Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Typically, networks operate on a best-effort delivery basis, which means that all traffic has equal priority and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an equal chance of being dropped.

Before traffic can be handled according to its unique requirements, it must be identified or labeled. There are numerous classification techniques for doing this. These include Layer 3 schemes such as IP precedence or the differentiated services code point (DSCP), Layer 2 schemes such as 802.1P, and implicit characteristics of the data itself, such as the traffic type using the Real-Time Transport Protocol (RTP) and a defined port range.

The control plane DSCP support for RSVP feature allows you to set the priority value in the type of service (ToS) byte/differentiated services (DiffServ) field in the Internet Protocol (IP) header for RSVP messages. The IP header functions with resource providers such as weighted fair queueing (WFQ), so that voice frames have priority over data fragments and data frames. When packets arrive in a router's output queue, the voice packets are placed ahead of the data frames.

The figure below shows a path message originating from a sender with a DSCP value of 0 (the default) that is changed to 5 to give the message a higher priority and a reservation (resv) message originating from a receiver with a DSCP of 3.

Figure 1

Control Plane DSCP Support for RSVP

Raising the DSCP value reduces the possibility of packets being dropped, thereby improving call setup time in VoIP environments.

• Benefits
  
• Restrictions
  

• Cisco 2600 series
• Cisco 3600 series (Cisco 3620, 3640, and 3660)
• Cisco 3810 multiservice access concentrator
• Cisco 7200 series
• Cisco 7500 route/switch processor (RSP) only
• Cisco 12000 series Gigabit Switch Router (GSR)

The network must support the following Cisco IOS feature before control plane DSCP support for RSVP is enabled:

• Resource Reservation Protocol (RSVP)

• Enabling RSVP on an Interface
  
• Specifying the DSCP
  
• Verifying Control Plane DSCP Support for RSVP Configuration
  

To monitor and maintain control plane DSCP support for RSVP, use the following command in EXEC mode:

Router# show ip rsvp interface detail

CBWFQ-- Class-based weighted fair queueing. A queueing mechanism that extends the standard WFQ functionality to provide support for user-defined traffic classes.

class-based weighted fair queueing --See CBWFQ.

differentiated services --See DiffServ.

differentiated services code point --See DSCP.

DiffServ --An architecture based on a simple model where traffic entering a network is classified and possibly conditioned at the boundaries of the network. The class of traffic is then identified with a DS codepoint or bit marking in the IP header. Within the core of the network, packets are forwarded according to the per-hop behavior associated with the DS code point.

DSCP --Differentiated services code point. The six most significant bits of the 1-byte IP type of service (ToS) field. The per-hop behavior represented by a particular DSCP value is configurable. DSCP values range between 0 and 63.

IP precedence --The three most significant bits of the 1-byte type of service (ToS) field. IP precedence values range between zero for low priority and seven for high priority.

latency --The delay between the time a device receives a packet and the time that packet is forwarded out the destination port.

marking --The process of setting a Layer 3 DSCP value in a packet.

QoS --Quality of service. A measure of performance for a transmission system that reflects its transmission quality and service availability.

quality of service --See QoS.

Resource Reservation Protocol --See RSVP.

RSVP --Resource Reservation Protocol. A protocol for reserving network resources to provide quality of service guarantees to application flows.

ToS --Type of service. An 8-bit value in the IP header field.

type of service --See ToS.

Voice over IP --See VoIP.

VoIP --Voice over IP. The ability to carry normal telephony-style voice over an IP-based internet maintaining telephone-like functionality, reliability, and voice quality.

weighted fair queueing --See WFQ.

weighted random early detection --See WRED.

WFQ --Weighted fair queueing. A queue management algorithm that provides a certain fraction of link bandwidth to each of several queues, based on relative bandwidth applied to each of the queues.

WRED --Weighted random early detection. A congestion avoidance mechanism that slows traffic by randomly dropping packets when there is congestion.