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Configuring Voice over IP
Configure RSVP for Voice
RSVP allows end systems to request a particular quality of service (QoS) from the network. Real-time
voice traffic requires network consistency. Without consistent QoS, real-time traffic can experience
jitter, insufficient bandwidth, delay variations, or information loss. RSVP works in conjunction with
current queuing mechanisms. It is up to the interface queuing mechanism (such as weighted fair queuing
or weighted random early detection) to implement the reservation.
RSVP can be equated to a dynamic access list for packet flows.
You should configure RSVP to ensure QoS if the following conditions exist in your network:
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Enable RSVP
To minimally configure RSVP for voice traffic, you must enable RSVP on each interface where priority
needs to be set.
By default, RSVP is disabled so that it is backward compatible with systems that do not implement
RSVP. To enable RSVP on an interface, use the following command in interface configuration mode:
ip rsvp bandwidth [interface-kbps] [single-flow-kbps]
This command starts RSVP and sets the bandwidth and single-flow limits. The default maximum
bandwidth is up to 75 percent of the bandwidth available on the interface. By default, the amount
reservable by a flow can be up to the entire reservable bandwidth.
On subinterfaces, this command applies the more restrictive of the available bandwidths of the physical
interface and the subinterface.
Reservations on individual circuits that do not exceed the single-flow limit normally succeed. If,
however, reservations have been made on other circuits adding up to the line speed, and a reservation is
made on a subinterface which itself has enough remaining bandwidth, it will still be refused because the
physical interface lacks supporting bandwidth.
Cisco 7200 series routers, Cisco 7200 VXR routers, Cisco 7301 router, Cisco 7401ASR routers, and
Cisco 7500 series routers running VoIP and configured for RSVP request allocations according to the
following formula:
bps = packet_size + ip/udp/rtp header size * 50 per second
For G.729, the allocation is 24,000 bps. For G.711, the allocation is 80,000 bps.
T1/E1 Digital Voice Port Adapter Installation and Configuration
4-10
Small-scale voice network implementation
Slow links
Links with high utilization
Links less than 2 Mbps
Need for the best possible voice quality
Chapter 4
Configuring the PA-VXA, PA-VXB, and PA-VXC
OL-3592-02
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Troubleshooting
