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Friday, May 28, 2010

spanning tree Trunk bpdus sent down access port caused blocking on other switch

We have 3 switches, A,B,C both B and C connect to A on a single connection, the connection from B to A is on a single vlan and just a normal access port, the connection from C to A was a trunk port with many vlans. For some reason when we added switch C, the access port on switch A went into blocking mode, the following errors were displayed

8w0d: %SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk GigabitEthernet0/23 VLAN196.

8w0d: %SPANTREE-7-BLOCK_PORT_TYPE: Blocking GigabitEthernet0/23 on VLAN0196. Inconsistent port type.

Is it right that the bpdu's from switch C's trunk will go out of the access port on the other switch and block the link?

you need to use
switchport mode access on both switches with switchport nonegotiate or you need to move to trunk configuration on both ends. using mode trunk on both sides. That is even if downstream switch is configured with switchport mode access it still talks DTP (switchport nonegotiate) if upstream switch is left at dynamic desirable it may turn on trunking, as a result of this it will start to send proprietary BPDUs that contain the vlan-id of the instance.

The other side doing a consistency check disables the port on receiving these unexpected BPDUs
8w0d: %SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk GigabitEthernet0/23 VLAN196.

Also do not disable STP on link but the link 802.1Q trunk on both ends.

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