Switch behavior with untagged interswitch links

[Johan de Greef]

Hello,

I was wondering during a discussion about tagged and untagged packets what happens to packets when they pass through a layer 2 interswitch link.

The example setup was basic and as follows:

2 HP Procurve switches directly connected

Vlan 1 and VLAN 2 present on both switches

Interswitch link configured with VLAN 1 untagged and VLAN 2 tagged

2 PC connected in VLAN 1, 1 on each switch

When the 2 PCs need communicate to communicate the packet reaches the switch untagged. The switch know that untagged packets belong to VLAN 1, basic switching begins and after all the arps and lookups the switch learns that the packets need to be forwarded out over the interswitch link. 

The interswitch link is, however, untagged for VLAN 1.

How does the switch handle this? I came up with a few possibilities, but I don't know which one is the correct one.

1. The packets get tagged with a VLAN 1 tag upon entering the switch and the tag gets stripped when leaving the switch out of the interswitch link interface.

2. The packets get tagged with a VLAN 1 tag upon entering the switch and the tag remains when leaving the interswitch link interface. The fact that VLAN 1 is untagged on the interswitchlink does not matter, this also allows tagged VLAN 1 packets.

3. The packet enters the switch, but does not get tagged, it remains untagged (very unlikely IMHO)

Has anyone ever done any research into HP switch behavior as far as tagging and untagging is concerned on interswitch inks? If so, is there any technical documentation on this behavior?

Thanks in advance!

[paulgear · Accepted solution]

Hi Johan,

It seems to me that there's another question behind this question, but I'll answer as best I can.

What happens is most closely identified with solution 1 that you've listed. Packets internally are handled with tags. When a packet comes in without a tag, it is assigned to the VLAN that is untagged on that port, and this stays with it. So if an untagged packet comes in on a port configured with VLAN 2 untagged, it will be tagged if it goes out the inter-switch link. If a packet is sent out a port where its VLAN is untagged, its tag is removed automatically by the switch on egress.

So the bottom line is as long as you have your ports set up in the VLANs you want, and your inter-switch links matching tagged and untagged ports (GVRP is by far the easiest way to do this), then you don't have to care about how it works - the switch will handle the tagging and untagging automatically.

I hope that answers your questions.

[Johan de Greef]

Hi Paul,

Thanks for the answer!

Regards,

Johan