10.2 Queuing Options
10.2.5 Configuring priority queuing
Step 1—Create an Output Priority Queuing List

A priority list is a set of rules that describe the way packets should be assigned to priority queues. You can establish queuing priorities based on the protocol type or on packets entering from a specific interface. 

You can set a priority queue either by protocol type or by incoming interface type.

Create an output PQ list with the priority-list protocol command :

Router(config)#priority-list list-number protocol protocol-name
{
high | medium | normal | low} queue-keyword keyword-value

You can also create an output PQ list with the priority-list interface command. Use this command to set queuing priorities for all traffic arriving on an incoming interface :

Router(config)#priority-list list-number interface interface-type
interface-number
{high | medium | normal | low}

Step 2—Assign a Default Queue

You must explicitly assign a queue for packets that were not specified in the priority list.
The default queue is normal. Use the
priority-list default command to assign packets to a queue if no other priority list conditions are met:

Router(config)#priority-list list-number default
{high | medium | normal | low}

Step 3—Specify the Queue Sizes (optional)

You can specify the maximum number of allowable packets in each queue. In general, it is recommended that the default queue sizes not be changed.

Use the optional priority-list queue-limit command to change the default maximum number of packets in each queue :

Router(config)#priority-list list-number queue-limit
high-limit medium-limit normal-limit low-limit

Step 4-Assign the Priority List Number to an Interface

Only one list can be assigned per interface. When assigned, the priority list rules are applied to all traffic that passes through the interface.

After you define the priority list, enter interface configuration mode, and enter the priority-group command to link a priority list to an interface :

Router(config-if)#priority-group list

In the configuration shown in Figure , priority-list 2 specifies the following:

  • Telnet (TCP port 23) traffic is assigned to the high-priority queue.
  • Traffic from source network 131.108.0.0 is assigned to the high-priority queue, as specified by access-list 1. 
  • The list 1 argument in the second line of the configuration specifies that access-list 1 be used to sort packets for placement in the high-priority queue.
  • All traffic arriving from Ethernet interface 0 is assigned to the medium-priority queue.
  • All other IP traffic is assigned to the normal-priority queue.
  • All other traffic not specified in priority-list 2 is assigned to the low-priority queue.
  • Queue-size limits have been changed from the default values to the following:
  • 15 datagrams for the high queue
  • 20 datagrams for the medium queue
  • 20 datagrams for the normal queue
  • 30 datagrams for the low queue

Notice in Figure that priority-list 2 is linked to interface serial 0 by the priority-group 2 command.
Lab Activity    
  Our company has given us the task of connecting our east coast office with the west coast office via ISDN. It has been determined that we need to have a strict queuing policy for traffic going across this link. Certain traffic should have a higher priority than other traffic. We have decided that all TCP traffic should have high priority, ICMP traffic should have medium priority, all UDP traffic should have low priority, and all other traffic should have normal priority. It is our responsibility to connect these two routers together and configure priority queuing correctly.