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Cisco LAN Switching Fundamentals by Basir Sakandar - CCIE No. 6040, David Barnes - CCIE No. 6563

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UniDirectional Link Detection

Although the UniDirectional Link Detection (UDLD) protocol falls outside of STP, UDLD has numerous benefits that make it essential in a Layer 2 network. UDLD's function is to prevent a one-way communication between adjacent devices. When UDLD detects one-way conversation, it can do one of two things depending on whether UDLD is configured in Normal mode or Aggressive mode. In Normal mode, UDLD simply changes the UDLD-enabled port to undetermined state if it stops receiving UDLD messages from its directly connected neighbor. Aggressive mode was introduced in Catalyst OS 5.4(3); it will make eight attempts to re-establish the UDLD neighbor relation before error disabling the port. Aggressive mode is the preferred ...

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