MTU Testing

KB Number:
000001630

Symptoms:
When the customer creates a new VPG and the creation completes successfully. The VPG is now going through a initial sync and the progress is not moving.
Or
The customer sees brief site disconnections.

Cause:
The MTU size is too big and needs to be adjusted to fully send the packets.

Solution:
  1. Log into the VRA on the source site
  2. ping -M do -s MTUsize IPaddress
    1.   Example: ping -M do -s 1472 192.168.1.2
       

If testing results return as “fragmentation needed”, continue decreasing the value set until pings transmit without fragmentation. The pings referenced above will not allow fragmentation (“-M do”, and “–f”) so the ping will report when you hit a number where packets don’t fragment.
Note that the -s and -l flags control the data size. Due to the IP + ICMP packet overhead, the packet size is equal to the data size + 28 bytes. For example, if testing shows that only pings with size 1444 can pass normally, the MTU should be set to 1444+28 = 1472
Once a ping works with a given MTU size, either the network can be adjusted to accommodate for larger MTU, or the VRAs’ MTU settings can be adjusted.
Another useful method of determining the proper MTU size with ICMP pings can be found here: http://muzso.hu/2009/05/17/how-to-determine-the-proper-mtu-size-with-icmp-pings

Changing MTU in 4.0 and later VRAs
sed -i -r 's/mtu [0-9]{4}/mtu xxxx/' /etc/network/interfaces         NOTE: this will not work if the VRAs have the network interface set to DHCP
In order to validate the changes you will need to reload the networking settings, using this following command:
/etc/init.d/networking reload
and verify the changes using ifconfig
 

The MTU can be changed manually by performing the following in vi or nano:
1) Login to VRA 
2) vi /etc/network/interfaces or nano /etc/network/interfaces
# ZeRTO IP settings
 #loopback
 auto lo
 iface lo inet loopback
 
 auto eth0
 iface eth0 inet static
 address 172.20.121.25
 netmask 255.255.255.0
 up route add -net default gw 172.20.121.254

3) Add a line with the following syntax:
mtu xxxx
(where xxxx is an MTU value) Use the delete button, and not the backspace to change the mtu. Use i to insert the mtu number
4) Save (:wq) if using vi  or CRTL X and Y if using nano
5) /etc/init.d/networking reload
6) Verify change with ifconfig

 


Affected Versions:
6.0 and later

Hypervisor:
VMware, Hyper-V

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