IronPort Systems ES-1528 - Manual (Page 133)

ES-1528 User’s Guide
133
C
H A P T E R
20
Dynamic ARP
This chapter describes how to activate dynamic Address Resolution Protocol (ARP) learning
and how to enter static ARP table entries.
20.1 ARP Table Overview
Address Resolution Protocol (ARP) is a protocol for mapping an Internet Protocol address (IP
address) to a physical machine address, also known as a Media Access Control or MAC
address, on the local area network.
An IP (version 4) address is 32 bits long. In an Ethernet LAN, MAC addresses are 48 bits
long. The ARP table maintains an association between each MAC address and its
corresponding IP address.
20.1.1 ARP Table Entries
The ARP table is populated with MAC and corresponding IP address mappings in two
different ways.
• DHCP Snooping - The switch listens to traffic from a DHCP server on a trusted port and
learns IP-to-MAC address bindings by parsing DHCP ACK packets.
• Static Entries - The switch administrator can enter static IP-to-MAC address mappings
via the web configurator.
20.1.2 How Dynamic ARP Works
When an incoming ARP packet destined for a host device on a local area network arrives at
the switch, the switch's ARP program looks in the ARP table and, if it finds the address, sends
it to the device.
If no entry is found for the IP address, dynamic ARP discards the ARP packet.
20.2 Enabling Dynamic ARP
Click Dynamic ARP > Settings in the navigation panel to open the following screen. Use the
Dynamic ARP screen to configure ARP filtering on the specified VLANs.