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Figure 8-33 RSVP Packet-Classification Criteria

It is important to note that RSVP classifies only voice bearer traffic, not signaling traffic. Another classification mechanism such as an ACL or DiffServ code point (DSCP) / IP precedence must still be used to classify the voice-signaling traffic if any treatment better than best effort is desired for signaling traffic.

Bandwidth per Codec

Both LLQ and RSVP see the Layer 3 IP packet. Layer 2 encapsulations (FR, MLPPP, and so on) are added after queuing, so the bandwidth allocated by both LLQ and RSVP for a call is based on the Layer 3 bandwidth of the packets. This number is slightly different from the actual bandwidth used on the interface after Layer 2 headers and trailers have been incorporated. RSVP bandwidth reserved for a call also excludes both cRTP and VAD. Table 8-21 summarizes the bandwidth RSVP allocates for calls using different Cisco IOS gateway codecs.

Table 8-21 RSVP Bandwidth Reservations for Voice Codecs

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When a DSBM client sends or forwards an RSVP path message over an interface attached to a managed segment, it sends the path message to the DSBM of the segment rather than to the RSVP session destination address, as is done in conventional RSVP processing. As part of its message-processing procedure, the DSBM builds and maintains a path state for the session and notes the previous Layer 2 or Layer 3 hop from which it received the path message. After processing the path message, the DSBM forwards it toward its destination address.

The DSBM receives the RSVP resv message and processes it in a manner similar to how RSVP handles reservation request processing, basing the outcome on available bandwidth. The procedure is as follows:

•If it cannot grant the request because of lack of resources, the DSBM returns a resverror message to the requester.

•If sufficient resources are available and the DSBM can grant the reservation request, it forwards the resv message toward the previous hops using the local path state for the session.

RSVP Configuration

The following three tasks are performed on a gateway to originate or terminate voice traffic using RSVP:

1Turn on the synchronization feature between RSVP and H.323. This is a global command and is turned on by default when Cisco IOS Release 12.1(5)T or later is loaded.

2Configure RSVP on both the originating and terminating sides of the VoIP dial peers. Configure both the requested QoS (req-qos) and the acceptable QoS (acc-qos) commands. The guaranteed-delay option must be chosen for RSVP to act as a CAC mechanism. Other combinations of parameters may lead to a reservation, but not offer CAC.

3Enable RSVP and specify the maximum bandwidth on the interfaces that the call will traverse.

Table 8-22 lists the commands used to define and enable RSVP.

Table 8-22 RSVP Profile, req-qos and acc-qos Commands

Resource-Based CAC 619

Table 8-22 RSVP Profile, req-qos and acc-qos Commands (Continued)

Example 8-17 demonstrates the configuration required to enable RSVP with LLQ.

Example 8-17 Enabling RSVP with LLQ

!Global command enabling RSVP as CAC, turned on by default.

!

!RSVP classification default profile for Cisco VoIP packets ip rsvp pq-profile voice-like

ip rsvp bandwidth 200 25

!

interface serial 0/0 service-policy output LLQ-policy

!

voice-port 1/0:0

!

dial-peer voice 100 pots destination-pattern 2......

port 1/0:0

!

dial-peer voice 300 voip destination-pattern 3......

session target ipv4:10.10.2.2

!Configures RSVP CAC for voice calls using the dial peer. req-qos guaranteed-delay

acc-qos guaranteed-delay

In the example, the call rsvp-sync command enables synchronization of H.323 and RSVP, which allows new call requests to reserve bandwidth using RSPV. The ip rsvp pq-profile command tells IOS to classify voice packets into a low-latency queue priority queue, assuming LLQ is configured on the same interface as RSVP. On interface serial 0/0, a service-policy command enables a policy map that defines LLQ, and the ip rsvp bandwidth command reserves a total of 200 kbps, with a per-reservation limit of 25 kbps. The req-qos and acc-qos commands tell IOS to make RSVP reservation requests when new call requests have been made.