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While troubleshooting a problem that a client reported I came across the IOS interface setting bandwidth. For whatever reason their interface was set to bandwidth 20. I understand that it doesn't limit actual bandwidth on the port like srr-queue does but it's a bit confusing for someone like me who works mostly on Layer 2. Cisco says it's a bandwidth limitation for communication with higher protocols such as EIGRP. This article goes into some detail about how it's used for calculating across interfaces.

Is this a command that should be used only on interfaces where backbone devices are connected, ex. a Ten Gig link between a building switch and a main router? Does it affect clients if bandwidth is set fairly low on an interface they're connected to, such as reaching intranet servers or external devices?

  • It's merely an advisory setting to tell various protocols what the actual usable bandwidth is (vs. link speed.) It's only important to some routing protocols and load-balancing scenarios. It has zero effect on how traffic flows through the interface. – Ricky Beam Apr 14 '15 at 22:46
  • It will affect EIGRP traffic across an interface. Also it will guide network monitoring systems and QoS configurations on the available bandwidth on an interface. – cpt_fink Apr 15 '15 at 4:30
  • Thank you for your responses. Since that appears to affect services that are not related to the client then I can rule this out as an indirectly related cause – Kamikaze Rusher Apr 15 '15 at 19:05
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It's a little complex, and also platform specific in some cases. The most general answer is that the bandwidth statement is used as an input to the EIGRP routing protocol.

There are also cases where the bandwidth command can be applied on a main or subinterface and impact QoS calculations.

In general, unless you're running EIGRP and specifically need it, my advice would be to not use the command at all.

  • I don't have direct control over most layer 3 or higher protocols. If at most the effect to the client might be QoS impact then it's unlikely to be an (in)direct cause, however it is good to know should something like QoS arise in the future with bandwidth present in the port config – Kamikaze Rusher Apr 15 '15 at 19:06

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