Trunk, STP and duplex


Obviously, I am missing something very basic.

I want to hook a new switch (3560) to an existing one (2950).

On the existing 2950, I define a port as trunk and I define an encapsulation mode. I define the corresponding port on 3560 with the same parameters.

I get all the VLan on the new switch and everything is fine. I am able to see the trunk port STP status on the 2950 as "forwarding".

Now the point I do not understand : whenever I put one of the 2 ports (on

2950 or 3560 end) in half duplex mode, the port on the 2950 will be force to "Blocking" state.

(And actually, I have troubles to get it back even forcing the ports in full duplex.)

Could explain me the root cause for it ? I feel that I am missing an important point.

Thanks in advance,

A puzzled newbie..

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Trunks must be full-duplex, and must be the same speed.

formatting link
In order to assign a port to a trunk, the port must comply with the following requirements:

=B7A layer 3 interface is not configured on the port.

=B7A VLAN is not configured on the port.

=B7The port is not assigned to a different trunk.

=B7An available MAC address exists which can be assigned to the port.

=B7Auto-negotiation mode is not configured on the port.

=B7The port is in full-duplex mode.

=B7All ports in a trunk must operate at the same rate.

=B7All ports in a trunk must have the same ingress filtering and tagged modes.

=B7All ports in a trunk must have the same back pressure and flow control modes.

=B7All ports in a trunk must have the same priority.

=B7All ports in a trunk must have the same transceiver type.

=B7All ports in a trunk must belong to the same module, that means they must be located on the same slot.

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Well that link was for link-aggregation, not vlan trunking, but I'm pretty sure the rules still apply. I can't decisively say that half- duplex wont work (as I have not tried it), but I'm fairly certain that full-duplex is a requirement for trunks. As soon as you move to half- duplex, the port is taken out of trunking. I'm not sure why it is shown as 'blocking', presuming you did a show spantree and that is what you are talking about. If the switches only have one interconnection, and this downstream switch does not have another link to elsewhere in the network, it should be forwarding on all ports in all VLANs. If it does have another link, then the blocking may make sense depending on root and spantree priorities.

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