From: swm@emanon.com
Date: Thu Feb 19 2009 - 02:34:13 ARST
It's not so much that access-mode works but dot1q tunnel mode doesn't...
What's happening is some broadcast/multicast is getting forwarded.
You did the right thing by removing the access vlan from any other trunk
links/switches so there would be no loop.
But looking at your configuration, did you perhaps NOT use the dot1q tag
native vlan command set? If you aren't tagging them, the intermediary
switches will receive and interpret the frames rather than double tagging
them. Try adding that and see what happens. Or equally, you may change your
native vlan to that same vlan which does not go across other trunk
ports/switches, and that may accomplish the same thing.
Keep in mind I'm just going from memory from when I wrote that lab. While it
was quite fun, I have no idea if anything was changed since my departure.
Cheers,
Scott
---- Message from joe_astorino@comcast.net at 2009-02-19 02:54:17 ------
>Hi guys,
>
>I am having a difficult time understanding why I am getting this problem.
Basically, I am working on IPexpert volume 1, lab 5 which is L2 tunneling.
Part of this lab involves tunneling a trunk from Cat1 --> Cat4 --> Cat2 -->
Cat3. If I set up my tunneling ports as access ports everything works fine (I
have already pruned the access vlan I am using from all other trunks).
However, if I change the ports to dot1q-tunnel mode instead of being access
ports, I keep getting ports going err-disabled due to loop detection. Nothing
else has changed in my configuration so I am confused.
>
>I understand the fundamental difference I think. I know dot1q-tunnel uses q-q
technology to encapsulate a tag inside another tag -- usually used in SP
environments. I just don't understand why this would cause a loop but the
access port mode does not, when nothing else has changed. Here is a rough
picture of what I have. All switches are dual connected to all other switches
over fa0/19-24
>
>
>Cat1-----------------------Cat3
>| |
>| |
>| |
>| |
>Cat2-----------------------Cat4
>
>
>I don't know how else to show it, but also Cat1/Cat4 are connected and
Cat2/Cat3 are connected.
>
>Cat1: Fa0/19-20 ---> Cat4
>Cat1: Fa0/21-22 ---> Cat3
>Cat1: Fa0/23-24 ---> Cat2
>
>Cat2: Fa0/19-20 ---> Cat3
>Cat2: Fa0/21-22 ---> Cat4
>Cat2: Fa0/23-24 ---> Cat1
>
>Cat3: Fa0/19-20 ---> Cat2
>Cat3: Fa0/21-22 ---> Cat1
>Cat3: Fa0/23-24 ---> Cat4
>
>Cat4: Fa0/19-20 ---> Cat1
>Cat4: Fa0/21-22 ---> Cat2
>Cat4: Fa0/23-24 ---> Cat3
>
>So the basic idea for this task is to make Cat1 trunk to Cat3 by taking the
path Cat1 Fa0/19 ---> Cat4 Fa0/19 ---> Cat2 Fa0/22 ---> Cat3 Fa0/19
>
>The VLAN I chose to add for tunneling was VLAN 13. I added it only on Cat2
and Cat4. It is ONLY trunked on fa0/22 between Cat2 and Cat4.
>
>If I make Cat1 fa0/19 and Cat3 fa0/19 static 802.1q trunk ports and make
Cat4/Cat2 ports Fa0/19 and Fa0/22 access ports in VLAN 13 with also
l2protocol-tunnel turned on, it accomplishes the task!
>
>However, if I change NOTHING else and make Cat4/Cat2 Fa0/19 and Fa0/22
dot1q-tunnel instead of access I get Fa0/19 on Cat4 and Cat2 going
err-disabled due to loop detection. I am utterly confused!
>
>- Joe A
>
>
>Blogs and organic groups at http://www.ccie.net
>
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