Priority Queuing Classification (cont.)

Priority Queuing Classification (cont.)

• Priority queuing also supports classification of other protocols with these options:
• Protocol-specific access list (if available for the specified protocol)
• Packet size (greater or smaller than specified)
• Some of the supported protocols are:
• IPX
• CLNS
• DECnet
• AppleTalk
• VINES
• DLSw

Priority Queuing Insertion Policy

• Each queue has a maximum number of packets that it can hold (queue size).
• After a packet is classified to one of the following queues, the router will enqueue the packet if the queue limit has not been reached (tail-drop within each class).

Scheduling Priority Queuing

• Packet in
• HIGH
• queue?

• Packet in
• MEDIUM
• queue?

• Packet in
• NORMAL
• queue?

• Packet in
• LOW
• queue?

• Hardware Q

• Yes

• Yes

• Yes

• Yes

• No

• No

• No

• No

• Dispatch packet
• and start checking the
• HIGH queue again

Benefits and Drawbacks of Priority Queuing

• Benefits
• Provides low-delay propagation to high-priority packets
• Supported on most platforms
• Supported in all IOS versions (above 10.0)
• Drawbacks
• All drawbacks of FIFO queuing within a single class
• Starvation of lower-priority classes when higher-priority classes are congested
• Manual configuration of classification on every hop