3Com CoreBuilder 7000, Operation Manual, Page: 61 / 264

E-IISP Hierarchical Routing Algorithm 61
The user part corresponding to the user part of the ATM destination
address of the receiving ATM user.
2 The CoreBuilder 7000 family ATM switch compares the ATM destination
address with the full ATM address of each of its UNI neighbors. If a match
is found, the SETUP message is sent on to the appropriate UNI port,
finishing the process successfully with a route established.
3
If no match is found among its UNI neighbors, the CoreBuilder 7000
family ATM 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 7000
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 will use 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* will route to B* because its network
prefix is the shortest 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 will use
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