AL; Reviewed:
SPOC 8/24/2006
Solution & Interoperability Test Lab Application Notes
©2006 Avaya Inc. All Rights Reserved.
2 of 23
JNPR-M7i-J4300
1. Introduction
These Application Notes describe a solution for configuring the Juniper Networks M7i and J4300
routers to support an Avaya IP Telephony infrastructure. A Main site and a Branch site each have an
Avaya Communication Manager server. The sites are connected together via a compressed Real-
Time Protocol (cRTP) enabled Point-to-Point Protocol (PPP) connection. An H.323 trunk is
configured to route calls between the two sites.
It is common to use the G.729 codec for calls across a low speed link to conserve valuable
bandwidth. The bandwidth per G.729 call is about 26.4 kbps over a Point-to-Point protocol link.
The bandwidth can be further reduced to around 12 kbps with cRTP. The cRTP feature is based on
RFC 2508. cRTP compresses the IP/UDP/RTP packet header from 40 bytes to 2 or 4 bytes. For a
G.729 call, these headers make up a considerable amount of overhead, relative to the G.729 voice
sample payload. As a result, the cRTP feature offers significant VoIP bandwidth savings when
using a low bandwidth codec such as G.729, but less percentage saving when using higher
bandwidth such as G.711.
Two methods were used in providing the necessary Quality of Service (QoS) for the VoIP traffic.
The Juniper routers first use the DiffServ Code-Point (DSCP) information embedded in the layer-3
IP header to prioritize VoIP traffic, and then use bandwidth reservation to guarantee the required
bandwidth for VoIP traffic. The combination of traffic prioritization and bandwidth reservation
ensure that VoIP traffic passes through the Wide Area Network (WAN) connection successfully in
the presence of network congestion.
In addition the Juniper Networks J4300 router has the capability of supporting Dynamic Host
Configuration Protocol (DHCP) server functionality necessary to support option 176 which is used
by the Avaya IP Telephones. This functionality was implemented in the network configuration to
support all of the Avaya IP Telephones.
