296 Chapter 4: Congestion Management

would be discarded due to policing, because the cumulative rate for all traffic would exceed 400 kbps. The policer would have no way to decide to discard just the extra voice, but not video, because the policer acts on all traffic in the class. However, with the two lower-latency queues configuration, only the voice calls would lose packets due to policing, and the video conference would not have any packets dropped by the policer.

In effect, with multiple low-latency queues, you get more granularity in what you police. In fact, the most typical reason to use multiple low-latency queues is to support voice in one queue, and video in another.

Queuing does not differ when comparing using a single low-latency queue with multiple lowlatency queues in a single policy map. IOS always takes packets from the low-latency queues first, as compared with the non-low-latency queues (those with a bandwidth command), just like before. However, IOS does not reorder packets between the various low-latency queues inside the policy map. In other words, IOS treats all traffic in all the low-latency queues with FIFO logic.

In short, using multiple low-latency queues in one policy map does enable you to police traffic more granularly, but it does not reorder packets among the various low-latency queues.

Unfortunately, the DQOS and QoS courses and exams disagree with each other about where you can have more than one low-latency queue. The DQOS course states that only one lowlatency queue is allowed in a single policy map, whereas the QoS course states that multiple low-latency queues are allowed. So, you may talk to colleagues who have seen some Cisco documentation, or taken the DQOS class, and swear that only one low-latency queue is allowed. Thankfully, the chances are low that you would see a question distinguishing this fine point. However, you should be aware of the differences in documentation and the courses, so we made it a point to draw attention to the differences here.

IP RTP Priority

Before Cisco created LLQ, Cisco created two other tools that added a single priority queue to another queuing tool. IP RTP Reserve was the first of these two tools to be introduced, followed by the IP RTP Priority feature. With current IOS releases, Cisco recommends not using these older tools, but instead suggests using LLQ. However, because IP RTP Priority is currently on QoS exams, a brief explanation is warranted.

IP RTP Priority provides many features like LLQ, but with a few differences. It adds a PQ to either WFQ or to CBWFQ. It classifies VoIP payload traffic by classifying packets in a range of UDP port numbers, from which it only selects traffic from the even-numbered (VoIP payload) ports. Like LLQ, the PQ created by IP RTP Priority always gets serviced first, but the bandwidth reserved for this queue is policed to prevent it from taking over all bandwidth. Table 4-14 summarizes the main features and compares the features with LLQ.

IP RTP Priority Configuration

To configure IP RTP Priority, you just add one additional configuration command to an interface that already uses WFQ. There are no additional show commands to list information about IP RTP Priority beyond the commands that apply to the underlying queuing tools (WFQ and CBWFQ). Table 4-15 lists the two configuration commands, and Example 4-9 lists several example configurations.

Table 4-15 Command Reference for IP RTP Priority

! Example 1

interface serial 1/0

description this interface will use WFQ and IP RTP Priority, with 30 kbps reserved

encapsulation hdlc

fair-queue

continues