The definition of interface tags in Advanced Routing and Forwarding (ARF) facilitates the use of virtual routers, which only use a part of the overall routing table. The interface tag for a packet received from another local router is set according to the following procedure:
- If the a packet's sender address is recognized as coming from an IP network which is defined in the device, then the interface tag for that IP address is used.
- If the interface receiving the packet is connected to just one IP network, then the interface tag for that IP network is used.
- If there is no unique result from steps 1 and 2, the device attempts to use the MAC address to determine the IP address of the next hop (reverse ARP lookup). The devices uses this IP address in an attempt to identify the relevant IP network, and thus the corresponding interface tag.
- If there is no unique result from options 1 to 3, then the device attempts to identify the relevant IP network (and interface tag) from the routing table.