XenServer - join host with different number of interfaces to existing pool
If your host has different number of interfaces then hosts in existing pool you need to make sure the names of interfaces are in order with those in pool. It means that if you use 8 NICs in pool and new host has 10 NICs you need to user first 8 NICs on that pool.
So if somehow you ended up with connecting eth2-eth9 for whatever reason on new host you need to rename them to become eth0-eth7. Especially if you use bonded interfaces in pool your NIC names have to be consistent. More detail on renaming interface on xenserver you can find on citrix page: https://support.citrix.com/article/CTX135809.
To rename interface you can use interface-rename command on host. So login on your new host localy using server console or remote console if applicable in your case.
Run command to see all your NICs:
interface-rename --list
Name MAC PCI ethN Phys SMBios Driver Version Firmware
eth0 90:1b:0e:eb:3e:d9 0000:01:00.0 eth0 em1 Intel i350 igb 5.3.5.3 1.63, 0x80000ec5
eth1 90:1b:0e:eb:3e:da 0000:01:00.1 eth1 em2 Intel i350 igb 5.3.5.3 1.63, 0x80000ec5
eth2 4c:52:62:01:72:57 0000:3d:00.0 eth2 em3 Flexible LOM Adapter i40e 1.3.46-k 3.33 0x800010ef 1.1876.0
eth3 4c:52:62:01:72:58 0000:3d:00.1 eth3 em4 Flexible LOM Adapter i40e 1.3.46-k 3.33 0x800010ef 1.1876.0
eth4 4c:52:62:01:72:59 0000:3d:00.2 eth4 em5 Flexible LOM Adapter i40e 1.3.46-k 3.33 0x800010ef 1.1876.0
eth5 4c:52:62:01:72:5a 0000:3d:00.3 eth5 em6 Flexible LOM Adapter i40e 1.3.46-k 3.33 0x800010ef 1.1876.0
eth6 b4:96:91:03:f6:74 0000:d8:00.0 eth6 p7p1 SLOT 07 igb 5.3.5.3 1.63, 0x80000ee4
eth7 b4:96:91:03:f6:75 0000:d8:00.1 eth7 p7p2 SLOT 07 igb 5.3.5.3 1.63, 0x80000ee4
eth8 b4:96:91:03:f6:76 0000:d8:00.2 eth8 p7p3 SLOT 07 igb 5.3.5.3 1.63, 0x80000ee4
eth9 b4:96:91:03:f6:77 0000:d8:00.3 eth9 p7p4 SLOT 07 igb 5.3.5.3 1.63, 0x80000ee4
According to this list you need to make eth2 into eth0, eth3 into eth1 etc. To make that use command interface-rename --update <target eth name>=MAC|PCI|Phys|SMBios. I will use 'Phys' names cause it's shorter to write. Result will be:
interface-rename --update eth0=em3 eth1=em4 eth2=em5 eth3=em6 eth4=p7p1 eth5=p7p2 eth6=p7p3 eth7=p7p4 eth8=em1 eth9=em2
INFO [2018-09-05 11:14:47] Performing manual update of rules. Not actually renaming interfaces
INFO [2018-09-05 11:14:47] All done
Now you need to reboot server for changes to take effect. When server is up again check NIC order:
interface-rename --list
Name MAC PCI ethN Phys SMBios Driver Version Firmware
eth0 4c:52:62:01:72:57 0000:3d:00.0 eth2 em3 Flexible LOM Adapter i40e 1.3.46-k 3.33 0x800010ef 1.1876.0
eth1 4c:52:62:01:72:58 0000:3d:00.1 eth3 em4 Flexible LOM Adapter i40e 1.3.46-k 3.33 0x800010ef 1.1876.0
eth2 4c:52:62:01:72:59 0000:3d:00.2 eth4 em5 Flexible LOM Adapter i40e 1.3.46-k 3.33 0x800010ef 1.1876.0
eth3 4c:52:62:01:72:5a 0000:3d:00.3 eth5 em6 Flexible LOM Adapter i40e 1.3.46-k 3.33 0x800010ef 1.1876.0
eth4 b4:96:91:03:f6:74 0000:d8:00.0 eth6 p7p1 SLOT 07 igb 5.3.5.3 1.63, 0x80000ee4
eth5 b4:96:91:03:f6:75 0000:d8:00.1 eth7 p7p2 SLOT 07 igb 5.3.5.3 1.63, 0x80000ee4
eth6 b4:96:91:03:f6:76 0000:d8:00.2 eth8 p7p3 SLOT 07 igb 5.3.5.3 1.63, 0x80000ee4
eth7 b4:96:91:03:f6:77 0000:d8:00.3 eth9 p7p4 SLOT 07 igb 5.3.5.3 1.63, 0x80000ee4
eth8 90:1b:0e:eb:3e:d9 0000:01:00.0 eth0 em1 Intel i350 igb 5.3.5.3 1.63, 0x80000ec5
eth9 90:1b:0e:eb:3e:da 0000:01:00.1 eth1 em2 Intel i350 igb 5.3.5.3 1.63, 0x80000ec5
Now just configure management interface on new NIC and reboot server. If you already configured management interface you will need to do it again cause it will stay on same interface as before.
In my case I configured management on interface eth0 before changing order though active link was on eth2. Then after changing NIC order and rebooting server it was reachable over eth0 interface.
Be careful not to use xe-reset-networking cause it will revert names back.