Nov 13, 2007

MPLS AToM: Overview

Feature Overview
Any Transport over MPLS (AToM) is a solution for transporting Layer 2 packets over an MPLS backbone. AToM enables service providers to supply connectivity between customer sites with existing data link layer (Layer 2) networks by using a single, integrated, packet-based network infrastructure — a Cisco MPLS network. Instead of separate networks with network management environments, service providers can deliver Layer 2 connections over an MPLS backbone.

With Cisco AToM technology, provisioning and connecting is straightforward. A customer using Ethernet in a building or campus in one location can connect through a service provider offering Ethernet over MPLS to the customer's Ethernet networks in remote locations.

AToM provides a common framework to encapsulate and transport supported Layer 2 traffic types over an MPLS network core. Service providers can use a single MPLS network infrastructure to offer customers connectivity for supported Layer 2 traffic, as well as customers' IP traffic in Layer 3 VPNs.

AToM supports the following transport types:

•ATM AAL5 over MPLS

•ATM Cell Relay over MPLS

•Ethernet VLAN over MPLS

•Frame Relay over MPLS

•PPP over MPLS

•HDLC over MPLS

ATM AAL5 over MPLS
How ATM AAL5 SDUs Move Between PE Routers
ATM AAL5 over MPLS encapsulates ATM AAL5 service data units (SDUs) in MPLS packets and forwards them across the MPLS network. Each AAL5 SDU is transported as a single packet. The following steps outline the process of encapsulating the SDU.

Ingress PE router

1. An ingress provider edge (PE) router receives an ATM AAL5 SDU and removes the header.

2. The PE router copies the control word elements from the header to the corresponding fields in the control word of the SDU. The control word contains:

–Explicit forward congestion indication (EFCI) bit—Used by ATM switches to indicate congestion experienced by forwarded data cells.

–Cell loss priority (CLP) bit—Indicates whether a cell should be dropped if it encounters extreme congestion as it moves through the ATM network.

3. The PE router adds a virtual circuit (VC) label and a label switched path (LSP) tunnel label to the packet for normal MPLS routing through the MPLS backbone. The core routers use the LSP tunnel label to move the packet through the MPLS backbone. A core router does not distinguish ATM AAL5 traffic from other types of traffic. The packet is handled just like other packets in the MPLS backbone.

Egress PE router

1. At the other edge of the MPLS backbone, the egress PE router receives the packet and copies the control word elements from the control word to the header.

2. The PE router removes the VC label and LSP tunnel label if one is present. If no LSP tunnel label is present, it is because the penultimate router removed that label.

3. The PE router adds an AAL5 header and sends the packet out the appropriate customer-facing interface.

Figure 1 illustrates this process.

Figure 1 ATM AAL5 Packets as They Traverse the MPLS Backbone



AAL5 Packets Containing OAM Cells
The Cisco 7200 and 7500 series routers support the transport of F5 end-to-end operational, administrative, and maintenance (OAM) cells. Only Mode 0 is supported. F5 OAM cells are transported over the MPLS backbone with the payload. The OAM cell fits into the payload of a single AAL5 packet.

Notes:

•PVC switching is not supported in OAM encapsulation.

•Both PE routers must be configured with the same VPI/VCI value.

•OAM transparency is not supported on the Cisco 12000 series routers.

OAM Cell Emulation
Supported Platforms:

This functionality is supported on the following platforms:

•Cisco 7200 series routers

•Cisco 7500 series routers

If a PE router does not support the transport of OAM cells across an LSP, you can use OAM cell emulation to locally terminate or loopback the OAM cells. You configure OAM cell emulation on both PE routers, which emulates a VC by forming two unidirectional LSPs. You use the oam-ac emulation-enable command on both PE routers to enable OAM cell emulation.

After OAM cell emulation is enabled on a router, you can configure and manage the ATM VC in the same manner as you would a terminated VC. A VC that has been configured with OAM cell emulation can send loopback cells at configured intervals toward the local CE router. The endpoint can be either of the following:

•End-to-end loopback, which sends OAM cells to the local CE router.

•Segment loopback, which responds to OAM cells to a device along the path between the PE and CE routers.

The OAM cells include the following:

•Alarm indication signal (AIS)

•Remote defect indication (RDI)

These cells identify and report defects along a VC. When a physical link or interface failure occurs, intermediate nodes insert OAM AIS cells into all the downstream devices affected by the failure. When a router receives an AIS cell, it marks the ATM VC down and sends an RDI cell to let the remote end know about the failure.

See the Configure OAM Cell Emulation for ATM AAL5 over MPLS section for information on configuring OAM cell emulation.

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