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Chapter 21
Configuring Dynamic ARP Inspection
Understanding Dynamic ARP Inspection
Interface Trust States and Network Security
Dynamic ARP inspection associates a trust state with each interface on the switch. Packets arriving on
trusted interfaces bypass all dynamic ARP inspection validation checks, and those arriving on untrusted
interfaces undergo the dynamic ARP inspection validation process.
In a typical network configuration, you configure all switch ports connected to host ports as untrusted
and configure all switch ports connected to switches as trusted. With this configuration, all ARP packets
entering the network from a given switch bypass the security check. No other validation is needed at any
other place in the VLAN or in the network. You configure the trust setting by using the ip arp inspection
trust interface configuration command.
Use the trust state configuration carefully. Configuring interfaces as untrusted when they should be
Caution
trusted can result in a loss of connectivity.
In
Figure
21-2, assume that both Switch A and Switch B are running dynamic ARP inspection on the
VLAN that includes Host 1 and Host 2. If Host 1 and Host 2 acquire their IP addresses from the DHCP
server connected to Switch A, only Switch A binds the IP-to-MAC address of Host 1. Therefore, if the
interface between Switch A and Switch B is untrusted, the ARP packets from Host 1 are dropped by
Switch B. Connectivity between Host 1 and Host 2 is lost.
Figure 21-2
ARP Packet Validation on a VLAN Enabled for Dynamic ARP Inspection
DHCP server
Switch A
Switch B
Port 1
Port 3
Host 1
Host 2
Configuring interfaces to be trusted when they are actually untrusted leaves a security hole in the
network. If Switch A is not running dynamic ARP inspection, Host 1 can easily poison the ARP cache
of Switch B (and Host 2, if the link between the switches is configured as trusted). This condition can
occur even though Switch B is running dynamic ARP inspection.
Dynamic ARP inspection ensures that hosts (on untrusted interfaces) connected to a switch running
dynamic ARP inspection do not poison the ARP caches of other hosts in the network. However, dynamic
ARP inspection does not prevent hosts in other portions of the network from poisoning the caches of the
hosts that are connected to a switch running dynamic ARP inspection.
In cases in which some switches in a VLAN run dynamic ARP inspection and other switches do not,
configure the interfaces connecting such switches as untrusted. However, to validate the bindings of
packets from nondynamic ARP inspection switches, configure the switch running dynamic ARP
inspection with ARP ACLs. When you cannot determine such bindings, at Layer 3, isolate switches
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
21-3
OL-8915-03
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