3Com CoreBuilder 9000, User Manual

Page: 344 / 468

344 C
HAPTER
12: E-IISP P
ROTOCOL
4 After an NNI neighbor is identified according to the procedure in step 3,
an internal link is established to the output port to which that neighbor is
attached, and the VPI/VCI identifier and the neighbor’s address are
recorded. This completes the virtual channel up to the output port.
5 To continue the virtual channel, a SETUP message is forwarded to the NNI
neighbor to establish an external link (connection) to it.
Hop Limit —
Avoiding Loops
One step of forwarding the SETUP message between NNI neighbors is
called a
hop . If the number of hops exceeds a preset limit, a looping
condition is declared and a RELEASE message is returned to the sender,
terminating the process unsuccessfully with no route established.
Selecting the Hop Limit
When a network is designed using the hierarchical method of E-IISP, the
hop limit is normally set to 7. For a very large network or for a network
that is not hierarchic, choose the hop limit to be the longest possible path
between two units in the network without repeating a section. To set the
hop limit, see “Update Maximum NNI Hops” on page 182.
For information about how to display internal links and examples, see
“Display Call Routed to ATM Address” on page 157.
Crankback The crankback mechanism in the CoreBuilder 9000 switch automatically
re-routes a call if a virtual channel cannot be established on the path fixed
by the routing algorithm. This can happen for a number of reasons:
No route to destination
No VCC available — Number of VCCs exceed a limit
No rate available — No available bandwidth
Resources unavailable — Node internal resource problem
The crankback mechanism eliminates the need for the original
transmitting station to release and reestablish the call.
The crankback mechanism works as follows: If a virtual channel fails to be
established along one of the links, a RELEASE message is sent back to the
switch that sent the SETUP message.