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Encapulation command -- What is a "second-dot1q" VLAN?

petehummon
Level 1
Level 1

Hi Cisco people,

 

   I've just inherited a Cisco L2 device running IOS XR 6.6.12.  This switch has a bundle connection to another identical switch.  I see this subinterface on the switch-to-switch bundle connection in the running config:

 

 

interface Bundle-Ether3.1000 l2transport
 description vlan 1000 to other switch
 encapsulation dot1q 1000 second-dot1q 123

 

 

I understand enough to know that the the above allows all traffic tagged with VLAN 1000 across the Bundle.  Easy enough.  But what is that "second-dot1q" VLAN 123?  Is that a double-tagged VLAN?  The "?" menu isn't much help:

 

RP/0/RP0/CPU0:SW-1#conf t
RP/0/RP0/CPU0:SW-1(config)#interface Bundle-Ether3.1000 l2transport
RP/0/RP0/CPU0:SW-1(config-subif)#encapsulation dot1q 1000 ?
  ,             <comma>
  exact         Do not allow further inner tags
  ingress       Perform MAC-based matching
  second-dot1q  IEEE 802.1Q VLAN-tagged packets
  <cr>
RP/0/RP0/CPU0:SW-1(config-subif)#encapsulation dot1q 1000

I'm not a Layer 2 guy, so I'm not sure what "IEEE 802.1Q VLAN-tagged packets" could mean here.  What is VLAN 123's relationship to VLAN 1000 in my example?  What traffic will be permitted across the bundle with this config?  Thank you!

1 Accepted Solution

Accepted Solutions

Reza Sharifi
Hall of Fame
Hall of Fame

The second tag is for Q-in-Q vlan traffic.

see link below for more info:

 

https://www.cisco.com/en/US/docs/ios/lanswitch/configuration/guide/lsw_ieee_802.1q.html

 

HTH

View solution in original post

1 Reply 1

Reza Sharifi
Hall of Fame
Hall of Fame

The second tag is for Q-in-Q vlan traffic.

see link below for more info:

 

https://www.cisco.com/en/US/docs/ios/lanswitch/configuration/guide/lsw_ieee_802.1q.html

 

HTH

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