE
message is sent back to the switch that sent the SETUP message.
E-IISP Hierarchic Routing Algorithm 343