Notes for the neutron GRE Mode

In GRE mode, if MTU and offloading are improperly configured, the network performance (https://ask.openstack.org/en/question/6140/quantum-neutron-gre-slow-performance/) is severely reduced ).

The following configurations (http://openstack.redhat.com/Using_GRE_tenant_networks) are recommended for rdo ):

MTU

When using GRE, set the MTU in the guest to 1400, this will allow for the GRE header and no packet fragmentation.

Hint: If you want to check if this answer will solve your issue, executeIfconfig eth0 MTU 1400On the instance and check the speed.

Edit the file:/Etc/quantum/dhcp_agent.iniAnd set:

dnsmasq_config_file=/etc/neutron/dnsmasq-neutron.conf

Create and edit file// Etc/neutron/dnsmasq-neutron.confAnd put this inside:

dhcp-option-force=26,1400

Restart DHCP:

service neutron-dhcp-agent restart

And reboot your instance. Verify that your MTU is 1400:

 [email protected]:~# ifconfig eth0      Link encap:Ethernet  HWaddr fa:16:3e:f0:6a:9f             inet addr:10.1.0.2  Bcast:10.1.0.255  Mask:255.255.255.0           inet6 addr: fe80::f816:3eff:fef0:6a9f/64 Scope:Link           UP BROADCAST RUNNING MULTICAST  **MTU:1400**  Metric:1           RX packets:934855 errors:0 dropped:0 overruns:0 frame:0           TX packets:207741 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000            RX bytes:1210256390 (1.2 GB)  TX bytes:226172592 (226.1 MB)

In fact, you only need to modify MTU to 1454: DHCP-Option-force = 26,1454

Offloading

You shoshould turn offloading such as Tso/LRO and GRO/GSO off on the instance physical machine for traffic to work.

This can be done with this command (replace ethx with your physical network interface name ):

ethtool -K ethX tso off lro off gro off gso off

You can modify the network script for this change to apply on startup:

/etc/sysconfig/network-scripts/ifcfg-eth0ETHTOOL_OPTS="-K ${DEVICE}  tso off lro off gro off gso off"