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Catalyst 2960 Switch SoftwareConfiguration Guide
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Chapter28 Configuring Network Securi ty with ACLs
Configuring IPv4 ACLs
To remove an ACL from a terminal line, use the no access-class access-list-number {in | out} line
configuration command.
Applying an IPv4 ACL to an Interface
This section describes how to apply IPv4 ACLs to network interfaces. Not e these guidelines:
•Apply an ACL only to inbound Layer 2 interfaces.
•When controlling access to an interface, you can use a named or numbered ACL.
Beginning in privileged EXEC mode, follow these steps to control access to an interface:
To remove the specified access group, use the no ip access-group {access-list-number | name} {in}
interface configuration command.
This example shows how to apply access list 2 to a port to filter packets entering the port:
Switch(config)# interface gigabitethernet0/1
Router(config-if)# ip access-group 2 in
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.
When you apply an undefined ACL to an interface, the switch acts as if the ACL has not be en applied to
the interface and permits all packets. Remember this behavior if you use undefined ACLs for network
security.
Hardware and Software Treatment of IP ACLs
ACL processing is primarily accomplished in hardware, but requires forwarding of some traffic flows to
the CPU for software processing. If the hardware reaches its capacity to store ACL configurations,
packets are sent to the CPU for forwarding. The forwarding rate for software-forwarded traffic is
substantially less than for hardware-forwarded traffic.
If ACLs cause large numbers of packets to be sent to the CPU, the switch perfo rmance can be negatively
affected.
When you enter the show ip access-lists privileged EXEC command, the match count displayed does
not account for packets that are access controlled in hardware. Use the show access-lists hardware
counters privileged EXEC command to obtain some basic hardware ACL statistics for switched packets.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Identify a specific interface for configuration, and enter interface
configuration mode.
Step3 ip access-group {access-list-number |
name} {in}
Control access to the specified interface.
Step4 end Return to privileged EXEC mode.
Step5 show running-config Display the access list configuration.
Step6 copy running-config startup-config (Optional) Save your entries in the configuration file.