Q n Q Tunneling

Q n Q Tunneling  
December 16, 2009

By David Clark, CCIE# 14742 (Routing and Switching), CCSI# 31937

Service providers often have multiple customers with overlapping VLAN ranges. Q-in-Q tunneling or 802.1Q tunneling is the mechanism that allows service providers to preserve customer VLANS being transported through the service provider networks. Q-in-Q tunneling uses a two-level VLAN tag structure. Each customer is assigned a unique VLAN within the service providers network, this unique tag is added to all incoming frames from the customer network. The original frame from the customer remains untouched. The inner frame is often referred to as the customer VLAN tag because the customer originally assigns it.

Q-in-Q tunneling significantly reduces the number of VLAN required within a customer network as each customer is assigned a unique VLAN. To the customer the service provider network appears as a transparent bridge connecting their sites together.

When configuring Q-in-Q tunneling the ports connected directly to the customer switch are configured with the command “switchport trunk encapsulation dot1q”. The customer ports are configured as per normal and require no extra configuration.

 

interface FastEthernet0/20

 description customer

 switchport trunk encapsulation dot1q

 switchport mode trunk

 switchport nonegotiate

end

 

Interface FastEthernet0/21

 description service provider

 switchport access vlan 21

 switchport mode dot1q-tunnel

 l2protocol-tunnel cdp

 l2protocol-tunnel stp

 l2protocol-tunnel vtp

 no cdp enable

end

 

In the above example Fast 0/20 resides on the customer switch, while Fast 0/21 is on the service provider switch and configured as a tunnel port. VLAN 21 becomes the outer tag and is added to all frames from Fast 0/20 sent down to Fast 0/21. The service provider port is specifically configured also to carry CDP, STP and VTP frames. The configuration can be verified with the command “show l2protocol-tunnel interface fast 0/21”

 

If the customer is running a negotiated EtherChannel a slightly different configuration is required. On the customer side a normal EtherChannel is configured.

interface Fastethernet0/1

switchport trunk encapsulation dot1q

switchport mode trunk

channel-group 1 mode desirable

 

interface Fastethernet0/2

switchport trunk encapsulation dot1q

switchport mode trunk

channel-group 1 mode desirable

 

The service provider configuration is slightly different from the previous example.

interface Fastethernet0/1

switchport access vlan 17

switchport mode dot1q-tunnel

l2protocol-tunnel point-to-point pagp

end

 

interface fastethernet0/2

switchport access vlan 18

switchport mode dot1q-tunnel

l2protocol-tunnel point-to-point pagp

end

 

In this case we use the “l2protocol-tunnel point-to-point pagp” command to carry the PagP frames across the service provider network. Also one customer VLAN is assigned per incoming from port from the customer switch.  For LACP the ““l2protocol-tunnel point-to-point LACP” would be used.

 

Article Source: http://www.ccbootcamp.com/support-resources/resources/articles-by-ccbootcamp.html

Tags: ccie, cisco, computer training, VLAN

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