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Cannot change br-int and br-tun MTU to >1500

asked 2014-06-28 02:52:50 -0500

Marwan AKIKI gravatar image

updated 2014-06-28 03:00:08 -0500

we have a server setup where openstack components are KVM-VMs:

  1. icehouse-controller.

  2. icehouse-network.

  3. icehouse-cinder.

All these components are attached to a bridge br-main-vm over subinterface (vlan 112) --> em2.112 of the main giga-ethernet interface em2.

Thus, VM --> br-main-vm --> em2.112 --> em2.

 - All interfaces have mtu=9000 configured in the interfaces file.

 - Further each VM has its interface configures to mtu=9000.

 - Pinging and passing data can be performed with packet > 1500.

However, br-int and br-tun (GRE) MTUs cannot be configured. They have by default 1500. I tried the veth_mtu = 9000 in the [agent] section of ml2_conf.ini but in vain.

I do not want to reduce the guest VM MTU; thus, how to proceed.

Hint: I tried to change the MTU in: nano /sys/devices/virtual/net/br-tun/mtu and nano /sys/class/net/br-tun/mtu but it did not work, ovs rewrite them.

Regards Marwan

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Did you try changing this with ifconfig? It might be as "simple" as writing a script that runs after startup to ifconfig mtu=blah. I don't see this option set in ovs natively. I know that there are ways to do this because people are able to configure it.

mpetason gravatar imagempetason ( 2014-06-30 13:42:04 -0500 )edit

Hi, I am also facing similar issue, were you able to find out the solution ?

prat gravatar imageprat ( 2014-10-07 04:28:58 -0500 )edit

2 answers

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answered 2014-06-30 10:36:04 -0500

granadol gravatar image

You change guest MTU by configuring this in the /etc/neutron/dhcp_agent.ini: dnsmasq_config_file=/etc/neutron/dnsmasq-neutron.conf

Then create the dnsmasq-neutron.conf with this:


This forces an MTU of 1400

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Sorry. Just read where you don;t want to reduce guest. Please disregard

granadol gravatar imagegranadol ( 2014-06-30 10:37:39 -0500 )edit

he same problem for me.

Instance running MTU 9000 I have set dhcp-option-force=26,9000 But still br-int and ovs qdhcp- interfaces is running mtu 1500.

JonasH gravatar imageJonasH ( 2014-09-18 00:27:51 -0500 )edit

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Asked: 2014-06-28 02:52:50 -0500

Seen: 1,445 times

Last updated: Jun 30 '14