Cisco Systems 3750E manual Configuring Network Security with ACLs, Understanding ACLs, 34-1

C H A P T E R 34

Configuring Network Security with ACLs

This chapter describes how to configure network security on the Catalyst 3750-E or 3560-E switch by using access control lists (ACLs), which in commands and tables are also referred to as access lists. Unless otherwise noted, the term switch refers to a Catalyst 3750-E or 3560-E standalone switch and to a Catalyst 3750-E switch stack.

Note Information in this chapter about IP ACLs is specific to IP Version 4 (IPv4). For information about IPv6 ACLs, see Chapter 35, “Configuring IPv6 ACLs.”

For complete syntax and usage information for the commands used in this chapter, see the command reference for this release, see the “Configuring IP Services” section in the “IP Addressing and Services” chapter of the Cisco IOS IP Configuration Guide, Release 12.2, and the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2.

This chapter consists of these sections:

•Understanding ACLs, page 34-1

•Configuring IPv4 ACLs, page 34-7

•Creating Named MAC Extended ACLs, page 34-27

•Configuring VLAN Maps, page 34-29

•Using VLAN Maps with Router ACLs, page 34-36

•Displaying IPv4 ACL Configuration, page 34-40

Understanding ACLs

Packet filtering can help limit network traffic and restrict network use by certain users or devices. ACLs filter traffic as it passes through a router or switch and permit or deny packets crossing specified interfaces or VLANs. An ACL is a sequential collection of permit and deny conditions that apply to packets. When a packet is received on an interface, the switch compares the fields in the packet against any applied ACLs to verify that the packet has the required permissions to be forwarded, based on the criteria specified in the access lists. One by one, it tests packets against the conditions in an access list. The first match decides whether the switch accepts or rejects the packets. Because the switch stops testing after the first match, the order of conditions in the list is critical. If no conditions match, the switch rejects the packet. If there are no restrictions, the switch forwards the packet; otherwise, the switch drops the packet. The switch can use ACLs on all packets it forwards, including packets bridged within a VLAN.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 34-1