Administrators may need to set certain values for MTU, or have services in place that reserve ICMP headers for transmission. Such overhead may cause packet fragmentation causing interruptions in replication using Zerto Virtual Replication. As such, the administrator will want to test for fragmentation as a troubleshooting step when interruptions in replication occur.


The default MTU used by the VRAs is 1492 in ZVR 3.5 and 1500 prior to ZVR 3.5. Depending on a given network configuration, these MTU sizes may be too large. Note that VRAs engage in PMTU discovery, but in some cases (MPLS VPN being the most common one) this mechanism can fail to detect the correct MTU, and as such MTU will need to be manually set in the VRA guest OS. Additionally, the network can be adapted to accommodate the larger MTU.


To test for the optimal MTU sizing between components, run the following commands in either a Windows or Unix/Linux shell, starting with larger values (1464, 1472, depending on ZVR version, which is the default value minus 28 bytes, as described below), and decreasing from there:


Linux : ping -M do -s MTUsize IPaddress


Example: ping -M do -s 1472


Windows : ping IPaddress -f -l MTUsize


Example: ping -f -l 1472


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. If the VRAs’ MTU settings needs to be adjusted, contact AssureStor Support to either facilitate the changes or enable sudo level user accounts on the VRA. If a sudo level user account already exists, proceed as follows:


  1. Login to the VRA

  2. $ sudo vi /etc/network/interfaces

#Zerto IP Settings
auto lo

iface lo inet loopback


auto eth0

iface eth0 inet static



up route add -net default gw

  1. Add a new line at the end of the file, where xxxx is an MTU value:
    mtu xxxx

  2. Save settings with :wq

  3. $ sudo /etc/init.d/networking reload

  4. Verify the change with ifconfig