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.
Beginning in privileged EXEC mode, follow these steps to configure an ARP ACL on Switch A. This procedure
is required in non-DHCP environments.
SUMMARY STEPS
1.
Configureterminal
2.
arp access-list acl-name
3.
permit ip host sender-ip mac host sender-mac
log
4.
exit
5.
ip arp inspection filter arp-acl-name vlan vlan-range
[
static
]
6.
interface interface-id
7.
no ip arp inspection trust
8.
end
9.
show arp access-list
acl-name
show ip arp inspection vlan vlan-range show ip arp inspection interfaces
10.
copy running-config startup-config
DETAILED STEPS
Purpose
Command or Action
Enter global configuration mode.
Configureterminal
Step 1
Define an ARP ACL, and enter ARP access-list configuration mode. By default, no
ARP access lists are defined.
arp access-list acl-name
Step 2
At the end of the ARP access list, there is an implicit
deny ip any mac any
command.
Note
Permit ARP packets from the specified host (Host 2).
permit ip host sender-ip mac
host sender-mac
log
Step 3
•
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 theip arp inspection vlan logging global configuration
command. For more information, see the section,
“
Configuring the Log Buffer."
Return to global configuration mode.
exit
Step 4
Apply the ARP ACL to the VLAN. By default, no defined ARP ACLs are applied to
any VLAN.
ip arp inspection filter
arp-acl-name vlan vlan-range
[
static
]
Step 5
•
For
arp-acl-name
, specify the name of the ACL created in Step 2.
Catalyst 2960-XR Switch Security Configuration Guide, Cisco IOS Release 15.0(2)EX1
OL-29434-01
203
Configuring Dynamic ARP Inspection
Configuring ARP ACLs for Non-DHCP Environments