Re: L2Protocol-Tunnel causes loopback errors IE Lab 20 posted 11/26/2007
I'm tunning cdp, stp, vtp, lacp. Which will do cause that?
switchport access vlan 100
switchport trunk encapsulation dot1q
switchport mode dot1q-tunnel
l2protocol-tunnel point-to-point lacp
no cdp enable
So in this case traffic is coming in and being tunneled in vlan 100,
and sent back via the trunk port to the same switch, is that correct?
Whats ever weirder is that another switch (SW2) that is not even being
tunneled is also having the issue.
Looks something like this:
D D D
o o o
t t t
1 1 1
q q q
SW1 -Tunnel - SW4 - Tunnel - SW3
SW1 --Dot1q-- SW4 -- Dot1q - SW3
On Nov 26, 2007 12:16 PM, Scott Morris <smorris@xxxxxxxxxxxx> wrote:
> 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?
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE-M
> #153, JNCIS-ER, CISSP, et al.
> VP - Technical Training - IPexpert, Inc.
> IPexpert Sr. Technical Instructor
> A Cisco Learning Partner - We Accept Learning Credits!
> Telephone: +1.810.326.1444
> Fax: +1.810.454.0130
> -----Original Message-----
> From: Gregory Gombas [mailto:ggombas@xxxxxxxxx]
> Sent: Monday, November 26, 2007 12:04 PM
> To: swm@xxxxxxxxxx
> 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@xxxxxxxxxx> 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
> > I'll assume the Brians handle the flow/explanation someplace in their
> > 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@xxxxxxxxxxxx
> > Telephone: +1.810.326.1444
> > Fax: +1.810.454.0130
> > http://www.ipexpert.com
> > -----Original Message-----
> > From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] 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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