25-9
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 25 Configuring Dynamic ARP Inspection
Configuring Dynamic ARP Inspection
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
po
rt 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.
Beginning in privileged EXEC mode, follow these steps to configure an ARP ACL on Switch A. This
p
rocedure is required in non-DHCP environments.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
arp access-list acl-name Define an ARP ACL, and enter ARP access-list configuration
mode. By default, no ARP access lists are defined.
Note At the end of the ARP access list, there is an implicit deny
ip any mac any command.
Step 3
permit ip host sender-ip mac host sender-mac
[log]
Permit ARP packets from the specified host (Host 2).
• For sender-ip, enter the IP address of Host 2.
• For sender-mac, enter the MAC address of Host 2.
• (Optional) Specify log to log a packet in the log buffer when
it matches the access control entry (ACE). Matches are logged
if you also configure the matchlog keyword in the ip arp
inspection vlan logging global configuration command. For
more information, see the “Configuring the Log Buffer”
section on page 25-13.
Step 4
exit Return to global configuration mode.
Step 5
ip arp inspection filter arp-acl-name vlan
vlan-range [static]
Apply the ARP ACL to the VLAN. By default, no defined ARP
ACLs are applied to any VLAN.
• For arp-acl-name, specify the name of the ACL created in
Step 2.
• For vlan-range, specify the VLAN that the switches and hosts
are in. You can specify a single VLAN identified by VLAN ID
number, a range of VLANs separated by a hyphen, or a series
of VLANs separated by a comma. The range is 1 to 4094.
• (Optional) Specify static to treat implicit denies in the ARP
ACL as explicit denies and to drop packets that do not match
any previous clauses in the ACL. DHCP bindings are not used.
If you do not specify this keyword, it means that there is no
e
xplicit deny in the ACL that denies the packet, and DHCP
bindings determine whether a packet is permitted or denied if
the packet does not match any clauses in the ACL.
ARP packets containing only IP-to-MAC address bindings are
c
ompared against the ACL. Packets are permitted only if the
access list permits them.
Step 6
interface interface-id Specify the Switch A interface that is connected to Switch B, and
enter interface configuration mode.
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