AI, ML, Development + Cisco Learning Blog Learning about Machine Learning, Artificial Intelligence, related devlopment topics and formerly Routing and Switching, Datacenter, Security and other topics, CCIE #23664, Frank Wagner

There are many different kind of QoS techniques around at Cisco. All of them are like a tollbox for managing traffic. Each technique has it’s preferred operational area. So here are some scenarios in which you would use a certain technique.

• Classification
  

You want to provide a prefered service to a type of traffic. The packet may be marked or not. Classification don only on one device, without marking the packet is described as a per-hop based classification. PQ (priority queing) and CQ (custom queing) are techniques used for this. Possible methods to identify certain traffic are ACL’s, policy based routing, comiited acces rate (CAR) or network-based application recognistion (NBAR).

• Congestion Management
  

What if an interface is accessed above it’s given bandwidth? Congestion occurs and priority queuing (PQ), custom queuing (CQ), weighted fair queuing (WFQ), and class-based weighted fair queuing (CBWFQ) are tools to mangage congestion.

• Queue Management
  

If a queue does fill up and buffers are flowing over, packets must be dropped. Which packets to drop, maybe packets with lower priority, to be able to deliver higher priority, this is done with Weighted early random detect (WRED).

• Link Efficiency
  

Some packets might be to large for efficent transport and it might be neccessary to compress these packets. RTP header compression (Compressed Real-Time Protocol header) can be used for this.

• Traffic shaping and policing

When shaping traffic, you would take care of a certain link not to exceed the configured bandwitdh or maybe another certain bandwith. Traffic is buffered then, with poicing it’s just discarded as other functions are similar for policing.

Queuing techniques, algorithms and when to use them.

• FIFO, First-in, first-out

Is the default queuing algorithm, and delivers packet in the same row it receives them, but could buffer them in between

• PQ, Priority queuing

PQ gives priority to traffic over other traffic, each packet is placed into one of four queues: high, medium, normal, low. There is absolut preferential treatment over low-priority queues.

• CQ, Custom queuing

is used to provide a garantied bandwidth, leaving the remaining bandwidth to other traffic. CQ does this by assigning a specific amount of queue space to each class of packet and then servicing the queues round-robin. PQ and CQ are statically configured. They don’t adapt network changes automatically.

• WFQ, Flow-based weighted fair queuing

provides consistent response time to congested networks, each queue ist serviced on a bye counted base. Each time 1000 bytes are serviced, one stream with 2×500 bytes it qually serviced, like the 1×1000 byte packet. It’s mostly used on serial interfaces. WFQ is IP-precedence aware

• CBWFQ, Class-based weighted fair queuing

CBWFQ is used to provide a minimum of bandwidth to a certain flow. It’s a garanteed amount of bandwidth. If it’s not used by the class other applications can use it.

Tools for congestion avoidance:

• WRED  Weighted random early detection is to avoid congestion before it becomes a problem. It’s an algorithm to drop packets if congestion is about to occuring. Senders themself then slows down transmitions speed.

Sources:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/qos.htm

QoS means, to give certain types of traffic in a network priority above other types of traffic or to garanty certain functionallity. And this could be done on many different kind of architectures.
But dependend on the type of traffic QoS can only be used, when certain demands are satisfied. That means, there has to be at least one field in the header of a packet.

Here is a list of Marking Fields

Field Location Length

IP Precedence (IPP) IP header 3 bits

IP DSCP IP header 6 bits

DS field IP header 1 byte

ToS byte IP header 1 byte

Cos ISL and 802.1Q header 3 bits

Discard Eligible (DE) Frame Relay header 1 bit

Cell Loss Priority (CLP) ATM cell header 1 bit

MPLS Experimental MPLS header 3 bits

Cisco has decided to give a more general approach to QoS with the Modular QoS CLI (MQC). This client defines a common set of configuration commands for the definition of for QoS features on a router or switch.

Cisco is in favor of configuring with MQC.

There are three steps that have to be configured within a MQC based setup.

1. class-map for matching packets into service classes, the match command can include QoS fields, ACL’s and MAC addresses, the match name is case sensitiv, the match any command matches any packet
2. policy-map PHB actions configured under policy-map
3. service-policy enabled on an interface
   

Here is an example:

class-map match-all SMTP-FROM-SERVER1
match access-group name
SMTP-FROM-SERVER1

policy-map CBWFQ
class SMTP-FROM-SERVER1
bandwidth 256

interface s0/0
bandwidth 512
service-policy output CBWFQ

ip access-list extended SMTP-FROM-SERVER1
permit tcp host 150.1.1.100 eq smtp any

Here the SMTP protocol is serviced with CBWFQ at interface serial0/0.

The class-map command is one of the new MQC based tools for classifying packets.

It possible to match many different kind of options. Including QoS fileds, ACL’s and MAC addresses. Be carefule the map names are case sensitiv.

Multiple match commands can be used in a class-map. The following points need to be considered.

1. Four CoS and IPP or eight DSCP values can be listed on a single match cos, match precedence, or match dscp command.
2. if a class map matches with multiple match commands, the match-any or match-all parameter on the class-map command defines whether a logical OR or a logical AND is used between the match commands. match-all means AND logic between the parameters, for example: class-map match-all name, match cos 3 4, means to match 3 and 4. With match-any only one match parameter has to be true for the rule to match. So 3 or 4 would suffice.
3. The match class name command is nesting the name of another class map logicaly.

Here is a nesting example:

class-map match-all nesting
match access-group 101
match precedence 10

class-map match-any morenesting
match class nesting
match cos 10

Source:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/qos.htm

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/hqos_c/qchintro.htm