3Com CoreBuilder 9000, User Manual

Page: 370 / 468

370 C
HAPTER
13: E
STABLISHING
ATM C
HANNELS
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, an 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 on how to display internal links and examples, see
“Display Call Routed to ATM Address” on page 157.
Crankback In case a virtual channel fails to be established along one path, a RELEASE
message is sent back to the node that sent the SETUP message.
The RELEASE message can have been sent due to any of the following
causes:
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
Automatic Retry
After receiving the RELEASE message, the source CoreBuilder 9000 again
attempts to establish a virtual channel but this time via an alternate path.
The alternate path is determined automatically by the CoreBuilder 9000
by means of the routing table that was established during the first
connection attempt. The routing table holds all the connection
alternatives. The CoreBuilder 9000 remembers which connection was
used in the failed attempt and chooses a different connection the second
time, resulting in a new and different path to the destination.
The availability of alternate paths for connections depends on the
addressing scheme that was used in setting up the network. Alternate
connections need to have the same match type. (See “Basic ILMI
Functions” on page 324.)
For more information and examples of the crankback mechanism, see
publications of the 3Com Technical Education Services.