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Configuring Voice over IP
To configure Multilink PPP and interleaving on a configured and operational interface or virtual
interface template, use the following commands in interface mode:
Command
Step 1
ppp multilink
Step 2
ppp multilink interleave
Step 3
ppp multilink fragment-delay
milliseconds
Step 4
ip rtp reserve lowest-UDP-port
range-of-ports
[maximum-bandwidth]
Note
The ip rtp reserve command can be used instead of configuring RSVP. If you configure RSVP, this
command is not required.
For more information about Multilink PPP, refer to the "Configuring Media-Independent PPP and
Multilink PPP" chapter in the Cisco IOS Release 12.0 Dial Solutions Configuration Guide.
Multilink PPP Configuration Example
The following example defines a virtual interface template that enables Multilink PPP with interleaving
and a maximum real-time traffic delay of 20 milliseconds, and then applies that virtual template to the
Multilink PPP bundle:
interface virtual-template 1
ppp multilink
encapsulated ppp
ppp multilink interleave
ppp multilink fragment-delay 20
ip rtp reserve 16384 100 64
multilink virtual-template 1
Configure RTP Header Compression
Real-Time Transport Protocol (RTP) is used for carrying packetized audio traffic over an IP network.
RTP header compression compresses the IP/UDP/RTP header in an RTP data packet from 40 bytes to
approximately 2 to 4 bytes (most of the time), as shown in
This compression feature is beneficial if you are running VoIP over slow links. Enabling compression
on both ends of a low-bandwidth serial link can greatly reduce the network overhead if there is a lot of
RTP traffic on that slow link.
Typically, an RTP packet has a payload of approximately 20 to 160 bytes for audio applications that use
compressed payloads. RTP header compression is especially beneficial when the RTP payload size is
small (for example, compressed audio payloads between 20 and 50 bytes).
T1/E1 Digital Voice Port Adapter Installation and Configuration
4-12
Chapter 4
Purpose
Enable Multilink PPP.
Enable real-time packet interleaving.
Optionally, configure a maximum fragment delay.
Reserve a special queue for real-time packet flows to specified
destination User Datagram Protocol (UDP) ports, allowing
real-time traffic to have higher priority than other flows. This is
only applicable if you have not configured RSVP.
Figure
Configuring the PA-VXA, PA-VXB, and PA-VXC
4-1.
OL-3592-02
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Troubleshooting
