Cisco WS-C3750-48PS-S Software Configuration Manual page 603

Chapter 31 Configuring Network Security with ACLs
Beginning in privileged EXEC mode, follow these steps to control access to an interface:
Command
Step 1 configure terminal
Step 2 interface interface-id
Step 3
ip access-group {access-list-number |
name} {in | out}
Step 4 end
Step 5 show running-config
Step 6
copy running-config startup-config
To remove the specified access group, use the no ip access-group {access-list-number | name} {in | out}
interface configuration command.
This example shows how to apply access list 2 to a port to filter packets entering the port:
Switch(config)# interface gigabitethernet1/0/1
Router(config-if)# ip access-group 2 in
When you apply the ip access-group interface configuration command to a Layer 3 interface (an SVI, a
Note
Layer 3 EtherChannel, or a routed port), the interface must have been configured with an IP address.
Layer 3 access groups filter packets that are routed or are received by Layer 3 processes on the CPU.
They do not affect packets bridged within a VLAN.
For inbound ACLs, after receiving a packet, the switch checks the packet against the ACL. If the ACL
permits the packet, the switch continues to process the packet. If the ACL rejects the packet, the switch
discards the packet.
For outbound ACLs, after receiving and routing a packet to a controlled interface, the switch checks the
packet against the ACL. If the ACL permits the packet, the switch sends the packet. If the ACL rejects
the packet, the switch discards the packet.
By default, the input interface sends ICMP Unreachable messages whenever a packet is discarded,
regardless of whether the packet was discarded because of an ACL on the input interface or because of
an ACL on the output interface. ICMP Unreachables are normally limited to no more than one every
one-half second per input interface, but this can be changed by using the ip icmp rate-limit unreachable
global configuration command.
When you apply an undefined ACL to an interface, the switch acts as if the ACL has not been applied to
the interface and permits all packets. Remember this behavior if you use undefined ACLs for network
security.
78-16180-02
Purpose
Enter global configuration mode.
Identify a specific interface for configuration, and enter interface
configuration mode.
The interface can be a Layer 2 interface (port ACL), or a Layer 3 interface
(router ACL).
Control access to the specified interface.
The out keyword is not supported for Layer 2 interfaces (port ACLs).
Return to privileged EXEC mode.
Display the access list configuration.
(Optional) Save your entries in the configuration file.
Catalyst 3750 Switch Software Configuration Guide
Configuring IP ACLs
31-21