XCP NIC Bonding
NIC bonding can improve server resiliency by using two physical NICs as if they were one. Two NICs may be bonded to create a single, high-performing channel that provides connectivity between VMs and your external network. 
You can create a NIC bond in XenCenter by using "Add Network" in the Networking tab and choosing Bonded network.
This is the default mode with traffic balanced between NICs using Source Level Balancing (SLB). This mode will only work properly on a single logical switch (e.g. a single physical switch, or multiple switches that are stacked with proprietary backplanes such that they appear as a single switch). The advantage of this mode is it will use both interfaces for increased capacity, and also will fail-over between ports if one fails.
If you're trying to setup simple fail-over redundancy with 2 separate switches, you probably want active/backup, which you can only configure from the CLI.
The recommended method is setup the bond first in XenCenter, then change the type to active/backup as below:
Pick out the bond's master UUID like master ( RO): 4b9c2fa4-daa0-f9e8-4313-90cfa57019f3
xe pif-param-set uuid=4b9c2fa4-daa0-f9e8-4313-90cfa57019f3 other-config:bond-mode=active-backup