Review/Perspective: Layer 2 protocols

2.1.b Implement and troubleshoot layer 2 protocols

  • 2.1.b [i] CDP, LLDP
  • 2.1.b [ii] UDLD



Cisco Discovery Protocol is a proprietary protocol that allows two Cisco devices to communicate device specific information to each other on the attached port.  This is useful for navigating Cisco networks and seeing what devices are connected to each port in addition to other information.

Link Layer Discovery Protocol is the standard version of CDP and can be used in multi-vendor network environments.




Unidirectional Link Detection is a layer 2 messaging protocol that serves as an echo mechanism to detect failure of transmit or receive between a pair of devices.  There are two modes for failure normal and aggressive, if a failure is detected and normal is configured no action is taken.  If aggressive mode is configured, the device will try to connect to the UDLD neighbor 8 times and if unsuccessful will put the port into an err-disable state.


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