Cisco 2950G 24 Software Configuration Guide - Page 428

Understanding UDLD Chapter 19 Configuring UDLD UDLD operates by using two mechanisms: • Neighbor database maintenance UDLD learns about other UDLD-capable neighbors by periodically sending a hello packet (also called an advertisement or probe) on every active interface to keep each device informed about its neighbors. When the switch receives a hello message, it caches the information until the age time (hold time or time-to-live) expires. If the switch receives a new hello message before an older cache entry ages, the switch replaces the older entry with the new one. Whenever an interface is disabled and UDLD is running, whenever UDLD is disabled on an interface, or whenever the switch is reset, UDLD clears all existing cache entries for the interfaces affected by the configuration change. UDLD sends at least one message to inform the neighbors to flush the part of their caches affected by the status change. The message is intended to keep the caches synchronized. • Event-driven detection and echoing UDLD relies on echoing as its detection mechanism. Whenever a UDLD device learns about a new neighbor or receives a resynchronization request from an out-of-sync neighbor, it restarts the detection window on its side of the connection and sends echo messages in reply. Because this behavior is the same on all UDLD neighbors, the sender of the echoes expects to receive an echo in reply. If the detection window ends and no valid reply message is received, the link is considered unidirectional, and the interface is shut down. Figure 19-1 shows an example of a unidirectional link condition. Figure 19-1 UDLD Detection of a Unidirectional Link Switch A TX RX Switch B successfully receives traffic from Switch A on this port. However, Switch A does not receive traffic from Switch B on the same port. UDLD detects the problem and disables the port. TX RX Switch B 43583 19-2 Catalyst 2950 Desktop Switch Software Configuration Guide 78-14982-01