19-20
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 19 Configuring Network Security with ACLs
Configuring Router ACLs
Note
The ip access-group interface configuration command is only valid when applied to a Layer 3
interface: an SVI, a 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.
If the input interface is configured to send ICMP Unreachable messages, these messages are sent
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.
Displaying ACLs and Access Groups
You can display existing ACLs and when you use the ip access-group interface configuration command
to apply ACLs to a Layer 3 interface, you can display the access groups on the interface. You can use
the privileged EXEC commands as described in
Table 19-3
to display this information.
Table 19-3 Commands for Displaying Access Lists and Access Groups
Command
Purpose
show access-lists [number | name]
Display the contents of one or all current IP and MAC address access lists
or a specific access list (numbered or named).
show ip access-lists [number | name]
Display the contents of all current IP access lists or a specific IP access list
(numbered or named).
show ip interface interface-id
Display detailed configuration and status of an interface. If IP is enabled
on the interface and ACLs have been applied by using the ip access-group
interface configuration command, the access groups are included in the
display.
show running-config [interface interface-id]
Displays the contents of the configuration file for the switch or the
specified interface, including all configured access groups, whether or not
IP is enabled on an interface.
