Trunk Port Tagging

[George B. Park, III]

Hello,

I have LACP trunks between a core and two downstream switches (all 5400s).

In reading I see alternate directions to either tag all VLANs between swtiches at the trunk level or untag VLAN1 or another VLAN.

What is the proper config?

Regards,
gp

[Mohieddin Kharnoub]

Hi

If you want Vlans to travel accross your switches, just tagg all Vlans on the trunks, and keep the default-vlan untagged.

Good Luck !!!

[André Beck]

George,

> In reading I see alternate directions to
> either tag all VLANs between swtiches at
> the trunk level or untag VLAN1 or another
> VLAN.

Both are correct. In fact, almost anything that you could do might be correct. It depends on what you interface to.

> What is the proper config?

For several reasons, having one VLAN untagged on a 802.1Q link is a good idea. It defines what should happen with untagged frames entering that port, and it establishes the highest level of inter-vendor compatibility (there are platforms which can't deal with it, though). The VLAN that is left untagged is often called the "native VLAN". Using the default VLAN (VID 1) as the native VLAN is another compatibility default. It is also very helpful in cases where ProCurve hardware falls back to port default configuration, like when you change from 1000BaseT to SFP or back on a 2626 dual personality port (silly concept, I could rant about it endlessly). If you have the switch management in VID 1, you still reach a box that broke this way and can fix it.

So it's a good idea to run VID 1 untagged and the others tagged, but other schemes will work as well as long as the peer side of the ISL thinks the same way about it.

BTW, you should always allow *all* existing VLANs on an ISL if you are running single instance STP. Leaving out certain VLANs arbitrarily (because you know they are not needed at the other end) can badly fail with SSTP.

FWIW, there is absolutely nothing link aggregation specific about that tagging. It applies to any ISL, aggregated or not.

BTW, I'm desperately trying to not use the term "trunk" as it means different things for different people ;)