Specifications

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Catalyst 2970 Switch Software Configuration Guide
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Chapter 26 Configuring Network Security with ACLs
Configuring IP ACLs
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 on Gigabit Ethernet interface 0/3 to filter packets entering
the interface:
Switch(config)# interface gigabitethernet0/3
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.
By default, the input interface sends ICMP Unreachable messages whenever a packet is discarded. 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.
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 performance 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.
IP ACL Configuration Examples
This section provides examples of configuring and applying IP ACLs. For detailed information about
compiling ACLs, refer to the Security Configuration Guide and the “IP Services” chapter of the Cisco
IOS IP and IP Routing Configuration Guide for IOS Release 12.1.
This example uses a standard ACL to allow access to the interface to a specific Internet host with the
address 172.20.128.64.
Switch(config)# access-list 6 permit 172.20.128.64 0.0.0
Switch(config)# end
Switch# show access-lists
Standard IP access list 6
permit 172.20.128.64 wildcard bits 0.0.0.0
Step 3
ip access-group {access-list-number |
name} {in}
Control access to the specified interface.
Step 4
end Return to privileged EXEC mode.
Step 5
show running-config Display the access list configuration.
Step 6
copy running-config startup-config (Optional) Save your entries in the configuration file.
Command Purpose