The goal of the BNG
architecture is to enable the BNG router to interact with peripheral devices
(like CPE) and servers (like AAA and DHCP), in order to provide broadband
connectivity to subscribers and manage subscriber sessions. The basic BNG
architecture is shown in the following figure.
Figure 1. BNG
The BNG architecture
is designed to perform these tasks:
the Customer Premise Equipment (CPE) that needs to be served broadband
subscriber sessions using IPoE or PPPoE protocols.
the AAA server that authenticates subscribers, and keeps an account of
the DHCP server to provide IP address to clients.
The four BNG tasks are
briefly explained in the following sections.
BNG connects to the
CPE through a multiplexer and Home Gateway (HG). The CPE represents the triple
play service in telecommunications, namely, voice (phone), video (set top box),
and data (PC). The individual subscriber devices connect to the HG. In this
example, the subscriber connects to the network over a Digital Subscriber Line
(DSL) connection. Therefore, the HG connects into a DSL Access Multiplexer
Multiple HGs can
connect to a single DSLAM that sends the aggregated traffic to the BNG router.
The BNG router routes traffic between the broadband remote access devices (like
DSLAM or Ethernet Aggregation Switch) and the service provider network.
Each subscriber (or
more specifically, an application running on the CPE) connects to the network
by a logical session. Based on the protocol used, subscriber sessions are
classified into two types:
session—The PPP over Ethernet (PPPoE) subscriber session is established using
the point-to-point (PPP) protocol that runs between the CPE and BNG.
session—The IP over Ethernet (IPoE) subscriber session is established using IP
protocol that runs between the CPE and BNG; IP addressing is done using the
with the RADIUS Server
BNG relies on an
external Remote Authentication Dial-In User Service (RADIUS) server to provide
subscriber Authentication, Authorization, and Accounting (AAA) functions.
During the AAA process, BNG uses RADIUS to:
subscriber before establishing a subscriber session
subscriber to access specific network services or resources
track usage of
broadband services for accounting or billing
The RADIUS server
contains a complete database of all subscribers of a service provider, and
provides subscriber data updates to the BNG in the form of attributes within
RADIUS messages. BNG, on the other hand, provides session usage (accounting)
information to the RADIUS server. For more information about RADIUS attributes,
connections with more than one RADIUS server to have fail over redundancy in
the AAA process. For example, if RADIUS server A is active, then BNG directs
all messages to the RADIUS server A. If the communication with RADIUS server A
is lost, BNG redirects all messages to RADIUS server B.
between the BNG and RADIUS servers, BNG performs load balancing in a
round-robin manner. During the load balancing process, BNG sends AAA processing
requests to RADIUS server A only if it has the bandwidth to do the processing.
Else, the request is send to RADIUS server B.
with the DHCP Server
BNG relies on an
external Dynamic Host Configuration Protocol (DHCP) server for address
allocation and client configuration functions. BNG can connect to more than one
DHCP server to have fail over redundancy in the addressing process. The DHCP
server contains an IP address pool, from which it allocates addresses to the
interaction between BNG and the DHCP server, BNG acts as a DHCP relay or DHCP
As the DHCP relay,
BNG receives DHCP broadcasts from the client CPE, and forwards the request to
the DHCP server.
As the DHCP proxy,
BNG itself maintains the address pool by acquiring it from DHCP server, and
also manages the IP address lease. BNG communicates on Layer 2 with the client
Home Gateway, and on Layer 3 with the DHCP server.
The DSLAM modifies
the DHCP packets by inserting subscriber identification information. BNG uses
the identification information inserted by the DSLAM, as well as the address
assigned by the DHCP server, to identify the subscriber on the network, and
monitor the IP address lease.