The reason for P2P Dispute can be that remote end (MARUTA encryptor) sent RSTP BPDU with Designated bit set (flag field in BPDU in RSTP encode the role and state of the port that originates the BPDU) but your 6509 think that he is root and sent best BPDU for segment. To solve this 6509 puts port in dispute/blk state to avoid situation when loop can occur due unidirectional connection or thmth else.
So in your case smth wrong with devices interpreting each other BPDU, or link from 6509 to MARUTA encryptor is unidirectional.
The dispute mechanism was a very nice enhancement that was added into spanning-tree. I'll try to provide some information on it and then we'll move on to how we can find out what's wrong and fix it.
Every segment, as you are probably aware, should have only one designated port. This port (and bridge) is responsible for generating BPDUs for the segment. Now, RSTP onwards, the BPDU format changed a little bit. It now includes the port role as well among other things. The port role is simply the spanning-tree role of the port.
A spanning-tree dispute happens when a designated port receives a BPDU in which the port role is also designated. The bridge that receives and detects this moves it's port to a discarding state - note that the port role still remains designated. The BPDU being received should also have the learning bit set for the dispute to occur.
This entire mechanism is quite helpful in detecting unidirectional situations which can lead to loops.
Let's come back to your scenario now. Could you post the show spanning-tree vlan please? As you said, this port goes to a Maruta encryptor. I'm not familiar with this device but I am assuming that it does not participate in spanning-tree. Is that correct? Are there any other switches connected to this device?
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...