From: Scott Morris (swm@emanon.com)
Date: Sun Nov 25 2007 - 23:04:47 ART
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!
Telephone: +1.810.326.1444
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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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