Dot1q Tunneling QinQ over AToM
Dotlq tunneling, or QinQ, refers to a technique whereby Ethernet frames are double tagged—that is, the Ethernet frames have two 802.1Q tags. This is done by configuring the interface in dotlqtunnel mode with an access VLAN instead of the normal access VLAN mode or trunk mode. The received frames on the interface that is configured for dot1qtunnel are left intact and another 802.1Q tag is put onto the frame. This 802.1Q tag refers to the VLAN number that is configured on the dot1qtunnel interface. If the frames that the router receives are already 802.1Q tagged, they receive another 802.1Q tag.
You can use dot1q tunneling to transport VLANs inside a VLAN across a switched network. You also can use dot1q tunneling to transport the customer VLANs across the MPLS network inside a VLAN of the service provider. If the PE router is configured with a dot1qtunnel interface to the CE router, and the CE router is configured with a trunk toward the PE router, the PE router adds a second 802.1Q tag to the Ethernet frames before labeling them and forwarding them onto the MPLS network.
In Figure 10-20, you can see that this technique carries the 50 customer VLANs from one site to another site across the MPLS network with the AToM solution. The CE router is configured with a trunk port carrying the 50 VLANs, whereas the CE-facing interface on the PE router is configured with the access VLAN 800 and dot1qtunnel. Therefore, all frames are tagged with VLANs 1 to 50 and are "encapsulated" into VLAN 800 of the service provider on the PE router. The result of this is that the service provider needs only one VLAN—VLAN 800—to carry all the VLANs from this customer for that site. The other sites have a configuration that is similar to this site.
Tunnel Label
VC Label
VLAN 800
VLAN 1-50
Ethernet Frame
Customer London Site
Asymmetrical Link
Customer VLANs 1 To SO
BG2.1Q Tunnel Port
VLAN 800
Example 10-15 shows a possible configuration for the CE and PE routers to have dot1q tunneling over AToM.
Example 10-15 Dot1q Tunnel Configuration Example "CË#
interface FastEthernet0/1 no ip address
interface FastEthernet0/1.1 encapsulation dotlQ 1 ip address 10.1.2.2 255.255.255.0
interface FastEthernet0/1.2 encapsulation dot1Q 2 ip address 10.1.4.2 255.255.255.0
Example 10-15 Dotlq Tunnel Configuration Example (Continued)
interface FastEthernet2/3 no ip address switchport switchport access vlan 800 switchport mode dotlqtunnel spanning-tree bpdufilter enable
interface Vlan800 no ip address mpls l2transport route 2.2.2.5 800
NOTE The mpls l2transport route destination vc-id command is the equivalent but older command for the command xconnect peer-router-id vcid encapsulation mpls.
Continue reading here: ToM Tunnel Selection
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