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

LLQ means priority a queue, to forward voice and video traffic before all other traffic.

If you have multiple LLQ queues, the difference between the single and multiple queue configuration is, that if you have at least two priority queues, both get policed. So if configured in a single police-map command, you will always policed the traffic at a maximum rate. Even if more bandwidth will be available, in case one queue fills up and the other still is not yet. The traffic will strictly be policed at the maximum rate.

The bandwidth percent gives the option to reserve a percentage of a link, also in case the link speed will change in the future. It will be calculated dependent on the actual link speed for the interface. This is changeable with the „bandwidth“ command on the interface.

The bandwidth remaining percent gives the option, to configure a remaining bandwidth on the actual link. If the link for example has a bandwidth of 1000 kbps and there is already different LLQ’s (100,200), then this is added to 300 kbps being already reserved. „max reserved-bandwidth“ will be per default 75% on an interface, which is 750 kpbs. So if you configure a reservation from the remaining percent, it will be calculated from

750 kbps

-300 kbps

_______

450 kbps.

So if you configure „bandwidth remaining percent 50“ you will get 225 kbps from the bandwidth of the interface.

Source:

Cisco Qos, Exam Certification Guide, Second Edition, Wendell Odom

------------------------------------------------------------------
|   Service     |  DSCP   |    DSCP     |       Application        |
|  Class name   |  name   |    value    |        Examples          |
|===============+=========+=============+==========================|
|Administration |  CS7    |   111000    | Heartbeats, SSH, Telnet  |
|---------------+---------+-------------+--------------------------|
|Network Control|  CS6    |   110000    | Network routing          |
|---------------+---------+-------------+--------------------------|
| Telephony     | EF,CS5  |101010,101000| IP Telephony             |
|---------------+---------+-------------+--------------------------|
| Multimedia    |AF41,AF42|100010,100100| Video conferencing       |
| Conferencing  |  AF43   |100110       | Interactive gaming       |
|---------------+---------+-------------+--------------------------|
| Multimedia    |AF31,AF32|011010,011100|Broadcast TV, Pay per view|
| Streaming     |AF33, CS4|011110,100000|Video surveillance        |
|---------------+---------+-------------+--------------------------|
| Low Latency   |AF21,AF22|010010,010100|Client/server transactions|
|   Data        |AF23, CS3|010110,011000|peer-to-peer signaling    |
|---------------+---------+-------------+--------------------------|
|High Throughput|AF11,AF12|001010,001100|Store&forward applications|
|    Data       |AF13, CS2|001110,010000|Non-critcal OAM&P         |
|---------------+---------+-------------+--------------------------|
|    Standard   | DF,(CS0)|   000000    | Undifferentiated         |
|               |         |             | applications             |
|---------------+---------+-------------+--------------------------|
| Low Priority  | CS1     |   001000    | Any flow that has no BW  |
|     Data      |         |             | assurance                |
------------------------------------------------------------------

Source:

http://tools.ietf.org/html/draft-baker-diffserv-basic-classes-01

• Custom Queuing has 16 queues available.
• All queues are serviced in a round-robin fashion.
• Bandwidth is specified in terms of byte count and queue length

The range for RTP/VOIP traffic packet could take is:

permit udp any any range 16384 32767

Also tcp port 1720 is used for voice control connetions similiar like port 21 with ftp

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version|  IHL  |Type of Service|          Total Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Identification        |Flags|      Fragment Offset    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Time to Live |    Protocol   |         Header Checksum       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Source Address                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Destination Address                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Options                    |    Padding    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The precedence value is presented in Type of service field in the tcp header. It can take up to 1 byte.

      0     1     2     3     4     5     6     7
+-----+-----+-----+-----+-----+-----+-----+-----+
|                 |     |           |     |     |
|   PRECEDENCE    | STRM|RELIABILITY| S/R |SPEED|
|                 |     |           |     |     |
+-----+-----+-----+-----+-----+-----+-----+-----+

The ip precedence value is set and carried over network boarders. It does not have to be set by each router on the path again.

Source:

http://certcities.com/editorial/columns/story.asp?EditorialsID=33

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