Normally, Cisco switches intercept and process a number of layer two protocols, including CDP, STP, VTP, and others. This can present problems when attempting to make a switch appear completely transparent, as is a common goal of service providers. Full transparency can be achieved with layer 2 protocol tunneling (not to be confused with L2TP).
Switches can be configured to forward CDP, STP, and VTP frames instead of intercepting them. The command to enable basic L2 tunneling is l2protocol-tunnel at interface configuration. By default, this one command disables CDP on the interface and enables tunneling for CDP, STP, and VTP, as seen in the running configuration:
interface GigabitEthernet0/1
l2protocol-tunnel cdp
l2protocol-tunnel stp
l2protocol-tunnel vtp
no cdp enable
show l2protocol-tunnel
interface GigabitEthernet0/1
l2protocol-tunnel cdp
l2protocol-tunnel stp
l2protocol-tunnel vtp
l2protocol-tunnel point-to-point pagp
l2protocol-tunnel point-to-point lacp
l2protocol-tunnel point-to-point udld
no cdp enable
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