Trunk and VLANS between switches

[Björn Gam]

So thanks for your support......and I hope the weather in sweden is better than in germany.

Bjoern

[RicN]

Hello Bjoern, I doubt it is.. very gray and rainy here. :)

Glad to be able to help!

(And if you want, you can attach points to posts that helped you out.) ;)

By the way, one more thing that is confusing regarding VLANs and trunks is, that if you have say VLAN 1,10 and 20 attached to port a1 and then make a trunk out of a1 and a2, then all VLAN configuration on a1 is removed, and you will manually have to re-enter it on the new "virtual" trunk port.

[Björn Gam]

Hello,

at this weekend I was trying to add the VLAN(s) at our default trunk-connection between the switches, but it was not working so well. Some of our clients can't connect anymore to the "storage-vlan". And the strange thing all other clients can't connect to the default gw.

What I did? I add the the same VLANS (ID) to every switch and I added the VLANS to every trunk-connection between the switches. And I also changed some local trunk-groups at the switches. I removed for example the trunk group 4 and I added the two ports of trunk group 4 to trunk group 1 they were useing all the same VLANS and so I belived it's a waste of trunk groups.

Any idea?
Bjoern

[André Beck]

Hi RicN,

just to point out a pitfall:

> You only need to add the VLANs that actually
> does exist on both side of the trunk, if you
> have some VLAN with ports only on switch B
> then you do not have do anything.

While in theory this is perfectly true, there is a nasty surprise lurking when doing that in a Single Spanning Tree topology. SSTP establishes a "merged" topology that essentially is the same as the physical topology and will happily set links to forwarding where some of your VLANs are not tagged, while setting others to blocking where they are tagged. In an SSTP topology, it's best to use an all-or-nothing approach for VLAN tagging to prevent accidentally isolating partitions of some VLANs. Without PVST, the only other option is MSTP, but it's not the nicest option ;)

HTH,
Andre.

[Björn Gam]

Oh I found my error....I was the reconfiguration of the trunk ports.

We're using four different trunk groups for our storage VLAN. And I removed one of the trkgrp and add it to a another trkgrp without disable the ports. So I create a broadcast storm.

Bjoern