[Openstack-operators] Virtual Instance can't obtain IP address (OpenStack Havana)

David Magda dmagda at ee.ryerson.ca
Wed Jan 29 15:35:19 UTC 2014

On Wed, January 29, 2014 10:25, Alvise Dorigo wrote:
> So, it seems that the DHCP client's request correctly originates from the
> VM, and is correctly trasmitted to the network node by mean of the data
> network the two br-int interfaces are connected to; but for some reason
> the dhcp-agent doesn't receive anything, as proved by the null traffic on
> its tap.
> Any idea about what and how I could more deeply investigate ?

On your network node, check the NIC settings for various offloading

    $ ethtool -k eth0 | grep offload
    tcp-segmentation-offload: off
    udp-fragmentation-offload: off [fixed]
    generic-segmentation-offload: off
    generic-receive-offload: off
    large-receive-offload: off [fixed]
    rx-vlan-offload: on
    tx-vlan-offload: on

We recently solved this problem at $WORK by disabling GRO on our network

The other issue we've come across that may be related is MTUs: since Open
vSwitch 1.9+ got rid of Path MTU Discovery, you may need to adjust your


There are two ways to fix this: if your network infrastructure has jumbo
frames (1500+) enabled, change the hardware NICs to have an MTU of 1546
(or larger). If you do not have jumbo frames enabled, configure the guests
to have an MTU of 1454 or smaller.

One or both of these things (GRO and MTU) will probably be needed for a
properly functioning virtual network.

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