From: Gregory Gombas (ggombas@gmail.com)
Date: Mon Nov 26 2007 - 14:04:20 ART
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
>
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> -----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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