Cisco 6500 Series Software Configuration Manual page 133

Chapter 9 Configuring VLANs
Understanding How VLANs Work
Figure 9-2 Undistributed TrCRFs
Switch A Switch B
ISL
TrBRF 3
TrCRF TrCRF
TrCRF
350 200
400
By default, Token Ring ports are associated with the default TrCRF (VLAN 1003, trcrf-default), which
has the default TrBRF (VLAN 1005, trbrf-default) as its parent. In this configuration, a distributed
TrCRF is possible (see Figure 9-3), and traffic is passed between the default TrCRFs located on separate
network devices if the network devices are connected through an ISL trunk.
Figure 9-3 Distributed TrCRF
Switch A Switch B
ISL
TrBRF 2
TrCRF
TrCRF
TrCRF
300 300
300
Within a TrCRF, source-route switching forwards frames based on either MAC addresses or route
descriptors. The entire VLAN can operate as a single ring, with frames switched between ports within a
single TrCRF.
You can specify the maximum hop count for All-Routes and Spanning Tree Explorer frames for each
TrCRF. When you specify the maximum hop count, you limit the maximum number of hops an explorer
is allowed to traverse. If a port determines that the explorer frame it is receiving has traversed more than
the number of hops specified, it does not forward the frame. The TrCRF determines the number of hops
an explorer has traversed by the number of bridge hops in the route information field.
If the ISL connection between network devices fails, you can use a backup TrCRF to configure an
alternate route for traffic between undistributed TrCRFs. Only one backup TrCRF for a TrBRF is
allowed, and only one port per network device can belong to a backup TrCRF.
If the ISL connection between the network devices fails, the port in the backup TrCRF on each affected
network device automatically becomes active, rerouting traffic between the undistributed TrCRFs
through the backup TrCRF. When the ISL connection is reestablished, all but one port in the backup
TrCRF is disabled. Figure 9-4 illustrates the backup TrCRF.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
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