Priority queuing some questions

[sarahr202]

Hi everybody.

I am watching this great Video on qos. Got few questions on Priority queuing.  Based on the  video lecture, we have 4 quietus in priority queuing system

They are:

High, medium, normal, ( i forgot the last one lol)

Anyway let say I have already classified my traffic as

traffic1,

traffic2,traffic3

traffic 4.

I want traffic 1 to use High priority, traffic 2 should use " Medium" priority queue.

How can I map these traffic classes to queues ?

thanks for your help

Have a great weekend.

[daniel.dib]

Priority queueing is a legacy type of queueing that uses four queues. They are high, medium, normal and low. Priority queueing is prone to queue starvation because it will entirely empty one queue before moving on to the next. That means that if there are constantly packets in the high queue then the other queues will never get served.

You can assign either protocols or use access-lists to map traffic to the different queues in the following way:

R1(config)#priority-list 1 protocol http medium

R1(config)#access-list 100 permit tcp host 1.1.1.1 host 2.2.2.2 eq 22

R1(config)#priority-list 1 protocol ip high list 100

Then assign it to an interface:

R1(config)#int f0/0

R1(config-if)#priority-group 1

R1(config-if)#do sh int f0/0

FastEthernet0/0 is administratively down, line protocol is down

  Hardware is Gt96k FE, address is c200.2bcc.0000 (bia c200.2bcc.0000)

  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

  Half-duplex, 10Mb/s, 100BaseTX/FX

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input never, output never, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: priority-list 1

  Output queue (queue priority: size/max/drops):

     high: 0/20/0, medium: 0/40/0, normal: 0/60/0, low: 0/80/0

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 0 bits/sec, 0 packets/sec

     0 packets input, 0 bytes

     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog

     0 input packets with dribble condition detected

     0 packets output, 0 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

The default is to allow 20 packets into high, 40 into medium, 60 into normal and 80 into low. The queue sizes can be tuned in the following way:

R1(config)#priority-list 1 queue-limit ?

  <0-32767>  High limit

R1(config)#priority-list 1 queue-limit 5 ?

  <0-32767>  Medium limit

R1(config)#priority-list 1 queue-limit 5 20 ?

  <0-32767>  Normal limit

R1(config)#priority-list 1 queue-limit 5 20 50 ?

  <0-32767>  Lower limit

R1(config)#priority-list 1 queue-limit 5 20 50 100

I would not expect to see this type of queuing today but it doesn't hurt to know it for reference.

Daniel Dib
CCIE #37149

Please rate helpful posts.

[sarahr202]

Thanks Daniel.

[Joseph W. Doherty]

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Posting

I just want to emphasis what Daniel already noted, which is in PQ each queue has absolute priority over any queues "below" it, which means, for example, "low" queue packets will only be transmitted when there are no "high", "medium" or "normal" packets waiting to be transmitted. So as Daniel also noted, PQ can easily "starve" lower queues of all bandwidth.

BTW, since PQ is just looking at queue contents, it also doesn't consume much device resource for its management.