Читать книгу CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies - Ben Piper, David Higby Clinton - Страница 78

When a non-edge port on a bridge transitions to the forwarding state—something that can happen if a new switch is added, for example—it notifies other bridges in the topology in order to trigger a reconvergence. The bridge flushes any MAC addresses associated with the port and begins sending BPDUs with the topology change (TC) bit set. It also begins a timer called the TC While timer that's twice the value of its Hello time (by default, the Hello time is 2 seconds, so the TC While timer would be 4 seconds). It continues sending BPDUs with the TC bit set until the TC While timer expires.

When another bridge receives a BPDU with the TC bit set, it clears its MAC address table for all ports except the port on which it received the BPDU. It begins its own TC While timer and sends BPDUs with the TC bit set out of all of its designated ports and its root port. Eventually, all bridges become aware of the topology change.