The interfaces of a UNIX gateway can be divided into two groups: logical (virtual) and physical interfaces.
Chapter 5, "Ethernet and VLANs," already introduced the concept of virtual LAN (VLAN) interfaces as an example for a logical interface, as well as alias (secondary) interface configuration. Data link layer address resolution works differently on WAN links. You must consider special Address Resolution Protocol (ARP) issues that are related to point-to-point or point-to-multipoint (Frame Relay) setups.
This chapter introduces the ARP operation as well as the gateway's view of its directly connected physical and logical interfaces. You have already learned in Chapter 5 how they become represented in the kernel routing and ARP tables, and how their deletion or disappearance affects these tables. As you will see, ARP operation can take on different manifestations, such as proxy ARP or gratuitous ARP, regular ARP and reverse/inverse ARP. A discussion of routing table tools (netstat, route, iproute2) concludes the chapter.