Cisco Systems 3750E manual Configuring ARP ACLs for Non-DHCP Environments, 23-8

Chapter 23 Configuring Dynamic ARP Inspection

Configuring Dynamic ARP Inspection

	Command	Purpose
Step 4
	interface interface-id	Specify the interface connected to the other switch, and enter
		interface configuration mode.
Step 5
	ip arp inspection trust	Configure the connection between the switches as trusted.
		By default, all interfaces are untrusted.
		The switch does not check ARP packets that it receives from the
		other switch on the trusted interface. It simply forwards the
		packets.
		For untrusted interfaces, the switch intercepts all ARP requests
		and responses. It verifies that the intercepted packets have valid
		IP-to-MAC address bindings before updating the local cache and
		before forwarding the packet to the appropriate destination. The
		switch drops invalid packets and logs them in the log buffer
		according to the logging configuration specified with the ip arp
		inspection vlan logging global configuration command. For
		more information, see the “Configuring the Log Buffer” section
		on page 23-12.
Step 6
	end	Return to privileged EXEC mode.
Step 7
	show ip arp inspection interfaces	Verify the dynamic ARP inspection configuration.
	show ip arp inspection vlan vlan-range
Step 8
	show ip dhcp snooping binding	Verify the DHCP bindings.
Step 9
	show ip arp inspection statistics vlan	Check the dynamic ARP inspection statistics.
	vlan-range
Step 10
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable dynamic ARP inspection, use the no ip arp inspection vlan vlan-rangeglobal configuration command. To return the interfaces to an untrusted state, use the no ip arp inspection trust interface configuration command.

This example shows how to configure dynamic ARP inspection on Switch A in VLAN 1. You would perform a similar procedure on Switch B:

Switch(config)# ip arp inspection vlan 1

Switch(config)# interface gigabitethernet1/0/1

Switch(config-if)#ip arp inspection trust

Configuring ARP ACLs for Non-DHCP Environments

This procedure shows how to configure dynamic ARP inspection when Switch B shown in Figure 23-2 on page 23-3does not support dynamic ARP inspection or DHCP snooping.

If you configure port 1 on Switch A as trusted, a security hole is created because both Switch A and Host 1 could be attacked by either Switch B or Host 2. To prevent this possibility, you must configure port 1 on Switch A as untrusted. To permit ARP packets from Host 2, you must set up an ARP ACL and apply it to VLAN 1. If the IP address of Host 2 is not static (it is impossible to apply the ACL configuration on Switch A) you must separate Switch A from Switch B at Layer 3 and use a router to route packets between them.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

23-8 OL-9775-02