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Software Configuration Guide—Release 12.2(25)EW
OL-6696-01
Chapter 35 Configuring Network Security with ACLs
Understanding ACLs
ACL Overview
An ACL is a collection of sequential permit and deny conditions that applies 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 permissions required to be forwarded, based on the conditions specified in
the access lists. It tests the packets against the conditions in an access list one-by-one. The first match
determines whether the switch accepts or rejects the packets. Because the switch stops testing conditions
after the first match, the order of conditions in the list is critical. If no conditions match, the switch drops
the packet. If there are no restrictions, the switch forwards the packet; otherwise, the switch drops the
packet.
Switches traditionally operate at Layer 2, switching traffic within a VLAN, whereas routers route traffic
between VLANs at Layer 3. The Catalyst 4500 series switch can accelerate packet routing between
VLANs by using Layer 3 switching. The Layer 3 switch bridges the packet, and then routed the packet
internally without going to an external router. The packet is then bridged again and sent to its destination.
During this process, the switch can control all packets, including packets bridged within a VLAN.
You configure access lists on a router or switch to filter traffic and provide basic security for your
network. If you do not configure ACLs, all packets passing through the switch could be allowed on all
parts of the network. You can use ACLs to control which hosts can access different parts of a network
or to decide which types of traffic are forwarded or blocked at router interfaces. For example, you can
allow e-mail traffic to be forwarded but not Telnet traffic. ACLs can be configured to block inbound
traffic, outbound traffic, or both. However, on Layer 2 interfaces, you can apply ACLs only in the
inbound direction.
An ACL contains an ordered list of access control entries (ACEs). Each ACE specifies permit or deny
and a set of conditions the packet must satisfy in order to match the ACE. The meaning of permit or deny
depends on the context in which the ACL is used.
The Catalyst 4500 series switch supports two types of ACLs:
• IP ACLs, which filter IP traffic, including TCP, the User Datagram Protocol (UDP), Internet Group
Management Protocol (IGMP), and Internet Control Message Protocol (ICMP).
• MAC (Ethernet) ACLs, which filter non-IP traffic.
Supported Features That Use ACLs
The switch supports two applications of ACLs to filter traffic:
• Router ACLs are applied to Layer 3 interfaces. They control the access of routed traffic between
VLANs. All Catalyst 4500 series switches can create router ACLs, but you must have a Cisco IOS
software image on your switch to apply an ACL to a Layer 3 interface and filter packets routed
between VLANs.
• Port ACLs perform access control on traffic entering a Layer 2 interface. If there are not enough
hardware CAM entries, the output port ACL is not applied to the port and a warning message is given
to user. (This restriction applies to all access group modes for output port ACLs.) When there are
enough CAM entries, the output port ACL might be reapplied.
If there is any output port ACL configured on a Layer 2 port, then no VACL or router ACL can be
configured on the VLANs that the Layer 2 port belongs to. Also, the reverse is true: port ACLs and
VLAN-based ACLs (VACLs and router ACLs) are mutually exclusive on a Layer 2 port. This
restriction applies to all access group modes.
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