The show fdb command is a global command. When you type it without a context, it shows all your current FDB entries regardless of which ports or VLANs they are specific to. MAC address tables in XOS are specific to VLANS and not VRs. So if you create a VLAN-1 and assign port 1,2 and 3 to it, any MAC addresses learnt on those ports belong to the VLAN-1 FDB as opposed to the VR. You are therefore able to type show fdb vlan that shows all MACs particular to the specified VLAN. You can also specify port, VPLS, etc BUT, you cannot specify VR