Unified ISSU: Abstracted fabric interfaces

Abstracted fabric (af) interfaces, configured for Junos Node Slicing, support unified in-service software upgrade (ISSU). Unified ISSU enables an upgrade between two Junos OS releases with no disruption on the control plane and with minimal disruption of traffic. An af interface is a pseudo interface that represents a first class Ethernet interface behavior. An AF interface facilitates routing control and management traffic between guest network functions (GNFs) through the switch fabric.