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Configuring IP Networks for Real-Time Voice Traffic

Release 12.0(7)XK

Note Multilink PPP should not be used on links greater than 2 Mbps.

Multilink PPP support for interleaving can be conﬁgured on virtual templates, dialer interfaces, and

ISDN BRI or PRI interfaces. To conﬁgure interleaving, you need to complete the following tasks:

• Conﬁgure the dialer interface or virtual template, as deﬁned in the relevant chapters of the

Cisco IOS 12.0 Dial Solutions Conﬁguration Guide.

• Conﬁgure Multilink PPP and interleaving on the interface or template.

To conﬁgure Multilink PPP and interleaving on a conﬁgured and operational interface or virtual

interface template, use the following commands in interface mode:

For more information about Multilink PPP, refer to the “Conﬁguring Media-Independent PPP and

Multilink PPP” chapter in the 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 priority 16384 16383 25

multilink virtual-template 1

Conﬁguring RTP Header Compression

Real-Time Transport Protocol (RTP)isusedforcarrying packetized audio trafﬁcoveran 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 Figure 1.

This compression feature is beneﬁcial if you are running Voice over IP 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).

Step Command Purpose

1 router(config-if)# ppp multilink Enable Multilink PPP.

2 router(config-if)# ppp multilink interleave Enable real-time packet interleaving.

3 router(config-if)# ppp multilink fragment-delay

milliseconds

Optionally, conﬁgure a maximum fragment delay.

router(config-if)# ip rtp priority

starting-rtp-port-number port-number-range

bandwidth

Reserve a strict priority queue for a set of RTP packet ﬂows

belonging to a range of UDP destination ports

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