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Cisco QoS Queuing Methods
Cisco Queuing Methods
First In First Out (FIFO) Queueing
First In, First Out (FIFO) Queueing Packets are forwarded in the same order in which they arrive at the interface.
Priority Queuing (PQ)
PQ ensures that important traffic gets the fastest handling at each point where it is used. It was designed to give strict priority to important traffic.
Weighted Fair Queuing (WFQ)
WFQ is one of Cisco’s premier queuing techniques. It is a flow-based queuing algorithm that does two things simultaneously: It schedules interactive traffic to the front of the queue to reduce response time, and it fairly shares the remaining bandwidth between high bandwidth flows.
Class-Based Weighted Fair Queueing (CBWFQ)
Class-based weighted fair queueing (CBWFQ) extends the standard WFQ functionality to provide support for user-defined traffic classes. For CBWFQ, you define traffic classes based on match criteria including protocols, access control lists (ACLs), and input interfaces.
Low Latency Queuing (LLQ)
The Low Latency Queueing feature brings strict priority queueing to Class-Based Weighted Fair Queueing (CBWFQ).
Internet Protocol Real-Time Protocol Priority (IP RTP Priority)
The Frame Relay IP RTP Priority feature provides a strict priority queueing scheme on a Frame Relay permanent virtual circuit (PVC) for delay-sensitive data such as voice.
CBWFQ
class-map match-all MatchEF
match dscp ef
!
policy-map VoicePolicy
class MatchEF
bandwidth percent 30
LLQ
class-map match-all MatchEF
match dscp ef
!
!
policy-map VoicePolicy
class MatchEF
priority percent 30
- cos is used at layer 2
- ip precedence is the old way for type of service(ToS)
- DSCP is the new way for type of service(ToS)
- DSCP is backward compatible with ip precedence
Below is a the IPV4 datagram packet
As you notice ToS is 8 bits
ip precedence use the first 3 bits , DSP use the first 6 bits (in fact the 6th bit is never used)
Ip Precedence (Major class)
It use a scale of 0 to 7 and will be typically used as below
- 0:Best Effort (000)
- 1:Medium priority(001)
- 2:Hight priority(010)
- 3:Call signals(011)
- 4:Video conference(100)
- 5:Voice(101)
- 6:Reserved (110)
- 7:Reserved(111)
again this is the old way to be used with ToS
Configuration Example:
Router(config)#class-map match-all IP-PRECEDENCE
Router(config-cmap)#match protocol rtp
Router(config-cmap)#exist
Router(config)#policy-map IP-PRECEDENCE-POLICY
Router(config-pmap)#class IP-PRECEDENCE
Router(config-pmap-c)#set ip precedence ?
<0-7> Precedence value
critical Set packets with critical precedence (5)
flash Set packets with flash precedence (3)
flash-override Set packets with flash override precedence (4)
immediate Set packets with immediate precedence (2)
internet Set packets with internetwork control precedence (6)
network Set packets with network control precedence (7)
priority Set packets with priority precedence (1)
routine Set packets with routine precedence (0)
Router(config-pmap-c)#set ip precedence critical
Router(config-pmap-c)#exit
Router(config)#interface gigabitEthernet 0/1
Router(config-if)#service-policy input IP-PRECEDENCE-POLICY
DSCP
The advantages of DSCP is that you may have more levels to define than “ip precedence”. For each level in “ip precedence” , there are 3 sub levels.at the same time it is backward compatible
default Match packets with default dscp (000000) (no sub level for level 0 or best effort)
af11 Match packets with AF11 dscp (001010)
af12 Match packets with AF12 dscp (001100)
af13 Match packets with AF13 dscp (001110)
Notice in the above level for af11 = 001|010 , as the 6th bit is not used this will become in reality 001|01 which is equal to 1|1 in decimal and the final result is af11
an old router which does not support DSCP will look at the first 3 bits , so the major classification will be understood by the old router .
The second part or the second 3 bits is called Drop Preference ,the higher it is , the more chance it has to be dropped . To resume af11 is better than af13 because af13 has more chance to be dropped than af11
Below are the rest of levels
af21 Match packets with AF21 dscp (010010)
af22 Match packets with AF22 dscp (010100)
af23 Match packets with AF23 dscp (010110)
af31 Match packets with AF31 dscp (011010)
af32 Match packets with AF32 dscp (011100)
af33 Match packets with AF33 dscp (011110)
af41 Match packets with AF41 dscp (100010)
af42 Match packets with AF42 dscp (100100)
af43 Match packets with AF43 dscp (100110)
ef Match packets with EF dscp (101110) (no sub levels for level 5 or critical level)
Configuring DSCP on a cisco router is the same as ip precedence the only difference will be in the following line
Router(config-pmap-c)#set ip dscp ef