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The network must support Resource Reservation Protocol (RSVP) before the Control Plane DSCP Support for RSVP feature is enabled.

Control plane DSCP support for RSVP can be configured on interfaces and subinterfaces only. It affects all RSVP messages that are sent out on the interface or that are present on any logical circuit of the interface, including subinterfaces, permanent virtual circuits (PVCs), and switched virtual circuits (SVCs).

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 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 or differentiated services (DiffServ) field in the 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), which is changed ito 5 to give the message a higher priority, and it shows 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.

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

Router> enable

Router# configure terminal

Router(config)# interface gigbitEthernet 0/0/0

Router(config-if)# ip rsvp bandwidth 23 43

Router> enable

Router# configure terminal

Router(config)# interface gigbitEthernet 0/1/0

Router(config-if)# ip rsvp signalling dscp 10

Router# show
 ip rsvp interface detail
Gi0/0/0:
   RSVP: Disabled
   Interface State: N/A
   Bandwidth:
     Curr allocated: 0 bits/sec
     Max. allowed (total): 0 bits/sec
     Max. allowed (per flow): 0 bits/sec
     Max. allowed for LSP tunnels using sub-pools (pool 1): 0 bits/sec
     Set aside by policy (total): 0 bits/sec
   Traffic Control:
     RSVP Data Packet Classification is ON
   Signalling:
     DSCP value used in RSVP msgs: 0x17
     Number of refresh intervals to enforce blockade state: 4
   Authentication: disabled
     Key chain:   <none>
     Type:        md5
     Window size: 1
     Challenge:   disabled 
   FRR Extension:
     Backup Path: Not Configured
   BFD Extension:
     State: Disabled
     Interval: Not Configured
   RSVP Hello Extension:
     State: Disabled
   RFC 3175 Aggregation: Disabled
     Role: exterior.