bandwidth and QOS

[huajie chen]

Hi Guys,

I get a 20Mbps lease line between two offices and it connects two cisco C4507R switches. I configured QOS on both of the switches, and as I know the QOS will take effect when the network congestion happens. But the ports which the lease line connected show 100Mbps on the switch. So I configured 'bandwidth 20480' command under the ports, whether this will help QOS active when the network flow is over 20Mbps?

my command under the interface:

interface GigabitEthernet1/38

no switchport
bandwidth 20480
ip address 10.81.16.4 255.255.255.248
service-policy output QOS-SH

[Pawan Raut]

No...Your switch port is Gig port so you can configured upto 1 Gbps BW but 20Mbps BW provided by your ISP so your ISP will restrict the traffic flow over 20 Mbps even if you configured BW  more than 20 Mpbs. So it is always standard practice to configured the BW as provided by your ISP. 

[huajie chen]

so what's the point to configure 'bandwidth' command under the interface? Will it make the QOS policy active when the network flow over the value configured by the 'bandwidth' command?

[Joseph W. Doherty]

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Posting

Setting bandwidth doesn't directly control QoS.  However, setting it to match your available bandwidth should "normalize" interface bandwidth utilization stats, e.g. 20 Mbps usage would show 100% utilization.  Also, if your running a dynamic routing protocol, some will adjust their metric based on the bandwidth setting.

[huajie chen]

Got it. Thanks.

[Joseph W. Doherty · Accepted solution]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Your 4500 QoS will only engage when the interface congests.

What you need is a shaper, with QoS support, that can match your provider's bandwidth rate.

Unfortunately, this isn't a feature of the 4500 series.

[huajie chen]

Hi Joseph,

May I configure the shape command under the 'class default' to make the interface congested when the network flow is over 20Mbps?

[Joseph W. Doherty · Accepted solution]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Again, don't believe that's possible on a 4500 series L3 switch.

On a router, yes, you would do that, and you could also add a child policy.