Advanced Queuing

Advanced queuing concepts typically combine one or more of the queuing methods previously discussed with new data-transfer techniques.

PQ-WFQ

PQ and WFQ have traditionally been the most popular ways to prioritize traffic (until the Voice over IP boom, that is).

The problem is that voice is highly time-sensitive, and although a router can examine various parts of a packet against an access list to see whether it is a voice packet, such a process take time.

PQ-WFQ is designed to add a priority function to a WFQ process, as shown in Figure 12.3. Essentially, it gives the router two WFQ systems, one for normal traffic and another for voice. The high-priority traffic is always serviced before the lower-priority traffic, but otherwise, there is no other prioritization. Everything is treated in a fair-queue fashion.

Figure 12.3. PQ-WFQ

You use the following command in interface configuration mode to tell the router to look for Real-Time Transport Protocol (RTP) packets and assign them priority:

 Router(config-if)#ip rtp priority starting-rtp-port-number port-number-range bandwidth 

The port number is the port that the voice application will begin using, and the range indicates how many ports can be used. The router first looks in the User Datagram Protocol (UDP) header to see whether it's a voice packet before digging deeper for the actual RTP header. If it doesn't see a port in the appropriate UDP port range, it treats the packet as any other traffic. The bandwidth option indicates how much bandwidth in Kbps can be used for the priority traffic.

Class-based WFQ

You would use class-based WFQ (CB-WFQ) for reasons similar to why you would use PQ-WFQ; the primary difference is the way traffic is handled. It too uses WFQ on different classes of traffic, but at the core of CB-WFQ lies the token bucket, which controls the rate at which data can cross the circuit, no matter how much bandwidth is available. When configured, a token bucket collects tokens that represent an amount of data transferred in a given time. Before data can be transferred, the sender needs to collect tokens out of the bucket, allowing it to send the data. This process ensures that the data being sent fits the proscribed bandwidth available. It can also be linked to a moving target, such as a Frame Relay committed information rate (CIR).

CB-WFQ Steps

You need take only a few steps when configuring CB-WFQ. First, you define the traffic that will apply. You typically do so by creating a map class and using an access list to strictly define the traffic. Another way uses protocol.

Next, you provide the traffic-specific information, such as average and peak bandwidth bit rates. You do so in a policy map. The policy map also contains the configured map class information. It's important to note that there can be several classes defined inside a single policy.

Last, you apply the whole thing to an interface. Before traffic of the specific type can leave the interface, the router needs to pull tokens out of the token bucket, representing how much data is crossing within a certain time frame.

Although you can configure CB-WFQ on serial, Asynchronous Transfer Mode (ATM), and Ethernet interfaces, among others, you cannot configure CB-WFQ on Ethernet subinterfaces, such as those routing virtual LAN (VLAN) information.

Configuring CB-WFQ

This section shows the commands that you can use to configure CB-WFQ and describes what each one is doing. The first line configures a map class called demo on the router and places the router into map class configuration mode:

 Router(config)#class-map demo 

The next command says that access list 102 will define the traffic that is interesting to this bandwidth segment:

 Router(config-cmap)#match access-group 102 

The following line defines a policy map on the router called circuit and places the router into policy map configuration mode:

 Router(config)#policy-map circuit 

The next command attaches the configured map class to the policy:

 Router(config-pmap)#class demo 

This line keeps the maximum bandwidth used at the CIR of 384,000 bits per second:

 Router(config-pmap-c)#shape average 384000 

The following command indicates how much bandwidth in Kbps is available for the traffic that matches the access list in the map class defined by demo:

 Router(config-pmap-c)#bandwidth 256 

The next command defines a second class called class-default. Not requiring a specific configuration, this class defines what to do with all other traffic that doesn't match an access list entry:

 Router(config-pmap-c)#class class-default 

The next command indicates that unclassified flows should be treated in a fashion consistent with WFQ:

 Router(config-pmap-c)#fair-queue 

The following line places the router into interface configuration mode for Serial 0/0:

 Router(config)#interface serial 0/0 

This command applies the policy to the interface. You can apply policies toward traffic either arriving or leaving:

 Router(config-if)#service-policy output circuit 

You can configure a number of other options as well. For example, you might want to define the precedence tag in the IP header for traffic that matches the access list entry. You could do so in the policy for the specific map class through the set ip precedence command and define an input policy on the interface instead of an output.

Low-Latency Queuing

The primary component missing from CB-WFQ is that of prioritization. There's no way to say that this one type of traffic always goes first without giving it a high amount of bandwidth. Prioritization with CB-WFQ directly depends on how much bandwidth you assign.

Designed for time-sensitive interactive traffic such as audio, low-latency queuing is a simple add-on to CB-WFQ. When complete, it allows the router to examine the IP packet, looking for RTP information inside. If it finds an RTP packet, the router puts the packet in the priority queue, allowing it to be forwarded before any other traffic.

The following command gives strict priority to a class within a policy map, and you use it with commands from CB-WFQ. The bandwidth is the amount in Kbps (that will be reserved for the high-priority traffic). Any amount beyond it will spill into the lower-priority queues, possibly down to the undefined traffic queues:

 Router(config-pmap-c)# priority bandwidth