From: Scott Morris (smorris@ipexpert.com)
Date: Mon Nov 26 2007 - 14:16:09 ART
I doubt your BPDU's are what's looped back. What other L2 frames are being
moved? (Or did you enable tunneling of STP as well?)
Will you see ANYTHING exit and enter back to the same switch?
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE-M
#153, JNCIS-ER, CISSP, et al.
CCSI/JNCI-M/JNCI-ER
VP - Technical Training - IPexpert, Inc.
IPexpert Sr. Technical Instructor
A Cisco Learning Partner - We Accept Learning Credits!
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-----Original Message-----
From: Gregory Gombas [mailto:ggombas@gmail.com]
Sent: Monday, November 26, 2007 12:04 PM
To: swm@emanon.com
Cc: Cisco certification
Subject: Re: L2Protocol-Tunnel causes loopback errors IE Lab 20
Hi Scott - thanks for replying. I too thought it might have something to do
with the bpdu's somehow being sent back, which is why I removed the tunnel
vlans from the allowed vlan list of all trunks.
What I don't understand is how the bpdu's can be looped back when the tunnel
vlans only exist on SW4 (the "provider" switch) ?!?
On Nov 25, 2007 9:04 PM, Scott Morris <swm@emanon.com> wrote:
> It happens because you have the VLAN coming back to you on some other
> trunk interface. And your switch is seeing traffic that it initiated
> coming back, so it's believing there's a loop that spanning tree
> missed. Those should be the interfaces that end up being blocked.
>
> Your switches are doing exactly what they are supposed to be doing,
> but it's really frustrating to have to sit and visualize the packet
> flow to make it make sense! :)
>
> But because you are "tweaking" things and doing the L2 tunneling on
> one side, think about how other switches handle that and where all it may
go.
>
> I'll assume the Brians handle the flow/explanation someplace in their
books.
> In the eBook Labs that I created for our ProctorLabs product, one of
> the labs has some interesting L2Tunnel stuff going on, and the
> solution guide walks through a big chunk of debugs and "thinking" of this
whole process.
> Once you figure it out though, it really does make sense! :)
>
>
> HTH,
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
> JNCIE-M #153, JNCIS-ER, CISSP, et al.
> CCSI/JNCI-M/JNCI-ER
> VP - Technical Training - IPexpert, Inc.
> IPexpert Sr. Technical Instructor
>
> A Cisco Learning Partner - We Accept Learning Credits!
>
> smorris@ipexpert.com
>
>
>
> Telephone: +1.810.326.1444
> Fax: +1.810.454.0130
> http://www.ipexpert.com
>
>
>
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of Gregory Gombas
> Sent: Sunday, November 25, 2007 7:03 PM
> To: Cisco certification
> Subject: L2Protocol-Tunnel causes loopback errors IE Lab 20
>
> Hi Experts,
>
> This is from IE lab 20 Task 1.1 and 1.2.
>
> Shortly after configuring the l2protocol tunnels on SW4, either SW1 or
> SW2 goes err-disable on the non tunneling ports:
>
> %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on FastEthernet0/20
> %PM-4-ERR_DISABLE: loopback error detected on Fa0/20, putting Fa0/20
> in err-disable state
>
> The strange thing is that the ports that are going err-disable are not
> even part of the tunnel configuration!
> The only way I could stop this from happening was to remove vlan 100
> and 200 (the access vlans used for tunneling) from the allowed vlan list
on SW4.
>
> Any ideas on why this happens? I can send you my configs if needed.
>
> Thanks,
> Greg
>
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