Cisco – increasing bandwidth between two Cisco switches

You need to use the following command to add your VLAN 30 to an existing Dot1Q trunk on a Cisco Catalyst switch:

switchport trunk allowed vlan add 30

Otherwise IOS just thinks you're trying to overwrite the existing configuration and you are left with an accidentally deleted set of allowed VLANs.

You could similarly use "remove" in place of "add" to remove only one VLAN. See the entire syntax below. (It is actually the same syntax in Cisco Nexus OS or IOS, FYI.)

SW-FOO(config-if)#switchport trunk allowed vlan ?
  WORD    VLAN IDs of the allowed VLANs when this port is in trunking mode
  add     add VLANs to the current list
  all     all VLANs
  except  all VLANs except the following
  none    no VLANs
  remove  remove VLANs from the current list

Another option is to put all of your allowed VLANs into the command, like so:

switchport trunk allowed vlan 10,20,30

This option is more time consuming but also works.

Is HP's version of "RSTP" a Cisco-like Rapid-PVST+, is it more of 802.1q RSTP or something like 802.1s MST?

In the former case, you have to make sure that "Switch 21" - although all it's user ports are in VLAN21, also has VLAN20 configured, and is actively participating as a bridge in VLAN20's spanning tree (with only two STP interfaces, in that case).

If the config is more MST-like, you'll have to make sure that same mapping of VLAN-to-MST-Instance(s) is applied on the given (set of) switche(s). Plus, you have to make sure that the VLAN used to talk MST from switch to switch is available/allowed on all inter switch links (I believe this is usually done in VLAN 1 and without tags).

Essentially, this boils down to the same problem, no matter which dialect of spanning-tree you are running.

Both members of a such a switch pair need to be actively participating as bridges in the spanning-tree(s) for both VLANs of the given switch pair. Of course, the inter switch links must be configured to allow both VLANs; your diagram suggest that this is the case.

Cheers Marc