802.1Q-in-Q Tunneling
Traditionally, VLANs have not extended beyond the WAN boundary. VLANs in one campus
extend to a WAN edge router, but VLAN protocols are not used on the WAN.
Today, several emerging alternatives exist for the passage of VLAN traffic across a WAN,
including 802.1Q-in-Q, Ethernet over MPLS (EoMPLS), and VLAN MPLS (VMPLS). While
these topics are more applicable to the CCIE Service Provider certification, you should at least
know the concept of 802.1 Q-in-Q tunneling.
Also known as Q-in-Q or Layer 2 protocol tunneling, 802.1Q-in-Q allows an SP to preserve
802.1Q VLAN tags across a WAN service. By doing so, VLANs actually span multiple
geographically dispersed sites. Figure 2-5 shows the basic idea
The ingress SP switch takes the 802.1Q frame, and then tags each frame entering the interface with
an additional 802.1Q header. In this case, all of Customer1’s frames are tagged as VLAN 5 as they
pass over the WAN; Customer2’s frames are tagged with VLAN 6. After removing the tag at
egress, the customer switch sees the original 802.1Q frame, and can interpret the VLAN ID
correctly. The receiving SP switch (SP-SW2 in this case) can keep the various customers’ traffic
separate based on the additional VLAN tags.
Using Q-in-Q, an SP can offer VLAN services, even when the customers use overlapping VLAN
IDs. Customers get more flexibility for network design options, particularly with metro Ethernet
services. Plus, CDP and VTP traffic passes transparently over the Q-in-Q service.
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