Cisco ASR 9000 Series Configuration Manual page 13

Broadband Network Gateway Overview
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 (DSLAM).
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.
Establishing Subscriber Sessions
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:
• PPPoE subscriber session—The PPP over Ethernet (PPPoE) subscriber session is established using the
point-to-point (PPP) protocol that runs between the CPE and BNG.
• IPoE subscriber 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 DHCP protocol.
Interacting 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:
• authenticate a subscriber before establishing a subscriber session
• authorize the 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, see
BNG supports 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.
During interactions 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.
Interacting 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 CPE.
During the interaction between BNG and the DHCP server, BNG acts as a DHCP relay or DHCP proxy.
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.
OL-26148-02
RADIUS Attributes, on page
Cisco ASR 9000 Series Aggregation Services Router Broadband Network Gateway Configuration Guide, Release
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BNG Architecture
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