Classification for Voice Packets into LLQ

As you learned in previous chapters, LLQ is one of the most important Cisco QoS mechanisms to ensure quality for voice conversations, because it prioritizes voice packets over data packets at the router egress interface. For this to work, voice packets must be classified such that they are placed in the priority queue (PQ) portion of LLQ.

Cisco IOS provides the service levels that RSVP accepts by working in conjunction with IOS queuing tools. As a general Cisco IOS feature, RSVP has its own set of reserved queues within Class-Based Weighted Fair Queuing (CBWFQ) for traffic with RSVP reservations. So, RSVP can create hidden queues that compete with the explicitly defined CBWFQ queues, with the RSVP queues getting very low weights—and remember, with all WFQ-like features, lower weight means better service.

You would just configure CBWFQ, and then RSVP, and not consider the two features together. However, the performance of the voice flows with RSVP reservations would actually suffer. The reason is that the reserved hidden RSVP queues, although they have a low weight, are separate from the PQ feature of CBWFQ/LLQ. Packets in reserved RSVP queues do not get strict priority over packets from other queues. It has long been known that this treatment, a low-weight queue inside WFQ, is insufficient for voice quality over a congested interface. Therefore, when RSVP is configured for a voice call, the voice packets need to be classified into the PQ.

IOS solves the problem by having RSVP put voice-like traffic into the existing LLQ priority queue on the interface. RSVP uses a profile to classify a flow of packets as a voice flow. The profile considers packet sizes, arrival rates, and other parameters to determine whether a packet flow is considered a voice flow. The internal profile, named voice-like, is tuned to classify all voice traffic originating from a Cisco IOS gateway as a voice flow without the need for additional configuration. Therefore, while RSVP makes the reservations, it then in turn classifies voice-like traffic into the LLQ PQ, and other non-voice-like traffic with reservations into RSVP queues, as shown in Figure 8-33.

To perform the extra classification logic shown in the figure, RSVP is the first egress interface classifier to examine an arriving packet. If RSVP considers the packet a voice flow, the packet is put into the PQ portion of LLQ. If the flow does not conform to the voice profile voice-like, but is nevertheless an RSVP reserved flow, it is placed into the normal RSVP reserved queues. If the flow is neither a voice flow, nor a data RSVP flow, LLQ classifies the packet as it normally would.

Although RSVP voice traffic can be mixed with traffic specified in the priority class within a policy map, voice quality can suffer if both methods are implemented simultaneously. The two methods do not share bandwidth allocation and therefore will lead to an inefficient use of bandwidth on the interface. As bandwidth is defined in the configuration for the egress interfaces, all the bandwidth and priority classes will be allocated bandwidth at configuration time. No bandwidth is allocated to RSVP at configuration time because RSVP requests its bandwidth when the traffic flow begins. RSVP therefore gets allocated bandwidth from the pool that is left after the other features have already allocated their bandwidth.

Figure 8-33 RSVP Packet-Classification Criteria

Egress Interface / PVC Queueing (LLQ)

Figure 8-33 RSVP Packet-Classification Criteria

Egress Interface / PVC Queueing (LLQ)

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.

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