3Com CoreBuilder 9000, User Manual

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342 C
HAPTER
12: E-IISP P
ROTOCOL
E-IISP Hierarchic
Routing Algorithm
The CoreBuilder 9000 ATM Enterprise Switch E-IISP protocol features a
proprietary hierarchic routing algorithm by which calls are routed
efficiently through a network with a hierarchic topology. The routing
algorithm determines the optimum output port of the switch on which to
transmit the call onwards towards its destination.
The efficiency of the algorithm is based on the hierarchic ATM addressing
system and on the fact that each switch knows the network prefix of the
ATM address of each of its NNI neighbors, as well as the full ATM address
of each UNI neighbor. In principle, the algorithm is always successful; a
route is always found to the destination. This is because, in a hierarchic
network topology, every node can be reached by every other node. For
information about hieracrchic ATM addressing, see “Hierarchic ATM
Addressing” on page 336.
The following description of the routing method assumes that an external
link of a virtual channel has already been established to an input port of
the switch. In addition, the call’s destination address, which was
embedded in the signaling SETUP message, is available to the switch. The
routing method now seeks to establish an internal link from the input
port to an optimum output port.
The routing method consists of the following steps:
1 The CoreBuilder 9000 ATM Enterprise Switch receives the ATM address
of the destination ATM user, the
ATM destination address, from the
signaling SETUP message that establishes the external link to the input
port. The ATM destination address is comprised of two parts:
The network prefix part that corresponds to the network prefix of the
CoreBuilder 9000 ATM Enterprise Switch that is connected to the
receiving ATM user
The user part that corresponds to the user part of the ATM destination
address of the receiving ATM user.
2 The CoreBuilder 9000 ATM Enterprise 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.