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QOS Commands

bartonr1998
Level 1
Level 1

Hi All,

 

I was hoping someone could assist me in explaining what the function is of the below commands on a cisco switch interface?

 

srr-queue bandwidth share 1 30 35 5
priority-queue out
mls qos trust cos
auto qos trust

 

Many Thanks!

 

Kind Regards,

Ryan

1 Accepted Solution

Accepted Solutions

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Ryan,

the QoS effective implementation on switches is platform dependent.

The commands that you have provided are likely on a Catalyst 3560.

 

In short this kind of switches have an hardware oriented QoS very different from the modular QoS of SW based routers.

 

When an ethernet frame carrying an IP packet in the payload is received on the input interface a switch like this performs a classification and associates a DSCP value ( it is in the IP header) to the frame.

On the outgoing inteface there is a limited number of hardware based queues , the switch uses a DSCP to COS mapping matrix to find the outgoing queue for the packet/frame.

More specifically the commands that you have listed do the following:

>> priority-queue out

It enables the priority queue that you can see like a low latency queue and it should be used for VOIP packets

>> mls qos trust cos

This command instructs the switch in how to dea with received frames. The CoS is a subfield of 802.1Q header so this command should appear only on a L2 trunk port or port-channel.

The possible values of CoS 0 to 7 are mapped to the 4 outgoing queues.

 

>> auto qos trust

Auto-QoS is a set of macro that allow to deploy QoS on swictches without manually entering each single command.

 

>>

srr-queue bandwidth share 1 30 35 5

 

There are four queues, one is set as priority and served first when the PQ is empty the interface will take frames / bytes from the three standard queues according to the relative weights specified in the command.

Each standard queue can be actually of two types : shared round robin allowed to take more then its weight if the interface is empty or it can be policed so it cannot go beyond its weight  (this is called Shaped round robin).

 

To be noted in more modern switches like C3650, C3850 and newer C9x00 the QoS is modular like in SW based routers and you can use policy-maps.

 

Hope to help

Giuseppe

 

View solution in original post

5 Replies 5

marce1000
VIP
VIP

 

 - Ref : https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12-2_50_se/configuration/guide/scg/swqos.html#wp1202482

  Have a look at this document, start with reading it, then you will also find the mentioned commands explained

 M.



-- ' 'Good body every evening' ' this sentence was once spotted on a logo at the entrance of a Weight Watchers Club !

Thanks Marc!

 

I will have a read over these documents!

 

Kind Regards,

Ryan

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Ryan,

the QoS effective implementation on switches is platform dependent.

The commands that you have provided are likely on a Catalyst 3560.

 

In short this kind of switches have an hardware oriented QoS very different from the modular QoS of SW based routers.

 

When an ethernet frame carrying an IP packet in the payload is received on the input interface a switch like this performs a classification and associates a DSCP value ( it is in the IP header) to the frame.

On the outgoing inteface there is a limited number of hardware based queues , the switch uses a DSCP to COS mapping matrix to find the outgoing queue for the packet/frame.

More specifically the commands that you have listed do the following:

>> priority-queue out

It enables the priority queue that you can see like a low latency queue and it should be used for VOIP packets

>> mls qos trust cos

This command instructs the switch in how to dea with received frames. The CoS is a subfield of 802.1Q header so this command should appear only on a L2 trunk port or port-channel.

The possible values of CoS 0 to 7 are mapped to the 4 outgoing queues.

 

>> auto qos trust

Auto-QoS is a set of macro that allow to deploy QoS on swictches without manually entering each single command.

 

>>

srr-queue bandwidth share 1 30 35 5

 

There are four queues, one is set as priority and served first when the PQ is empty the interface will take frames / bytes from the three standard queues according to the relative weights specified in the command.

Each standard queue can be actually of two types : shared round robin allowed to take more then its weight if the interface is empty or it can be policed so it cannot go beyond its weight  (this is called Shaped round robin).

 

To be noted in more modern switches like C3650, C3850 and newer C9x00 the QoS is modular like in SW based routers and you can use policy-maps.

 

Hope to help

Giuseppe

 

Thanks for you explanation Giuseppe I appreciate it!

 

Kind Regards,

Ryan

Joseph W. Doherty
Hall of Fame
Hall of Fame
BTW, assuming your configuration commands are from an older series Catalyst switch, there's a global configuration command that enables/disables QoS for the whole switch. I.e. if QoS is disabled (which on those switches it is, by default), then although you can "see" QoS commands, both globally and per interface, they are inactive.

Also on such switches, once QoS is globally enabled, the switch has a default QoS configuration, which also, by default, erases/resets ingress frame/packet CoS/ToS markings unless configured otherwise. Further note, the default QoS configuration often increases port drop rates.

Lastly, although the newer switches that have QoS commands much like Cisco routers, they are often a subset of what's found on a router and, "under the covers", those QoS commands are programmed into dedicated hardware (which is often why their QoS features are so much more limited than a software based router's).
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