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Compatibility issues with an STP that differs from the one on the edge switches in the service provider network

Core issue

Spanning Tree Protocol (STP) is a Layer 2 (L2) protocol that runs on bridges and switches. The specification for STP is IEEE 802.1D. The primary purpose of STP is to ensure loops are not created when redundant paths are present in the network. Loops are deadly to a network.

Refer to Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches for more information on STP and how to configure STP.


Usually the dot1q tunneling feature is incompatible with L2 protocols. Tunnel ports usually drop STPs and processed Cisco Discovery Protocols (CDPs). Whether STP is dropped or STP from the tunnel ports are seen, .1w STP falls back to .1d compatibility mode.

In order to resolve this issue, try these options:

  • Disable the STP on the edge dot1q tunnel switch.   
  • Configure L2 protocol tunneling. 

    Note: This protocol was developed to enhance dot1q tunneling performance and allow the establishment of a transparent connection between two ends of the tunnel.  

Refer to Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling for more information.

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Last update:
‎06-18-2009 04:01 PM
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