how to check if switch is close to its switching capacity?

[netone.cisco]

Hello all.

In our company network we currently have switch models: 2960, 3750 and 4507. Their datasheets and the performance guides present their switching capacities in Mpps or Gbps, backplane capacity and in the case of 4507 the linecards themselves have forwarding capacity.

What commands can I use on switches to check if they are reaching their capacity? On routers I learned to check the CPU and the processes but I cant find a way to check this on switches. please help.

thanks very much

[Shashank Singh]

Hi Mario,

As switches do forwarding in hardware, CPU utilization is not an indication of the throughput on the switch. Throughput again is not a conclusive evidence on lower end platforms (2960, 3750 etc) where oversubscription comes into picture.

I am not aware of any command that tells the real time aggregate throughput on any switch (mpps/mbps). Interfacewise results can be seen in show interface outputs though.

As a general observation, switches normally do not hit their capacity as long as they have free space in the (TCAM) memory to store mac-addresses (L2) and routes (L3). To check on the available memory we can use the following commands:

show tcam count (4500, 6500)

show platform tcam utilization (2960, 3750)

Another very informative comamnd on the 6500 switches is "sh platform hardware capacity" that gives granular information about a lot of resources on the switch, like CPU usage per slot, TCAM available, RAM usage etc.

Hope this helps,

Shashank

P.S Please rate helpful posts

[glen.grant · Accepted solution]

  On the 2960 and the 3750 you can use the command "show controllers utilization"  and this will tell you how busy the switch fabric is.  Unfortunetly I do not know an equivalent for the 4500's series , as that comand will not work on the 4500 which is too bad as it is a very useful command on the non chassis switches.

[netone.cisco]

Hi Glen and Shashank. thanks very much for your replies. Later in the day I will check these commands on the equipment itself.

Shashank, can you please explain better the following:

"...Throughput again is not a conclusive evidence on lower end platforms (2960, 3750 etc) where oversubscription comes into picture..." ?? Could not understand it.

thanks

[Shashank Singh]

Hi Mario,

On most of the 2960, 3750 (pizzabox switches) models, one ASIC controls more than one port. This means that an oversubscribed fastethernet port will provide near to 100 mbps only if other ports in the same port group are not being used heavily.

Oversubscription actually makes 'available bandwidth per port' a fuzzy concept. There is no fixed value which can be calculated as it will depend on how heavily are the other ports in the port group are being utilized. For eg. with a populated port group on a gigabit line card with OSR of 8:1 it may seem that each port has 1/8 gig of actual bandwidth. Now I am not denying the math but this infact is the minimum value and holds true only when all other 7 ports in that port group are being utilized at 100% (which seldom happens). The actual available bandwidth keeps floating between 1/8 gig and 1 gig depending on how much traffic is being pumped across the remaining seven ports.

Marketing specs for the devices tend to ignore oversubscription on the device and the numbers presented are often the maximum values (which are never acheived practically). Hence even though the throughput on the switch may be less than the spec in the data sheet, the switch may still be getting overutilized.

Following is a good discussion on oversubscription we had some time back: https://supportforums.cisco.com/message/3272513

Hope this helps,

Shashank

P.S. Please rate the helpful posts.

[netone.cisco]

Hi Shashank thanks for the explanation on oversubscription. the discussion you linked to was a bit complex for me but I certainly got the principles.

now back to my question. Today I had the chance to try the commands on some switches:

2960 and 3560 (not 3750 as i stated): the "show controllers utilization" command I believe is just the right command, especially the last lines that show switch receive, transmit and fabric percentage utilization. check < http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_50_se/command/reference/cli2.html#wp3573524 >

The "show platform tcam utilization" also works for them and many tcam subcommands

For the 4500 its a completely different story. show controllers is very very limited (no utilization subcommands) , there is no "show platform tcam...".

I tried show modules / tcam / overs / utiliz / asic / ... and nothing. I even tried a "show tech-support" and search for the words "utilization" and "subsc" (from oversubscription) and still got nothing