# Overview

Access Node Control Protocol (ANCP), also known as Layer 2 Control (L2C), is based on a subset of the General Switch Management Protocol (GSMP), as defined in the GSMPv3 Base Specification (draft-ietf-gsmp-v3-base-spec-06.txt). GSMP is a general purpose protocol used to control a label switch.

GSMP enables a controller to establish and release connections across the switch, add and delete leaves on a multicast connection, manage switch ports, request configuration information, request and delete reservation of switch resources, and request statistics. It also enables the switch to inform the controller of asynchronous events such as a link going down.

Deploying value-added services across digital subscriber line (DSL) access networks requires special attention to quality of service (QoS) and service control. This control depends on tighter coordination between network elements in the broadband access network while not causing added burden to the operations support system (OSS) layer.

ANCP is an extension to GSMPv3 that functions as a control plane between a service-oriented layer 3 edge device (the Broadband Remote Access Server) and a layer 2 access node. In this role, ANCP performs QoS-related, service-related, and subscriber-related operations. These operations include the following:

- Dynamic discovery of the access topology and enabling an authentication server to retrieve this information
- Subscriber and service data retrieval from the OSS by the Broadband Remote Access Server (B-RAS) and sending that information to the access node to simplify service management (referred to as line configuration)
- An optimized, layer 2 multicast (IGMP) replication
- On-demand access-line testing (ANCP Operation, Administration, and Maintenance)

JunosE Software supports the use of RADIUS attributes to monitor ANCP-related information, such as upstream and downstream data rates. For information about using RADIUS attributes see JunosE Broadband Access Configuration Guide.

## Access Topology Discovery

Many queuing or scheduling mechanisms must avoid congestion within the access network while contending with multiple flows and distinct QoS requirements. These mechanisms require that B-RAS devices obtain information about the access network topology, the links within that network, and their rates.

Operations support systems cannot enforce the consistency of this gathered information in a reliable and scalable way. ANCP discovery enables the automated discovery of the access network topology, resolving this problem.

## Line Configuration

Following access topology discovery, the B-RAS can query a subscriber management OSS component (for example, a RADIUS server) to retrieve subscriber authorization data. This type of query is typically managed by the B-RAS, but in some cases (for example, DSL-related enforcement) it can be useful to push service information to the access node for local enforcement of the corresponding subscriber line. Using the line configuration feature provides a more flexible way to achieve this on-demand service.

| Note: JunosE Software supports only a CLI version of this feature.

## Transactional Multicast

IP multicasting in access networks often involves an access server replicating the same multicast stream to multiple subscribers. This type of replication wastes access bandwidth when multiple subscribers access network services using the same access node. The amount of multicast replication is based on the number of subscribers, rather than the number of access nodes.

The access node sending a single copy of the multicast stream to a specific access node is a more efficient use of the bandwidth. Using this method, the access node performs the multicast replication for subscribers that reside beyond the access node.

ANCP transactional multicast enables the E Series router to set up a multicast replication state in the access node. In Asynchronous Transfer Mode (ATM) access networks, the B-RAS can use the ANCP to set up point-to-multipoint cross-connects in the access nodes.

## OAM

A simple solution based on ANCP provides B-RAS with an access-line test capability. When enabled through the CLI, the B-RAS uses an ANCP message to trigger the access-node to perform a loopback test on the local loop (between the access node and the CPE). The access node reports the results of the test by means of another ANCP message.

## Retrieval of DSL Line Rate Parameters

The router retrieves the DSL line rate parameters from ANCP and reports this information to the SRC software with the corresponding COPS messages. If the router cannot retrieve the DSL line rate parameters from ANCP, it retrieves the DSL information in the following ways:

- From AAA layer—For PPP interfaces, the router retrieves the DSL line rate parameters from the AAA layer and reports this information to the SRC software.
- From DHCP options—For DHCP external server and DHCP local server in equal-access mode, the router retrieves the DSL line rate parameters from DHCP options and reports this information to the SRC software. For more information about retrieval of DSL line rate parameters from DHCP options, see DHCP Overview Information.

## Learning the Partition ID from an Access Node

ANCP running on the network access server can learn the partition ID from an access node and use this partition ID to communicate with the access node. This learning option of the ANCP in the virtual router enables the network access server to learn the partition IDs from all the access nodes. This learned partition ID can be used to send messages to establish adjacency.