Tag: Layer 2

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.



Review/Perspective: Switch Administration

2.1.a Implement and troubleshoot switch administration

  • 2.1.a [i] Managing MAC address table
  • 2.1.a [ii] errdisable recovery
  • 2.1.a [iii] L2 MTU


Managing MAC address table

The MAC Address table found in layer 2 IOS contains the MAC address of every known device on a network, and the port their traffic is coming in on.  As a frame enters the switch, it examines the ethernet frame and records the MAC address and the port it comes in on and maintains a database of MACs:ports so it knows where to forward frames as needed.  Otherwise if a destination MAC is missing from the MAC table, switches will flood that frame out all ports in a last ditch effort to find the host the frame is destined for.

Commands to view the MAC address table can be found in the BeThePackets Wiki at:



errdisable recovery

Should a port kick into errdisable, you can recover it typically by bouncing the port using the shut/no shut commands.




MTU is the Maximum Transmission Unit which defines the largest size of frames an interface can transmit without the need to fragment.  there are three types of MTU that are recognized when configuring a switch:

  • 10/100Mbps switch interfaces
  • 1000Mbps switch interfaces
  • Routed and SVI interfaces


MTU mismatches can occur if the interface MTU configuration is lower than the originally sized ethernet frame.  You can adjust this by configuring the offending interface with the correct size MTU.