QoS | Priority Queueing

PQ allows certain traffic to be strictly-prioritized, multiple queue, allow priority. By default is define 4 queue are high, medium, normal, low, if you not define queue type default use normal. Each of queue is 20, 40, 60, 80 packets by default.

PQ use round-robin sequence occurs for every single packet. Therefore, if there are consistently packets in upper queues, packet in lower queues will never get serviced. PQ like CQ no direction option is applied, as queuing is always outbound.

Task configuration of PQ

#define priority list (required)

R1(config)#priority-list ?
  <1-16>  Priority list number

R1(config)#priority-list 1 ?
  default      Set priority queue for unspecified datagrams
  interface    Establish priorities for packets from a named interface
  protocol     priority queueing by protocol
  queue-limit  Set queue limits for priority queues

R1(config)#priority-list 1 protocol ?
  aarp              AppleTalk ARP
  appletalk         AppleTalk
  arp               IP ARP
  bridge            Bridging
  bstun             Block Serial Tunnel
  cdp               Cisco Discovery Protocol
  clns              ISO CLNS
  clns_es           ISO CLNS End System
  clns_is           ISO CLNS Intermediate System
  cmns              ISO CMNS
  compressedtcp     Compressed TCP (VJ)
  decnet            DECnet
  decnet_node       DECnet Node
  decnet_router-l1  DECnet Router L1
  decnet_router-l2  DECnet Router L2
  dlsw              Data Link Switching (Direct encapsulation only)
  http              HTTP
  ip                IP
  ipv6              IPV6
  ipx               Novell IPX
  llc2              llc2
  pad               PAD links
  pppoe             PPP over Ethernet
  qllc              qllc protocol
  rsrb              Remote Source-Route Bridging
  snapshot          Snapshot routing support
  stun              Serial Tunnel

R1(config)#priority-list 1 protocol http ?
  high    
  medium  
  normal  
  low     

R1(config)#priority-list 1 protocol http high ?
  gt  Prioritize packets greater than a specified size
  lt  Prioritize packets less than a specified size
  

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

#assigning priority list to an interface (required)

R1(config-if)#priority-group ?
  <1-16>  Priority group

R1(config-if)#priority-group 1 ?
  

#monitoring (optional)
- default queue FIFO on interface FastEthernet

...
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
...

- use PQ

...
  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
...

#final config

...
interface FastEthernet0/1
 ip address 192.168.100.1 255.255.255.0
 duplex auto
 speed auto
 priority-group 1

priority-list 1 protocol http high
priority-list 1 protocol ip medium
priority-list 1 protocol pppoe normal
priority-list 1 default low
...

The concept config PQ almost like CQ just change 'queue-list' with 'priority-list' in global configuration IOS command and applied on an interface use 'priority-group'. CQ and PQ still don't need MQC (modular QoS CLI) IOS command, next time may be I will explain about example Congestion Management with MQC.