One of my personal favorite L2 subjects to discuss. When I was at the service provider this was a very cost effective Metro solution to extend customer VLANs. No routing protocols on the CPE and no expensive EoMPLS hardware required from the SP perspective. Simple and effective. You will hear many names for the outer tag (S-TAG, metro tag, etc…) just remember that the outer tag is the unique SP ID for that customer and the inner tags are the customers tags. This extra instance of .1q requires an additional 4 bytes so, make sure your system/global MTU is at least 1504 bytes to transmit a data frame size of 1500 bytes.

Combine this with L2TP and you can easily tunnel VTP,STP,and CDP frames.

Here is a brief example:

SW1: 

interface FastEthernet0/1
des connection to Customer_SITE_A
 switchport access vlan 100
 switchport mode dot1q-tunnel
 l2protocol-tunnel cdp
 l2protocol-tunnel stp
 l2protocol-tunnel vtp
 no cdp enable

SW4:

interface FastEthernet0/4
 des Cucstomer_SITE_B
 switchport access vlan 100
 switchport mode dot1q-tunnel
 l2protocol-tunnel cdp
 l2protocol-tunnel stp
 l2protocol-tunnel vtp
 no cdp enable

Now to expand upon this concept you can create an Etherchannel/Portchannel between the customer CE devices using separate SP tag for each link.

SW1: CPE_Site_A

interface fa 0/12
sw tr en dot
sw mo tr
channel-protocol LACP
channel-group 1 mode active

interface fa 0/13
sw tr en dot
sw mo tr
channel-protocol LACP
channel-group 1 mode

SW2: SP_SW1

int fa 0/12

 switchport access vlan 100
 switchport mode dot1q-tunnel
 l2protocol-tunnel cdp
 l2protocol-tunnel lldp
 l2protocol-tunnel stp
 l2protocol-tunnel vtp
 l2protocol-tunnel point-to-point lacp

int fa 0/13

 switchport access vlan 200
 switchport mode dot1q-tunnel
 l2protocol-tunnel cdp
 l2protocol-tunnel lldp
 l2protocol-tunnel stp
 l2protocol-tunnel vtp
 l2protocol-tunnel point-to-point lacp

int fa 0/24
sw tr en dot
sw mo tru

SW3: SP_SW2

int fa 0/12

 switchport access vlan 100
 switchport mode dot1q-tunnel
 l2protocol-tunnel cdp
 l2protocol-tunnel lldp
 l2protocol-tunnel stp
 l2protocol-tunnel vtp
 l2protocol-tunnel point-to-point lacp

int fa 0/13

 switchport access vlan 200
 switchport mode dot1q-tunnel
 l2protocol-tunnel cdp
 l2protocol-tunnel lldp
 l2protocol-tunnel stp
 l2protocol-tunnel vtp
 l2protocol-tunnel point-to-point lacp

int fa 0/24
sw tr en dot
sw mo tru

SW4: CPE_Site_B

interface fa 0/12
sw tr en dot
sw mo tr
channel-protocol LACP
channel-group 1 mode active

interface fa 0/13
sw tr en dot
sw mo tr
channel-protocol LACP
channel-group 1 mode active