Bandwidth / data limits

Unanswered Question
Jan 21st, 2009
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Hi,

Is it possible for the data usage to exceed the physical rate of the interface or the rate capped by the upstream provider?

Reason being, we hav a 4 Mbps mpls link between 2 of our sites on a serial interface,when we view the application usage on Netscout, it shoots to more than 7 meg at times.Just confused if the physical link restriction(serial) or the alloted link capacity by provider affects the usage to go above the limit.

Gurus, pls throw more light on this.

Thanks in advance!


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Elly Bornstein Wed, 01/21/2009 - 21:44
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There are a lot of variables here.


First off, you cannot go over the physical rate of an interface. A gig port can only transmit 1000Mbps. On some switches you cannot even achieve that speed, for packets require processing which can slow down the transmission.


But a Gig port is clocked in a fashion that it can transmit 1000Mbps.


If your ISP says you have a 4Mbps link, they are rate-limiting you using some sort of policer, which probably does not care what type of traffic is coming into their interface. They simply drop excessive packets over your 4Mbps rate.


Now I am a little confused about what Netscout is reporting. When you say 7Meg does that mean it is sending a 7Meg file or did you mean it is able to get a transmission rate of 7Mbps?


I would probably investigate what Netscout is doing at a lower layer, by downloading wireshark (freeware capture tool) and looking at what type of packets Netscout is sending across the ISP.


One key thing to look for is retransmission in TCP packets. If you see these it is probably due to the ISP dropping some TCP packets and the NetScout having to retransmit them.

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