From: Bob Sinclair (bsin@cox.net)
Date: Sat Nov 29 2003 - 21:50:41 GMT-3
Juan,
I put a sniffer on a 3550 dot1q trunk and observed the following when I
removed Vlan 1 from the dot1q trunk, but left it as the native vlan:
1. All traffic leaving the port is tagged
2. VTP, CDP and DTP traffic leave the port with Vlan 1 tags
3. No other Vlan 1 traffic is seen leaving the port (including no Vlan 1
BPDUs)
4. All BPDUs are PVST+ encapsulated, to address 01-00-0c-cc-cc-cd
This should not cause a problem as long as all of your switches are Cisco
and similarly configured:
a. The Cisco switches recognize that CDP, VTP and DTP are not to be
forwarded
b. The Cisco switches recognize the encapsulated BPDUs
You very well might have an STP issue if you connect such a port to a
non-Cisco switch, because the brand X switch will not see any recognizable
BPDUs, and the Cisco switch may not recognize the untagged BPDUs on the
native vlan coming from the Brand X switch. It would seem prudent to allow
the native vlan across the trunk.
But then, you would never put a Brand X switch in your network...
would you?
-Bob Sinclair
CCIE #10427, CISSP, MCSE
bsinclair@netmasterclass.net
----- Original Message -----
From: <jfaure@sztele.com>
To: <ccielab@groupstudy.com>
Sent: Saturday, November 29, 2003 12:55 PM
Subject: Catalyst 802.1q trunking issues
> Hi all:
>
> -From the 6500 CCO configuration guide, about removing vlan1 from the
> trunk:
>
> "You can remove VLAN 1. If you remove VLAN 1 from a trunk, the trunk
> interface continues to send and receive management traffic, for example,
> Cisco Discovery Protocol (CDP), VLAN Trunking Protocol (VTP), Port
> Aggregation Protocol (PAgP), and DTP in VLAN 1. "
>
> -But also, in the same document:
>
> "Disabling spanning tree on the native VLAN of an 802.1Q trunk without
> disabling spanning tree on every VLAN in the network can cause spanning
> tree loops. We recommend that you leave spanning tree enabled on the
native
> VLAN of an 802.1Q trunk. If this is not possible, disable spanning tree on
> every VLAN in the network. Make sure your network is free of physical
loops
> before disabling spanning tree"
>
> And then my question is:
> If you have several dot1q trunks configured in your swiched network in
such
> a way that these trunks don't allow pass the vlan1, the vlan1 is the
native
> vlan for them (you can see this doing a "sh int trunk") and the interface
> vlan 1 is in shutdown state in all the switches (but no STP disabled on
> this vlan 1), can you have any stp issues like to be unable to block some
> loops?
> What happens with the STP control traffic if vlan 1 isn't included on the
> trunks? Having vlan 1 in shutdown state maybe interpreted by the system as
> it has stp disabled for this vlan? I'm usign RAPID PVSTP
>
> Any thoughs will be greatly apreciated.
>
>
>
>
>
> Juan Faure Ferrer
> email: jfaure@sztele.com
>
> Lmnea de Negocio de Telematica y CC
> Ingeniero de Integracisn de Redes y Sistemas
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