Troubleshooting Complex Layer 2 Issues

CDP messages are generated every 60 seconds as Layer 2 multicast messages on each of a device’s active interfaces. CDP is a Data Link Layer utility found in IOS that resides at Layer 2 of the OSI model; as such, CDP is not routable and can only operate over directly connected interfaces. no cdp run command globally disables CDP while the no cdp enable command disables CDP on an interface. Disabling CDP globally and enabling it on individual interfaces is not possible.

show cdp entry Router1

To support non-Cisco devices and to allow for interoperability between other devices, IOS also supports the IEEE 802.1AB Link Layer Discovery Protocol (LLDP) which is similar to CDP. This protocol runs over the Data Link Layer, which allows two systems running different network layer protocols to learn about each other.LLDP supported devices can use TLVs to receive and send information to their neighbors.

Troubleshooting Using Basic Interface Statistics 

Duplex mismatch might be caused by hard-coding one side of the link to full duplex but leaving the other side to autonegotiate duplex. A 10/100 interface will default to half duplex if the other side is 10/100 and does not negotiate. Watch for the following types of errors:
   -Runts: Runts are frames smaller than 64 bytes.
   -CRC errors: The frame’s cyclic redundancy checksum value does not match the one calculated by the switch or router.
   -Frames: Frame errors have a CRC error and contain a noninteger number of octets.
   -Alignment: Alignment errors have a CRC error and an odd number of octets.
   -Collisions: Look for collisions on a full-duplex interface (meaning that the interface operated in half-duplex mode at some point in the past), or excessive collisions on a half-duplex interface.
   -Late collisions on a half-duplex interface: A late collision occurs after the first 64 bytes of a frame.

Troubleshooting Spanning Tree Protocol  

When BPDU Guard is enabled, a port is error-disabled if it receives a BPDU. You can check this with the  show interfaces status err-disabled command. Switching loops can result if the  spanning-tree portfast trunk  command is enabled on a trunk port toward another switch, or an interface has a duplex mismatch. A port protected by Root Guard is put in a root-inconsistent state if it tries to become a Root Port: a Root and Alternate Port with Loop Guard configured is put in a loop-inconsistent state if it stops receiving BPDUs. You can check this with the  show spanning-tree inconsistent ports command. 

Troubleshooting VTP 

To prevent your VLAN database from being altered when adding a VTPv1 or VTPv2 switch to the VTP domain, follow these steps:   

   Step 1. Change VTP mode to Transparent, which will reset the configuration revision number to 0.  

   Step 2. Configure the remaining appropriate VTP parameters.   

   Step 3. Configure trunking.   

   Step 4. Connect the switch to the network.    

Some useful commands

show interfaces status err-disabled   

traceroute mac source-mac destination-mac 

show spanning-tree detail