I’ve already written about setting the MTU (Maximum Transmission Unit) under Linux including Jumbo frames (FreeBSD specific MTU information is here).

With this quick tip you can increase MTU size to get a better networking performance.

How do I setup MTU under XEN xenbr0 interface?

You can easily set MTU under XEN (xenbr0) using ifconfig or ip command as follows:
# /sbin/ifconfig xenbr0 mtu 1400
OR
# /sbin/ip link set dev xenbr0 mtu 1400
Replace 1400 with required MTU value such as 1500. You can display current mtu with ip command or by visting /sys/class/net/xenbr0/mtu file:
# cat /sys/class/net/xenbr0/mtu
or
# /sbin/ip link show xenbr0

Make changes permanent

Append following line to /etc/rc.local:
# vi /etc/rc.local
Add following line, type:
/sbin/ip link set dev xenbr0 mtu 1400
Save and close the file.