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New Member

Blocking Port on the Cat 3548

Hi all,

i have the following setup.

Ethernet1 - (Router-Bridge) -TR1 ---Serial---TR2

now i change my setup to

Ethernet1 - (Router-Bridge) -TR1 ---Serial---TR2 - (Router-Bridge) - Ethernet2

-my problem is that the port of the link to the Ethernet2 goes to (STP) blocking.

this is the message on the Ethernet2 Switch


Received 802.1Q BPDU on non trunk FastEthernet0/1 on vlan 1"

I have configured DLSW peers from end to end.

I have run STP IEEE on all devices.

Know someone this problem too?




Re: Blocking Port on the Cat 3548

This is a well known problem. Somewhere in your network is misconfigured. A switch (say Switch A) is configured as trunk port; while the router (say Router B) which connects to Switch A is not. At the same time, Router B is a DLSw peer to the router which connects to Ethernet2 in your diagram. As a result, PVST+ BPDU is transported from Switch A to Ethernet2 Switch by DLSw. When Ethernet2 Switch receives PVST+ BPDUs, it thinks that it is a problem. It blocks the port. Please note that Router B and Switch A may not in the diagram you shown.

One of the easiest solutions is to configure dlsw icannotreach saps aa on the router connecting to Ethernet2 switch. However, DLSw will not transport SNAP frames anymore.

I have written a web page to resolve this problem. The page should be posted to CCO soon. The page contains detailed troubleshooting procedure. Please send me an e-mail to if you want me to send you the page. Of course, you are welcome to open a TAC case.

New Member

Re: Blocking Port on the Cat 3548

Hi, dixho,

Please send me your troubleshooting procedure to me as we met some similar problems and need sloved in urgent.

If you may send me asap,it will very appreciated.

my email addr:

Matthew Lee

New Member

Re: Blocking Port on the Cat 3548

Hi Rainer,

i haven`t the Solution for you, but i have the same Problem.

if you have a solution for this problem, could you tell me what

the solution is?

Thanks in advance


New Member

Re: Blocking Port on the Cat 3548

Hi Lorenz,

i use the workaround "icannnotreach saps aa" for the dlsw peer

between the ethernet networks and

it works!

Best regards



Re: Blocking Port on the Cat 3548

This is a well known issue. Please check the switch configuration and confirm that you do not misconfiguration. Please see the following URL for details: