20-2
Cisco 10000 Series Router Software Configuration Guide
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Chapter 20 Configuring L2 Virtual Private Networks
Using the Label Distribution Protocol (LDP), an AToM circuit session is identified by a unique VC
(virtual circuit) between two PE routers. When a Layer 2 frame is received by the imposition PE router,
it is encapsulated in an MPLS packet with a VC label, IGP label, and possibly other labels. When the
MPLS packet reaches the disposition PE router, the packet is converted back into its Layer 2
encapsulation.
AToM encapsulates Layer 2 frames at the ingress (or imposition) provider edge (PE) router, and sends
them to a corresponding PE router at the other end of the connection. The corresponding router is the
egress (or disposition) PE router, and it removes the encapsulation and sends out the Layer 2 frame.
The successful transmission of the Layer 2 frames between PE routers is due to the configuration of the
PE routers. You set up the connection, called a
pseudowire
, between the routers. An AToM circuit is one
type of pseudowire connection.
Benefits of Enabling Layer 2 Packets to Send in an MPLS Network
Some of the benefits of enabling Layer 2 packets to be sent in the MPLS network include:
•
The AToM product set accommodates many types of Layer 2 packets, including Ethernet and Frame
Relay, 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 the
“Standards and RFCs” section on page 20-5
for the specific standards that AToM follows.) This
benefits the service provider who wants to incorporate industry-standard methodologies in the
network. Other Layer 2 solutions are proprietary, which can limit the service provider’s ability to
expand the network and can force the service provider to use only one vendor’s equipment.
•
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.
A control word (also referred to as a
shim
header) can be added at the imposition router and, if so, this
control word is removed at the disposition router.
Cisco 10000 series router supports up to 8000 attachment circuits (ACs). An AToM circuit use one AC
and a LS circuit use two ACs. Therefore, Cisco 10000 series router supports 8000 AToM connections
or 4000 LS connections or any combination of both AToM and LS connections that sums up to 8000
ACs. Also, Tunnel selection allows you to specify the path that AToM traffic uses. See the
“Any
Transport over MPLS—Tunnel Selection” section on page 20-47
.
This chapter contains the following topics:
•
Feature History for L2VPN, page 20-3
•
Supported L2VPN Transport Types, page 20-3
•
Prerequisites for L2VPN: AToM, page 20-4
•
Restrictions for L2VPN, page 20-5
•
Standards and RFCs, page 20-5
•
MIBs, page 20-6
•
NSF and SSO—L2VPN, page 20-6
•
L2VPN Local Switching—HDLC/PPP, page 20-10
•
Configuration Tasks for L2VPN, page 20-12
•
Monitoring and Maintaining L2VPN, page 20-43
•
Configuration Example—Frame Relay over MPLS, page 20-44
