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I have a dumbbell topology with two hosts and two switches. Host1 is connected to Switch1, Switch1 is connected to Switch2, and Switch2 is connected to Host2. To make things more interesting Host1 also has 3 VMs with bridged addresses.

Each switch interface with a hot link is a trunk, except for an access link between Switch1 and Switch2. Switch1 and Switch2 also have another trunk interface between them.

There are only the default vlans in my set up at the moment. I issued commands to rate limit Vlan 1 to 0.5MBs on Switch1.

(config)# interface vlan1
(config-if)# rate-lime output 4000000 2000 2000 conform-action transmit exceed-action drop` 
(config-if)# rate-lime output 4000000 2000 2000 conform-action transmit exceed-action drop`

I'm trying to understand why when i run iperf to generate traffic from Host2 to Host1 I'm observing the maximum allowed bandwidth of ~1Gbs. I would have assumed that rate-limiting one switch would introduce a bottle-net within my system.

In my experiment I will need to control the switches from the VMs on host 1. Maybe there's an access controll issue here that I am overlooking. Still I'm puzzling over the inability to enforce rate limits on a switch between to arbitrary hosts.

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    Is the traffic actually hitting the SVI? Or is it simply L2 traffic? If the latter, then no, the rate limits you have applied will have no effect.
    – YLearn
    Mar 10, 2018 at 2:54
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Apr 1, 2018 at 22:37

1 Answer 1

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If both devices are connected to VLAN 1, then that command has no effect. That command would be for traffic leaving the interface, but if both devices are on that VLAN, the traffic never leaves the VLAN.

There are several options for limiting the traffic rate on switches. You can limit on the physical switch interface with that command by applying it to the physical interface. Remember that if you have multiple links between two switches, STP will block all but one link, otherwise you will end up with a broadcast loop that brings down your LAN.

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  • This is helpful, but I want to increase bandwidth through the interface by adding new vlans to a trunk. If this trunk is a fiber interface then there must be a way to add new wavelengths on the fiber between two switches. Maybe vlans is not what I need here.
    – Matt Hall
    Mar 10, 2018 at 3:28
  • "I want to increase bandwidth through the interface by adding new vlans to a trunk. I think you are confused. The bandwidth of a link is the bandwidth of a link. You can create a port channel to increase the bandwidth between two switches, but only if you have multiple flows between the switches, the bandwidth of a single flow will be limited to the bandwidth of a single link. In aggregate, all the flows across the port channel will have more bandwidth than a single link.
    – Ron Maupin ♦
    Mar 10, 2018 at 3:31
  • But can I limit the rate at which data enters a link? Maybe by setting a limit on the interface, and the increasing it when more demand presents itself
    – Matt Hall
    Mar 10, 2018 at 3:37
  • Yes, but you seem to be working on an X-Y Problem. The simple solution seems to be a port channel. The real solution is probably proper QoS.
    – Ron Maupin ♦
    Mar 10, 2018 at 3:41

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