Optimizing OpenStack Traffic Flow

Deciding whether you need to change the BIG-IQ VE MTU value

If your BIG-IQ VE needs to pass network traffic between an internal tenant (private) network and an external (public) network using an OpenStack virtual machine, the traffic must be directed through a router. The typical OpenStack configuration routes this through an Open-vSwitch Router. When data blocks pass through the router in this scenario, the traffic incurs a 50 byte overhead. For a BIG-IQ VE that sends and receives network traffic through interfaces that connect to internal tenant and external public networks, you must reduce the VE instance’s MTU value by 50 to account for that overhead.

Note: This requirement applies to other virtual machine types (such as BIG-IP and Linux) not just BIG-IQ Virtual Edition.

A common indication that you need to adjust the MTU value is when you can make an SSH connection to the BIG-IQ, but you cannot make a web connection over HTTPS. If everything else is configured correctly, then check to see if the MTU setting is causing the issue. Correct configuration (in this case) means:

• Ports 22 and 443 are open to receiving traffic.
• All required system services appear to be running normally.
• There is no abnormal consumption of system resources (RAM and CPU).

To verify that the MTU setting is causing the issue, you can log in to the BIG-IQ using SSH and run the following command: ifconfig eth0 mtu 1450. Then try connecting to the BIG-IQ again using your browser. If you can connect successfully, then you know that the MTU setting is the issue, and you need to permanently modify the MTU setting to make sure the user interface performs properly.

If you change the MTU value on the BIG-IQ using the command line, the value resets to 1500 whenever the VE reboots. So you need to edit the custom configuration file so that any time the VE restarts, the config file sets the correct value.

Set BIG-IQ VE MTU values to pass traffic between tenant and external networks

You edit the custom configuration file so that any time the VE restarts, it sets the MTU value correctly.

1. Access the BIG-IQ bash command shell.

   You can access this shell from the OpenStack console or you can use SSH to log in to the BIG-IQ VE.

2. In the command shell, type the command sudo vim /config/startup.

3. Type the user password when prompted.

   The config startup file opens for editing.

4. Type I to enter insert (edit) mode and add either of these two commands to the end of the file:

   • ifconfig eth0 mtu 1450
   • ip link set eth0 mtu 1450

5. Press the Esc key to exit insert mode.

6. Type :w! and press Enter to commit changes.

7. Type :q to exit the editor and return to the command prompt.

8. To verify the change was saved, type cat /config/startup.

9. To reboot the VE, type tmsh reboot.

10. When the reboot completes, return to the command shell and type ifconfig eth0 and verify that the new MTU value persists.

11. Access the BIG-IQ bash command shell run, and then type the following commands to change the MTU value for the default route in the device route table.

    1. tmsh modify sys management-route default {mtu 1450}

    2. tmsh save sys config

12. To verify the change, type ip route.

    This change to the local route table is persistent, so you do not need to make a change to the /config/startup file.