Like Rick says,
for vlan tagging both sides of the link better be matched (no different from speed/duplex).
when an untagged packet comes in the port. it's internally added a tag. Then within the switch it is propagated within vlan100.
- to tagged ports for vlan100 (packet is forwarded with tag)
- to untagged ports for vlan100 (tag is stripped before it's sent out)
when a tagged packet for vlan-100 is comming into this port it may also be accepted and forwarded.
for tagged packets with another vlan-number than 100 other lines in the switch config need to be examined.
But it's not really clear what's the issue here.
>>>Now when these devices are connected to the HP switch i cannot ping past the Riverbed appliance<<<
seems allright to me!
You may need to look in the appliance config to see if this is allowed, not in the vlan config.
>>>I reckon that the VLAN we are having an issue with resides on the core switch.
<<<
I assume the three devices in question connected to this coreswitch?