Configure RSVP for Voice

Resource Reservation Protocol (RSVP) enables routers to reserve enough bandwidth on an interface for reliability and quality performance. RSVP allows end systems to request a particular 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 WRED) to implement the reservation.

RSVP works well on PPP, HDLC, and similar serial line interfaces. It does not work well on multi-access LANs. RSVP can be equated to a dynamic access list for packet flows.

You should configure RSVP to ensure QoS if the following conditions describe your network:

• Small scale voice network implementation
• Links slower than 2 Mbps
• Links with high utilization
• Need for the best possible voice quality

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 backwards compatible with systems that do not implement RSVP. To enable RSVP for IP on an interface, use the following interface configuration command:

Router(config-if)# 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, RSVP applies to 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. However, if reservations have been made on other circuits adding up to the line speed, and a reservation is made on a subinterface that itself has enough remaining bandwidth, it will still be refused because the physical interface lacks supporting bandwidth.

A Cisco 1750 running VoIP and configured for RSVP requests allocations using the following formula:

bps=packet_size+ip/udp/rtp header size * 50 per second

For G.729, the allocation works out to be 24,000 bps. For G.711, the allocation is 80,000 bps.

For more information about configuring RSVP, refer to the "Configuring RSVP" chapter of the Network Protocols Configuration Guide, Part 1 for Cisco IOS Release 12.0T.

RSVP Configuration Example:

The following example enables RSVP and sets the maximum bandwidth to 100 kbps and the maximum bandwidth per single request to 32 kbps (the example presumes that both VoIP dial peers have been configured):

Router(config)# interface serial 0/0
Router(config-if)# ip rsvp bandwidth 100 32
Router(config-if)# fair-queue
Router(config-if)# end

After enabling RSVP, you must also use the req-qos dial-peer configuration command to request an RSVP session on each VoIP dial peer. Otherwise, no bandwidth is reserved for voice traffic.

Router(config)# dial-peer voice 211 voip
Router(config-dial-peer)# req-qos controlled-load
Router(config)# dial-peer voice 212 voip
Router(config-dial-peer)# req-qos controlled-load