Specifications

CHAPTER 3

Spanning Tree

Root Guard

Root Guard is meant to prevent the wrong switch from becoming the Spanning Tree root. It is enabled on ports other than

the root port and on switches other than the root. If a Root Guard port receives a BPDU that might cause it to become a

root port, the port is put into “root-inconsistent” state and does not pass traffic through it. If the port stops receiving these

BPDUs, it automatically reenables itself. To enable and verify Root Guard use the following commands:

(config-if)# spanning-tree guard root

# show spanning-tree inconsistentports

Unidirectional Link Detection

A switch notices when a physical connection is broken by the absence of Layer 1 electrical keepalives. (Ethernet calls

this a link beat.) However, sometimes a cable is intact enough to maintain keepalives but not to pass data in both direc-

tions. This is a Unidirectional Link. Operating at Layer 2, Unidirectional Link Detection (UDLD) detects a unidirectional

link by sending periodic hellos out to the interface. It also uses probes, which must be acknowledged by the device on the