default vlan tagged vs untagged on ISL

[Andyo]

Hello, Gents!

Recently I faced messy deployment of several 4000m switches. Say it was SW-1, SW-2, SW-3. Very simple Layer 1 chainy connections between them with SW-2 in centre. This triplet is running STP, having SW-2 as its root bridge. Interesting point there were interswitch link between SW-1 and SW-2 running multiple VLANs switching. But from SW-1 side port configured for only to carry VLANs with ID 10 & 20 (default vlan 1 is prohibited ;), while SW-2's port is configured to carry all three (default vlan, 10 & 20) VLANs tagged. There also none untagged vlan on that port. Generally STP BPDUs are carried untagged (default vlan). STP is definitly running over ISL. Finally is there explanation of HOW BPDU frames cross the ISL which one side prohibits default vlan ;) and other sends default vlan tagged? 

P.S. This thread has been moevd from Switches, Hubs, Modems (Legacy ITRC forum) to ProCurve / ProVision-Based. - Hp Forum Moderator

[Andyo]

difficult Q i know :) unfortunately I cannot currently port-monitor this ISL. I'll inform U as soon as I do it :)