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Configuring Dynamic ARP Inspection
This chapter describes how to configure dynamic Address Resolution Protocol inspection (dynamic ARP
inspection) on the Catalyst 3750 switch. This feature helps prevent malicious attacks on the switch by
not relaying invalid ARP requests and responses to other ports in the same VLAN. Unless otherwise
noted, the term switch refers to a standalone switch and to a switch stack.
To use this feature, you must have the enhanced multilayer image (EMI) installed on your switch.
For complete syntax and usage information for the commands used in this chapter, refer to the command
Note
reference for this release.
This chapter consists of these sections:
•
•
•
Understanding Dynamic ARP Inspection
ARP provides IP communication within a Layer 2 broadcast domain by mapping an IP address to a MAC
address. For example, suppose Host B wants to send information to Host A but does not have the MAC
address of Host A in its ARP cache. Host B generates a broadcast message for all hosts within the
broadcast domain to obtain the MAC address associated with the IP address of Host A. All hosts within
the broadcast domain receive the ARP request, and Host A responds with its MAC address. However,
because ARP allows a gratuitous reply from a host even if an ARP request was not received, an ARP
spoofing attack and the poisoning of ARP caches can occur. After the attack, all traffic from the device
under attack flows through the attacker's computer and then to the router, switch, or host.
A malicious user can attack hosts, switches, and routers connected to your Layer 2 network by poisoning
the ARP caches of systems connected to the subnet and by intercepting traffic intended for other hosts
on the subnet.
78-16180-02
Understanding Dynamic ARP Inspection, page 22-1
Configuring Dynamic ARP Inspection, page 22-5
Displaying Dynamic ARP Inspection Information, page 22-14
Figure 22-1
shows an example of ARP cache poisoning.
C H A P T E R
Catalyst 3750 Switch Software Configuration Guide
22
22-1
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