Читать книгу CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies - Ben Piper, David Higby Clinton - Страница 67

Now we come to perhaps the easiest part of Spanning Tree: determining which remaining ports to block. Consider the connections between SW1 and SW4. Both are directly connected to the root, SW3, and have no need of a link to each other. Because SW1 has the higher bridge priority, it will block both its ports, like so:

SW1#show spanning-tree vlan 1 | i Gi2/0|Gi2/1 Gi2/0 Altn BLK 4 128.9 P2p Gi2/1 Altn BLK 4 128.10 P2p

Blocked ports don't forward traffic, but they still receive and process BPDUs. This ensures that every bridge can learn about changes to the Spanning Tree topology. SW4 has a lower bridge priority and has both its ports forwarding:

SW4#show spanning-tree vlan 1 | i Gi2/0|Gi2/1 Gi2/0 Desg FWD 4 128.9 P2p Gi2/1 Desg FWD 4 128.10 P2p

Any non-root ports that aren't blocking are called designated ports. The job of designated ports is to forward BPDUs from the root bridge so that Spanning Tree can reconverge in the event of a topology change.