Address Resolution Protocol; How Arp Works - Juniper IGP - CONFIGURATION GUIDE V11.1.X Configuration Manual

JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide

Address Resolution Protocol

Sending IP packets on a multiaccess network requires mapping from an IP address
to a MAC address (the physical or hardware address).
In an Ethernet environment, Address Resolution Protocol (ARP) is used to map a
MAC address to an IP address. ARP dynamically binds the IP address (the logical
address) to the correct MAC address. Before IP unicast packets can be sent, ARP
discovers the MAC address used by the Ethernet interface where the IP address is
configured.
Hosts that use ARP maintain a cache of discovered Internet-to-Ethernet address
mappings to minimize the number of ARP broadcast messages. To keep the cache
from growing too large, an entry is removed if it is not used within a certain period
of time. Before sending a packet, the host searches its cache for Internet-to-Ethernet
address mapping. If the mapping is not found, the host sends an ARP request.
NOTE: For information about MAC address validation, see "MAC Address Validation"
on page 22.

How ARP Works

Figure 8 on page 19 and Figure 9 on page 19 show how ARP works where host 1
sends an IP packet to host 2 on a different subnet. To complete this transmission,
host 1 needs the MAC address of router 1, to be used as the forwarding gateway.
A typical scenario is:
1.
2.
3.
18
Address Resolution Protocol
host1(config-if)#interface serial 2/1
host1(config-if)#encapsulation ppp
host1(config-if)#profile acton
Use the no version to remove the assignment from the interface.
See profile
Host 1 broadcasts an ARP request to all devices on subnet 1, composed by a
query with the IP address of router 1. The IP address of router 1 is needed
because host 2 is on a different subnet.
All devices on subnet 1 compare their IP address with the enclosed IP address
sent by host 1.
Having the matching IP address, router 1 sends an ARP response, which includes
its MAC address, to host 1.