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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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Using Priority Queuing

Problem

You want to enable strict priority queues on an interface so that the router always handles high priority packets first.

Solution

To enable priority queuing on an interface, you must first define the priority list, and then you can apply it to the interface:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#access-list 101 permit ip any any precedence 5 tos 12
Router(config)#access-list 102 permit ip any any precedence 4
Router(config)#access-list 103 permit ip any any precedence 3
Router(config)#priority-list 1 protocol ip high list 101
Router(config)#priority-list 1 protocol ip medium list 102
Router(config)#priority-list 1 protocol ip normal list 103
Router(config)#priority-list 1 default low
Router(config)#interface Ethernet0
Router(config-if)#priority-group 1
Router(config-if)#exit
Router(config)#end
Router#

Discussion

As we discuss in Appendix B, priority queues strictly ensure that high priority packets are always handled before lower priority packets. We stress that using pure priority queuing like this is usually a bad idea because the higher priority traffic can take all of the available bandwidth and completely starve all other network traffic. You only want to use this style of queuing when you can be absolutely certain that the aggregate bandwidth of all high priority traffic will never consume the available link bandwidth. This could be the case, for example, if the high priority traffic is ...

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