From: Patrick Laidlaw (plaidlaw@3sgi.com)
Date: Mon Feb 05 2007 - 16:39:00 ART
Hello Mohammad,
As I understand and have played with dot1q tunneling I believe it works
this way:
You your self have multiple vlans coming through a trunk to lets say a
service provider. You tag vlan 5 10 15 and 20 coming through this
trunk. If your service provider has designated you as being on vlan
dot1q trunk vlan 100 a frame coming your vlan 5 would now be transported
across the Service providers trunk with both vlan tag 100 and 5. When
your traffic arrives at the remote site there vlan tag 100 is removed
leavening on the vlan 5 tag from your remote site trunk to see. Now
after going through this you need to remember that when you turn on a
dot1q tunnel you need to increase the MTU size to 1504 to allow for a
additional 4 bytes of tagging.
The thing about this is the Service provider only sees the first vlan
tag and the rest of your frame it doesn't pay attention to.
Patrick
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Mohammad Saeed
Sent: Monday, February 05, 2007 9:07 AM
To: Cisco certification
Subject: dot1q tunnel mode
Hi Evereybody,
When we configure this on an interface on 3550 switch:
switchport trunk encapsulation dot1q
switchport mode trunk
that interface will tag all the frames coming from respective vlans
except the native vlan.
But when we create dot1q tunnel and we configure on one interface that
is connected to Customer site to provide dot1q tunneling:
switchport mode dot1q-tunnel
switchport access-vlan 1000
Now as this port will be receiving traffic from multiple vlans, will
this interface tag all traffic with VLAN ID 1000 (theoratically it
shall not as we are not giving switchport trunk encapsulation dot1q)
or leave the traffic tagged with its original vlan. I am not clear in
what format this interface will receiving the traffic, will that be
normal ethernet frame or ethernet frame tagged with vlan id using
dot1q encapsulation. If any one can expalin a little bit, I will
appreciate that.....
To me it looks like, all traffic will already be encapsulated using
dot1q and this interface will mark those frames with VLAN ID 1000 and
will pass on to other end of dot1q tunnel.....
Regards,
Mohammad Zahed Saeed
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