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I am a-bit confused here.

I have a FW connected to L2 switch then to an ISP router. My ISP link has a 4Mb capped on the egress. 

[My FW] <-1Gb link-> [p0 L2 Switch p1] <-1Gb link-> [ISP Router] <- 4Mb link ->

Doing a SNMP capture on switch2's port1 (facing ISP router), it is reflecting a 4Mb maximum bandwidth usage.

But what I don't understand is this as below ->

You might say, the ISP maximum bandwidth is capped at 4Mb, thus the bandwidth capture is correct.

But the link between the switch and the router is a 1Gb link and there isn't any policing/shaping down at the ingress port at the Router.

The FW doesn't know the Router has a 4Mb capped, it will just send all the traffic it needs over to the Router at full bandwidth of 1Gbps.

So why isn't the bandwidth capture reflect at port1 on the LAN side showing more then 4Mbps ?

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  • Do you mean 4 MB(yte) or 4 Mbit??
    – Zac67 ♦
    Jul 2, 2018 at 18:10
  • sorry, thanks for pointing it out, everything is in bits. 1Gb lan link and 4Mb capped at ISP side
    – Noob
    Jul 2, 2018 at 18:13

1 Answer 1

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So why isn't the bandwidth capture reflect at port1 on the LAN side showing more then 4Mbps ?

Because your traffic is mostly TCP, and TCP will slow down as the ISP drops traffic (due to the cap). So your flows are adjusting to the actual bandwidth.

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  • thanks for reply. Does that means if I have pure UDP traffic, the bandwidth capture will actually reflect the actual bandwidth usage instead of the ISP's 4Mbps capped ?
    – Noob
    Jul 2, 2018 at 18:10
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    @Noob Using UDP, you can transmit any bandwidth you can manage towards the ISP router and it'll simply drop the excess.
    – Zac67 ♦
    Jul 2, 2018 at 18:13
  • @Zac67 - what I mean is with UDP, if I am sending at full link rate of 1Gbps, the bandwidth capture (at switch p1) in my setup should be reflecting 1Gbps instead of the WAN limitation of 4Mbps - right ?
    – Noob
    Jul 2, 2018 at 18:14
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    Yes. Sending 1 Gbit/s to the router will make it drop 996 Mbit/s when forwarding.
    – Zac67 ♦
    Jul 2, 2018 at 18:19
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    Yes. Why don't you just try it?
    – Zac67 ♦
    Jul 3, 2018 at 6:17

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