Enabling Fast Convergence

In addition to sending periodic general queries, an active device sends general queries when it detects a new port. However, because the device does not recognize the other device's port up event, multicast traffic might still require up to the query-interval time to resume after a topology change. Fast convergence allows the device to listen to topology change events in Layer 2 protocols such as spanning tree, and then send general queries to shorten the convergence time.

If the Layer 2 protocol cannot detect a topology change, fast convergence may not work in some cases. For example, if the direct connection between two devices switches from one interface to another, the rapid spanning tree protocol (802.1w) considers this optimization, rather than a topology change. In this example, other devices will not receive topology change notifications, and will be unable to send queries to speed up the convergence. Fast convergence works well with the regular spanning tree protocol in this case.

The following configuration example enables fast-convergency for VLAN 70.

device# configure terminal
device(config)# vlan 70
device(config-vlan-70)# multicast fast-convergence