Catalyst switches also have a mechanism to monitor the use of TrCRFs or ring numbers within a domain of switches. The Duplicate Ring Protocol (DRiP) collects and maintains the status of TrCRFs that are interconnected by TrBRFs. This information is used for the following purposes:
• Preventing duplicate ring numbers from being assigned to TrCRFs.
• Filtering All-Routes Explorer (ARE) frames from reentering TrCRFs that they have already visited.
• Operating the backup TrCRF function when an ISL trunk link goes down.
Every switch participating in Token Ring switching sends a DRiP advertisement out all ISL trunk ports every 30 seconds. Advertisements are sent to multicast address 01:00:0C:CC:CC:CC and are sent only on the default VLAN 1. When a switch receives the multicast advertisements, the switch does not forward the advertisements on to other switches over ISL links unless the advertisements contain new information. As well, a switch compares advertisements to the information in its own configuration. If it detects that a TrCRF has already been configured elsewhere, the local TrCRF configuration will be denied.
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