3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 2 ARP Configuration3Com Corporation2-72.4 Map between WAN Interface IP Address and Link LayerProtocol AddressIn a router, you shall maintain both the mapping from an Ethernet interface IP addressto an MAC address, and that from a WAN interface IP address to a link layer protocoladdress. Namely there are the following types:z On an interface encapsulated with X.25, the mapping between an IP address andX.121 address is maintained by the command x25 map ip.z On an interface encapsulated with Frame Relay, mapping between an IP addressand a virtual circuit number (DLCI) is maintained by the command fr map ip.The above mapping tables are also called second routing, which is essential to thenormal working of the router. For details, refer to related chapters in "Link LayerProtocol" of this manual.2.5 Authorized ARP Configuration2.5.1 Introduction to Authorized ARPAuthorized address resolution protocol (authorized ARP) enables the DHCP server orother modules to automatically add authorized ARP entries to the ARP table based oncertain conventions. Allowing only static entries and entries with IP addresses from theDHCP server to be added to the ARP table, authorized ARP can stop a DHCP serverlearning dynamically from illegal ARP responses, making illegal clients unable toaccess the Internet. Authorized ARP also provides a probing mechanism called ARPping. It can identify a log-off DHCP client and notify the DHCP Server of that. Bydeploying authorized ARP, you can achieve both enhanced network security and quickdetection of a client going down.Note:z Currently, authorized ARP can only be implemented on a router with the DHCPserver function enabled.z Currently, authorized ARP supports only the scenario that the DHCP server andDHCP clients are on the same segment.2.5.2 Basic Conceptsz ARP cacheEach host has an ARP cache, in which the recently learned mappings between the IPaddresses and hardware addresses are kept. By default, the lifetime of each entry inthe cache is 20 minutes.
