After the connection between two switches could not be established I went to see the log of the uplink switch and the first thing I have noticed was this:

%LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
%SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk FastEthernet0/1 VLAN300.
%SPANTREE-7-BLOCK_PORT_TYPE: Blocking FastEthernet0/1 on VLAN0300. Inconsistent port type.

The two switches were connected via access ports and this message was on my side, since the other side was the service provider and I was not given the technical details of “the other side”.


In short, interface was receiving BPDU’s which weren’t supposed to be there. This was the interface configuration:

interface FastEthernet0/1
description UPLINK
switchport access vlan 300
switchport mode access
spanning-tree portfast

Portfast port is designed to be connected to a device where BPDU’s are not expected. When BPDU guard is enabled (globally or per interface), the port will shutdown and enter a errdisable state. BPDU guard global enable is a great solution to protect portfast ports on a access switch where you don’t expect a switch to be plugged in.

In short, interface was receiving BPDU’s which weren’t supposed to be there!

The solution is the “spanning-tree bpdufilter enable” command which disables spanning tree on a port and that is made by restricting (filtering) sending and receiving BPDU’s. When enabled on a global level, BPDU filter will apply to all portfast ports.

Here is the configuration:

switch1(config)#spanning-tree portfast bpdufilter default

switch1(config)#int fa0/1
switch1(config-if)#spanning-tree bpdufilter enable


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  1. Hey Guys, I appreciate your post so much, after a long while troubleshooting my Catalyst 2950 series, I was able to get things working from your post here. Nice one

    1. Hey Guys, It is working now after running this command.Appreciated your post.Thanks alot

  2. Our connection runned over month without any issues, stopped working yesterday. My log showed that errormessage, i called the ISP if they changed things. Answer.. NO we did nothing.
    Anyway, that command saved my day. Thanks for sharing.

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