Re: 802.1q trunking between C2950 and 3Com Superstack III 4400
I'm not a 3Com expert, but I'd recommend:
1) configure VLAN1 on 3Com trunk port as untagged.
If you leave native VLAN default value on the Cisco side, VLAN1 is used as native and not tagged on 802.1q Cisco trunk.
2) I'm not sure if 3Com supports trunk negotiation (I don't think so).
C2950(config-if)#switchport mode trunk
to stop the trunk negotiation (default mode is desirable) and force the interface to become a trunk.
To your CDP note:
Does CDP work in both directions? I.e. do you see your C2950 on the other Cisco-devices connected to the 3Com switchstack via sh CDP nei?
My understanding is it should not be seen. CDP frames are sent always in VLAN1on trunks. In your case C2950 sends CDP frame untagged (native VLAN is VLAN1 by default) to 3Com. But VLAN1 is configured as tagged on 3Com, so the CDP frame is discarded.
In the opposite direction:
Cisco device sends CDP packet untagged to 3Com on access port (I suppose VLAN1, too). 3Com doesn't recognize the special Cisco destination address and treats it as an ordinary multicast - floods it to all ports. On the trunk port it's sent tagged in VLAN1. C2950 receives a CDP frame tagged in VLAN1. But Cisco 802.1q implementation is able to receive native VLAN frames both untagged and tagged (it's used in some VoIP topologies) and so CDP frame is accepted.
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...