Cisco Cisco IOS Software Release 12.2(14)SZ

This feature allows you to connect two VLAN networks that are in different locations, without using 
expensive bridges, routers, or switches at the VLAN locations. You can enable the MPLS backbone 
network to accept Layer 2 VLAN traffic by configuring the label edge routers (LERs) at both ends of the 
MPLS backbone. 

Adding a point-to-point virtual circuit (VC) requires you to configure the two VC endpoints at the two 
PE routers. Only the two PE routers at the ingress/egress points of the MPLS backbone know about the 
VCs dedicated to transporting Layer 2 VLAN traffic. All other routers do not have table entries for those 
VCs.

PE routers connected to the MPLS backbone perform label imposition and disposition. The imposition 
PE router encapsulates the Layer 2 VLAN packet into an MPLS PDU to transport it across the backbone 
to the disposition PE router. The disposition PE router takes the MPLS PDU, de-encapsulates the MPLS 
PDU, and delivers the Layer 2 VLAN packet to the correct interface.

When the imposition PE router encapsulates a Layer 2 VLAN packet to route it across the MPLS 
backbone, it includes a label stack with two levels of labels:

An Interior Gateway Protocol (IGP) stack, also known as a tunnel label 

A VC-based label

The MPLS backbone uses the IGP labels to transport the VLAN packet between the PE routers. The 
disposition PE router uses the VC-based label to select the outgoing interface for the VLAN packet.

The following list explains some of the benefits of enabling the transport of Layer 2 packets over the 
MPLS network:

The Any Transport over MPLS product set accommodates many types of Layer 2 packets, including 
Ethernet and AAL5, across multiple Cisco router platforms. This enables the service provider to 
transport all types of traffic over the backbone and accommodate all types of customers.

AToM adheres to the standards developed for transporting Layer 2 packets over MPLS. (See 
“Standards and Drafts Upon Which AToM Is Based” for the specific standards that AToM follows.) 
This benefits the service provider who wants to incorporate industry-standard methodologies in the 
network. 

Upgrading to AToM is transparent to the customer. Because the service provider network is separate 
from the customer network, the service provider can upgrade to AToM without disruption of service 
to the customer. The customers assume that they are using a traditional Layer 2 backbone.

The following restrictions apply to the Cisco 7304 router:

Modular QoS: The match input interface command cannot be used to match based on 
subinterfaces when using Ethernet over MPLS. The match qos-group command is not supported in 
Ethernet over MPLS.