QoS and what to manage with what

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.



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