Alternate Routing And Crankback - 3Com CoreBuilder 7000 Operation Manual

Alternate Routing
and Crankback
A.1.1
A.1.2
A.2.2
A.1.3
A.3.1
A.3.2
Peer Group Leader
Source
Destination
Occasionally, after the decision has been made by the switch to open a
channel in a certain direction, the link may fail or the attributes may
change. When the switch detects this change, it does not close the
connection and send a disconnect message. Instead it hops backwards,
or cranksback. In crankback, hops are decreased. The signal will
crankback to a point where an alternate route exists and will open the
rest of the channel in that direction. Figure 32 shows the same signal
with a bad connection between two switches.
A.2.1
A.2.3
B.2.1
X
Figure 32 Signal Failure in a Hierarchical Network
The signal between nodes B.2.1 and B.2.2 has failed. B.2.1 will send a
PTSE message that will be flooded to all nodes in the network. At the
same time, it will crankback the signal to the fist node that has an
alternate route, so that an alternate route can be chosen. Figure 33
shows the crankback path.
B.1.3
B.1.2
B.1.1
B.1.4
B.2.2
B.2.3
Place Where Signal Failed
Chosen Path to Take
Physical Link
PNNI Signaling Protocol
C.1.1
C.1.2
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