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New Member

Checking Bandwidth Usage

I have a LAN extention network (All l2 arpa encap) with L3 switches on each of the sites. Users are complaining of slow speeds.

I have ruled out misconfigs, interface errors, Processor Peaking, Log anomoies, Loops and routing instability, acl etc.

The only thing I can think of is that the delay could be is the 10Mbps LES circuits are not enough. How do I measure if the bandwidth is being maxed on the uplinks at the busy points of the day?

Do I need to run some IP Traffic analysis software such as cisco netflow or can I determine if we need more bandwidth from local switch commands?

Please advise!

1 ACCEPTED SOLUTION

Accepted Solutions
Super Bronze

Re: Checking Bandwidth Usage

"What does the /255 mean? How long a timeframe is this over? "

/255 is the divisor, so 21/255 = 8.24% (255 was likely chosen since it's what a single byte can contain)

The timeframe, on most interfaces, defaults to 5 minutes. This can often be adjusted using the load-interval command within the interface configuration.

16 REPLIES
Hall of Fame Super Bronze

Re: Checking Bandwidth Usage

NetFlow will be your best bet but keep in mind, NetFlow isn't supported in all L3 switches.

You can also tell the amount of traffic being used on a link by viewing the show interface output on the egress interface.

HTH,

__

Edison.

New Member

Re: Checking Bandwidth Usage

Thanks Edison.

How do I obtain Netflow?

Also, is the line I want to be looking out for in the sh int cmd this one;

reliability 255/255, txload 21/255, rxload 4/255

What does the /255 mean? How long a timeframe is this over?

Many thanks,

Carlton

Super Bronze

Re: Checking Bandwidth Usage

"What does the /255 mean? How long a timeframe is this over? "

/255 is the divisor, so 21/255 = 8.24% (255 was likely chosen since it's what a single byte can contain)

The timeframe, on most interfaces, defaults to 5 minutes. This can often be adjusted using the load-interval command within the interface configuration.

New Member

Re: Checking Bandwidth Usage

Thank you Joseph.

Hall of Fame Super Bronze

Re: Checking Bandwidth Usage

Carlton,

NetFlow is part of the IOS. It isn't a separate tool that you need to install. When I mentioned some L3 switches do not support NetFlow is due to the hardware limitation on those switches, not the software or tool limitation.

What type of switches are you running?

As for the values you've posted, they don't provide as much value to determine interface usage. The best way to gauge interface usage is to view the input/output packets/bytes to determine how much packets/bytes you are sending within the specified interval. By default, most interfaces will display a 30 second interval. Other interfaces display a 5 minute interval. The load interval can be modified per interface with the load-interval interface command.

HTH,

__

Edison.

New Member

Re: Checking Bandwidth Usage

If you just want to check bandwidth on a few links there is a program called "asithappens".

All you need to do is supply the address, the SNMP read string, and the interval. Then select the interface and a graph shows up.

If you want long term monitoring look into NMIS or Cacti.

If you want to build your own map of the network with link monitors check out the program "The Dude". Setting this product up takes some patience. You should add devices manually and don't scan any networks. This product is also SNMP heavy reading from every device every 30 seconds. Althoug it is very nice once setup it takes quite a bit of time to understand the potential and there are a few bugs...

All these programs read the counters on the ports from SNMP.

NetFlow on the other hand keeps counters for source, destination and port/protocol. Although 3750 switches with routing software (as indicated before) do not support NetFlow so you might not be able to record all your interfaces with NetFlow.

Look at Scrutinizer free version for NetFlow. It is a good program but only keeps 24 hours of data resetting at 00:00 every night. There are many good NetFlow recorders.

New Member

Re: Checking Bandwidth Usage

Thanks for some valuable info - a great post :-)

New Member

Re: Checking Bandwidth Usage

Jbarger - Thanks again.

I am using netflow on a 1700 with scrutinzer -A great piece of software and giving me the info I need. And for free too :-)

New Member

Re: Checking Bandwidth Usage

I see, so the values I provided are just a snapshot bandwicth of the time when I executed the command?

If I set up the load-interval to the maximum allowed would it incur and performance loss at all?

I thought that you had to pay for Netflow and presumed it was software like Cisco Works. Do you know of any good sources of info for setting it up? The switches I am using are 3550's and some 2950's.

Thanks Edison.

Carlton

Purple

Re: Checking Bandwidth Usage

The 2950's do not support netflow as that is a layer 3 function , not sure the 3550's do either. routers and your higher end switches like 4500's and 6500's support it . Fixed configuration switches like 3750 and 3560 do not support it either which leads me to believe your 3550 would not be supported as the 3560's are just an updated version of the 3550.

Hall of Fame Super Bronze

Re: Checking Bandwidth Usage

Carlton,

NetFlow comes with regular IOS, no need to purchase any additional software/license to activated. However, in switches in rely in the hardware and neither model you have support NetFlow. I'm afraid you need to look into 3rd party tools to monitor bandwidth in your links.

The ones mentioned in this thread are pretty good and there are freeware such as PRTG/MRTG.

As for the load-interval query - no, changing the value will not create any performance issue in the switch.

HTH,

__

Edison.

New Member

Re: Checking Bandwidth Usage

Edison,

Thanks for the info - once again.

I have found a 1700 router in which the traffic I am interested in traverses. I have enabled netflow exportation on the 1700 and I am using free software (scrutinizer) in order to graph the IP info. It's working great!

What a shame that the 3550/3560/3750's don't support netflow. They let you enter all the required command's

ip route cache (int)

ip flow-export source/destination/version

and nothing happens! Why include the commands in the first place?!

Carlton

Hall of Fame Super Bronze

Re: Checking Bandwidth Usage

and nothing happens! Why include the commands in the first place?!

Because they run IOS and IOS supports NetFlow :)

The issue is the lack of hardware support on this case.

__

Edison.

New Member

Re: Checking Bandwidth Usage

I thought CEF switches were superior to routers though? With the benefits of packet switching in ASIC rather than software? Just surpised that you can do it on low end 1700/2600 but not L3 switches.

Hall of Fame Super Bronze

Re: Checking Bandwidth Usage

With the benefits of packet switching in ASIC

Bingo, hence those switches need NetFlow in ASIC to maintain the speed. This component is not added in low end switches to keep the cost down.

__

Edison.

New Member

Re: Checking Bandwidth Usage

That makes sense :-)

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