Introduction To Authorized Arp; Basic Concepts; Map Between Wan Interface Ip Address And Link Layer Protocol Address; Authorized Arp Configuration - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide
2.4 Map between WAN Interface IP Address and Link Layer
Protocol Address
In a router, you shall maintain both the mapping from an Ethernet interface IP address
to an MAC address, and that from a WAN interface IP address to a link layer protocol
address. Namely there are the following types:
On an interface encapsulated with X.25, the mapping between an IP address and
X.121 address is maintained by the command x25 map ip.
On an interface encapsulated with Frame Relay, mapping between an IP address
and 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 the
normal working of the router. For details, refer to related chapters in "Link Layer
Protocol" of this manual.

2.5 Authorized ARP Configuration

2.5.1 Introduction to Authorized ARP

Authorized address resolution protocol (authorized ARP) enables the DHCP server or
other modules to automatically add authorized ARP entries to the ARP table based on
certain conventions. Allowing only static entries and entries with IP addresses from the
DHCP server to be added to the ARP table, authorized ARP can stop a DHCP server
learning dynamically from illegal ARP responses, making illegal clients unable to
access the Internet. Authorized ARP also provides a probing mechanism called ARP
ping. It can identify a log-off DHCP client and notify the DHCP Server of that. By
deploying authorized ARP, you can achieve both enhanced network security and quick
detection of a client going down.
Note:
Currently, authorized ARP can only be implemented on a router with the DHCP
server function enabled.
Currently, authorized ARP supports only the scenario that the DHCP server and
DHCP clients are on the same segment.

2.5.2 Basic Concepts

ARP cache
Each host has an ARP cache, in which the recently learned mappings between the IP
addresses and hardware addresses are kept. By default, the lifetime of each entry in
the cache is 20 minutes.
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2-7
Chapter 2 ARP Configuration