Troubleshooting the LAN Switching Data Plane

Confirm the Network Diagrams with CDP commands

show cdp neighbors
show cdp neighbors detail
show cdp entry name
show interfaces interface status

Isolate Interface Problems with Interface show commands

show interfaces status
show interfaces
show interfaces description
Show interface (interface port number fa0/1)
Things to look for
Interfaces in a down state that should be operational
Speed and Duplex issues
Runts, collisions, giants, frames in a show interface interface command

Isolate Filtering and Port Security Problems

Find all interfaces on which port security is enabled
Determine if a security violation has taken place

show port-security interface interface
show interfaces interface status
show port-security
show running-config

Violation States

Isolate VLAN and Trunking Problems

Check vlan and vlan trunks

  1. identify all access interfaces and their assigned vlans, reassign to the correct vlans if necessary
  2. Verify that all vlans exist on each switch, if not add them .
  3. Identify the operationally trunking interfaces on each switch and determine the vlans that can be forwarded over each trunk

show vlan brief
show vlan
show vlan id num
show interfaces interface switchport
show mac address-table dynamic

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