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Asked: 2016-06-30 12:17:04 +0800 CST

How to add latency and bandwidth-limit interface using tc?

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I know I can add latency to an interface using this command:

# tc qdisc add dev eth0 root netem delay 50ms

In addition to latency, I want to limit the bandwidth of this interface to 100kbps.

How can I do this?

I've seen some examples of how to limit the bandwidth. For example:

# tc qdisc add dev eth0 root handle 1: cbq avpkt 1000 bandwidth 10mbit 

# tc class add dev eth0 parent 1: classid 1:1 cbq rate 512kbit \
  allot 1500 prio 5 bounded isolated 

# tc filter add dev eth0 parent 1: protocol ip prio 16 u32

But if I try to run these commands after setting up the latency delay, I get errors that look like this:

RTNETLINK answers: File exists

For example:

# tc qdisc add dev eth0 root netem delay 97ms
# tc qdisc add dev eth0 root handle 1: cbq avpkt 1000 bandwidth 10kbit
RTNETLINK answers: File exists

I really want to be able to do both: Limit the bandwidth AND introduce a latency delay. Does anyone know how to accomplish this?

traffic-shaping tc

1 Answers

  1. In your example, you are trying to add two root qdiscs on the egress. The second command fails because you already have a root qdisc, and only one is allowed. Instead, create the root qdisc, then a child class for it, and then add your second qdisc as a child of the class. For example:

    tc qdisc add dev eth0 root handle 1:0 cbq avpkt 1000 bandwidth 10kbit
tc class add dev eth0 parent 1:0 classid 1:1 cbq rate 512kbit \
    allot 1500 prio 5 bounded isolated
tc qdisc add dev eth0 parent 1:1 netem delay 97ms

    In this example, we assigned the root qdisc the handle 1:0. We then specified the class as a child of 1:0 and gave it the handle 1:1. We then added the second qdisc as a child of 1:1.

    After you execute these commands, running tc qdisc should show both the cbq qdisc and the netem qdisc. The Linux Advanced Routing & Traffic Control HOWTO provides more information about qdiscs.

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