Priority Queueing

Priority queue uses 4 queues: high, medium, normal and low. These queues have pre-assigned priority that cannot be changed. Packets in these queues are dequeued in the order that means that you can starve the lower priority queues.

Priority Queue commands are processed like an access-list, they are read from top to the bottom. Once a match is found, the processing stops. Therefore the order is very important in PQ.

Why Use Priority Queueing?

PQ provides absolute preferential treatment to high priority traffic, ensuring that mission-critical traffic traversing various WAN links gets priority treatment. In addition, PQ provides a faster response time than do other methods of queueing.

Although you can enable priority output queueing for any interface, it is best used for low-bandwidth, congested serial interfaces.

EXAMPLE

Apply legacy Priority Queue to the interface using the following requirements:

• All RIP packets going out the serial link should be sent first
• If there are no RIP packets, ftp packets should be sent next
• If there are no RIP or FTP packets, web traffic should be sent next
• As a last resort, send all BGP traffic.

Priority Queue is configured using the priority-list command:

priority-list 1 protocol ip high udp rip
priority-list 1 procotol ip medium tcp ftp
priority-list 1 procotol http normal
priority-list 1 procotol ip low tcp bgp

Once the priority-list is created, we can apply this to the interface using the priority-group command as follows:

interface serial 0/0
priority-group 1

Priority Queue Verification:

show queueing interface serial 0/0
show queueing priority