From: Waters, Kivas (UK72) (Kivas.Waters@xxxxxxxxxxxxx)
Date: Thu Nov 08 2001 - 06:56:35 GMT-3
IEEE 802.1Q Configuration Considerations
IEEE 802.1Q trunks impose some limitations on the trunking strategy for a
network. The following restrictions apply when using 802.1Q trunks: Make
sure the native VLAN for an 802.1Q trunk is the same on both ends of the
trunk link. If the native VLAN on one end of the trunk is different from the
native VLAN on the other end, spanning-tree loops might result.
http://www.cisco.com/univercd/cc/td/doc/product/lan/c2900xl/29_35xu/scg/kivl
an.htm#xtocid2442336
regards
Ki
-----Original Message-----
From: Erick B. [mailto:erickbe@yahoo.com]
Sent: 08 November 2001 03:49
To: Bryan Osoro; steven.j.nelson@bt.com; cchurch@USTA.com;
ccielab@groupstudy.com
Subject: RE: VLAN trunking and 802.1Q
Guys,
I'm aware that dot1Q VLAN1 is untagged, but in my lab
and question I was pinging between VLAN2 and VLAN3
which are tagged VLANs. It didn't work until I added
another sub-interface configured for VLAN1 on the
router. After it worked, I removed this sub-interface
and it continued to work. I then rebooted router and
switch and it still worked. I have seen similar/same
odd behavior in some customers networks I've worked on
from time to time. I'm not sure how easy this problem
would be to produce on the fly as I haven't been able
to get it to act that way today.
On the switch side, it was to set to 802.1q and none
of the auto-stuff was on. Spanning tree was off to. I
just made changes on router side and it started
working even though router config was good already.
As for code, router IOS I was using was 12.0(7)T but I
also saw similar behavior with a recent 12.1(x) image.
I'm picky when I see strange behavior and try to get
to the bottom of it.
Thanks, Erick
--- Bryan Osoro <bosoro@hotmail.com> wrote:
> Reason for this happening is normal
>
> Dot1q in Cisco's implementation does NOT tag VLAN
> traffic on the "Native
> VLAN" Native VLAN on ports is 1, by default.
> Therefore VLAN 2 and 3
> traffic is getting to the router with Dot1q
> information in every frame,
> and VLAN 1 is NOT getting tagged. The router
> considers VLAN 1 to not be
> "encapsulated," and I bet if you had a debug ip
> packet detail on you'd
> see encapsulation failed when the PC sends the ping
> to the router. Up
> until 12.1 IOS could not do anything to fix this,
> now there is a native
> vlan command that makes the router aware of the
> switches behavior.
> Other fix is to change the native vlan on the switch
> to be some number
> that is not in use (i.e. 888) or some random number.
>
> -Bryan
>
> -----Original Message-----
>
> Guys
>
> This should do the trick, note the table regarding
> dot1q support and IOS
> versions.
>
> http://www.cisco.com/warp/customer/473/50.shtml
>
> Thanks
>
> Steve
>
> -----Original Message-----
>
> Erick,
>
> Do a 'sh trunk':
>
> WP-4006-1> sh trunk
> * - indicates vtp domain mismatch
> Port Mode Encapsulation Status
> Native vlan
> -------- ----------- ------------- ------------
> -----------
> 1/1 on dot1q trunking
> 1
> 1/2 auto dot1q trunking
> 1
> 2/1 auto dot1q trunking
> 1
> 2/2 auto dot1q trunking
> 1
>
> If your trunk port doesn't show 'trunking' as a
> status, it won't work.
> Try
> setting the mode on both ends to on, rather than
> auto or desirable. Are
> you
> running fairly recent GD code on the router and
> switch?
>
> Chuck
>
> -----Original Message-----
> From: nobody@groupstudy.com
>
> Hello there,
>
> I've been playing around with some VLAN stuff lately
> and have seen some odd behavior with trunking and I
> can't find a answer so thought I'd see if anyone
> here
> knew, because it is bugging me.
>
> Real basic setup.
>
> Cisco router with FE connecting to a switch. VLANs
> 1,
> 2, and 3. Workstation on switch in VLAN 2 and VLAN
> 3.
>
> Dot1Q trunk with all VLANs defined. Sub-interfaces
> on
> router for dot1q. Ping from PC in VLAN2 to VLAN3 and
> don't get further then router interface for that
> VLAN.
> Change to ISL encaps and everything works fine.
> Change
> back to Dot1Q and things continue to work. Save
> configs and reload switch and routers and things
> continue to work.
>
> I also had a similar thing when I didn't configure a
> VLAN1 sub-interface on the router (just 2 and 3).
> Once
> I added VLAN1 thing started working. Took VLAN1 away
> for kicks and things still worked. Weird.
>
> Any one have any thoughts???
>
> Thanks, Erick
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