Relative Priority Of Arp Acls And Dhcp Snooping Entries; Configuring Arp Acls For Non-Dhcp Environments - Cisco Catalyst 2960-X Security Configuration Manual

Configuring Dynamic ARP Inspection
Feature
Log buffer
Per-VLAN logging

Relative Priority of ARP ACLs and DHCP Snooping Entries

Dynamic ARP inspection uses the DHCP snooping binding database for the list of valid IP-to-MAC address
bindings.
ARP ACLs take precedence over entries in the DHCP snooping binding database. The switch uses ACLs only
if you configure them by using the ip arp inspection filter vlan global configuration command. The switch
first compares ARP packets to user-configured ARP ACLs. If the ARP ACL denies the ARP packet, the
switch also denies the packet even if a valid binding exists in the database populated by DHCP snooping.

Configuring ARP ACLs for Non-DHCP Environments

This procedure shows how to configure dynamic ARP inspection when Switch B shown in Figure 2 does 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.
Follow these steps to configure an ARP ACL on Switch A. This procedure is required in non-DHCP
environments.
OL-29048-01
Relative Priority of ARP ACLs and DHCP Snooping Entries
Default Settings
When dynamic ARP inspection is enabled, all denied
or dropped ARP packets are logged.
The number of entries in the log is 32.
The number of system messages is limited to 5 per
second.
The logging-rate interval is 1 second.
All denied or dropped ARP packets are logged.
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