With VPLS, you can tunnel certain Layer 2 control protocols. As you can see in Example 11-6, you can configure the PE router to tunnel Cisco Discovery Protocol (CDP), STP, or VLAN Trunking Protocol (VTP). In that case, the control packets of those protocols are transparently tunneled across the MPLS network, without the PE routers participating in those protocols. To tunnel these protocols across VPLS, configure the command l2protocol-tunnel on all the customer-facing physical Ethernet interfaces on the PE routers.
VPLS-PE-3#conf t Enter configuration commands, one per line. End with CNTL/Z. VPLS-PE-3(config)#int fas 8/2 VPLS-PE-3(config-if)#l2protocol-tunnel ? cdp ...