How can a switch support QoS?

[Ichinisan]

I've seen switches that claim to support QoS. I always thought QoS was a routing feature.

I thought a switch has a small amount of memory to associate MAC IDs with each port, and simply delivers packets. Basically, a MUCH simpler device than a router except that it has many ports. If a switch can support QoS, wouldn't it require a CPU and larger buffer, making it just as complex / unreliable as a router w/ integrated switch?

 

[jlazzaro]

just as complex / unreliable as a router w/ integrated switch?

router w/ integrated switch is not complex or unreliable, unless you're talking about a specific product. look into 802.1p for QoS at layer 2, http://en.wikipedia.org/wiki/IEEE_P802.1p

 

[imagoon]

Switches can respect the QoS settings in the packets. It isn't just a routing thing. If there is congestion the switch will forward higher priority packets first while delaying or dropping lower priority. QoS is only "unreliable" in the sense that most people don't have a clue how to configure it. You need control of both ends of links / all the network gear to make it work.

 

[spidey07]

Enterprise switches have per-port buffers and different tx/rx queues which you can manage, assign bandwidth thresholds to, advanced drop preference/characteristics. How those queues are serviced provides very granular QoS. Processor has little to do with the queuing/marking/scheduling, all done in hardware.

SOHO switches are a large shared memory pool/buffer and likely implement some rudimentary form of QoS (still in hardware) like strict priority queuing.

 

[drebo]

^^ Yep.

Switches support QoS in the same way that routers do...they queue traffic per port to make sure that the important traffic makes its way to the top of the egress queue. Most will also read the L3 QoS markings, rather than just the L2 QoS markings.

Very flexible.

Although, Cisco's Catalyst QoS is shit and obtuse.

 

[Railgun]

Although, Cisco's Catalyst QoS is shit and obtuse.

most people don't have a clue how to configure it.

Maybe slightly out of context, but point all the same.

 

[drebo]

Maybe slightly out of context, but point all the same.

I know what you're implying, but that's not the case.

Cisco's MOS QoS on their Catalyst switches is very poorly implemented. Juniper's is far superior to configure.

On the other hand, Cisco's CBFWQ implementation on IOS routers is very easy to use, and outside of some stupid limitations, quite functional. Did you know that you can't use CBWFQ on VLAN subinterfaces on an IOS router?

 

[drebo]

To clarify, on Catalyst switches, there is no consistency. All their switch models have different numbers of queues per port, the queues are addressed differently (queue 1 on one platform would be a priority queue, whereas queue 4 on another platform would be the priority queue), and there's very little consistency in the direction of the CoS to DSCP maps and visa versa.

It may work well once it's configured properly, but the configuration of it is such an obtuse amalgam of bullshit that it makes you want to shoot yourself when you have to touch it.

 

[Pheran]

To clarify, on Catalyst switches, there is no consistency. All their switch models have different numbers of queues per port, the queues are addressed differently (queue 1 on one platform would be a priority queue, whereas queue 4 on another platform would be the priority queue), and there's very little consistency in the direction of the CoS to DSCP maps and visa versa.

It may work well once it's configured properly, but the configuration of it is such an obtuse amalgam of bullshit that it makes you want to shoot yourself when you have to touch it.

To be fair, Auto QoS works pretty well on the 3750/2960 platforms. But on 6500, I'm totally with you, the QoS config is crap. It looks like they are finally fixing some of the mess on 6500 with IOS 15 though.