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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter19 Configuring Network Security with ACLs
Understanding ACLs
Switches traditionally operate at Layer 2 only, switching traffic within a VLAN, whereas routers route
traffic between VLANs. The Catalyst 3550 switch with the enhanced multilayer software image installed
can accelerate packet routing between VLANs by using Layer 3 switching. The switch bridges the
packet, the packet is then routed internally without going to an external router, and then the packet is
bridged again to send it to its destination. During this process, the switch can access-control all packets
it switches, including packets bridged within a VLAN.
You configure access lists on a router or Layer 3 switch to provide basic security for your net work . If
you do not configure ACLs, all packets passing through the switch c ould be allowed onto all parts of the
network. You can use ACLs to control which hosts can access different parts of a network or to de cide
which types of traffic are forwarded or blocked at router interfaces. For example, yo u can al low e- mai l
traffic to be forwarded but not Telnet traffic. ACLs can be configured to block inbound tra ffic, outbound
traffic, or both.
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 switch supports two types of ACLs:
•IP ACLs filter IP traffic, including TCP, User Datagram Protocol (UDP), Internet Group
Management Protocol (IGMP), and Internet Control Message Protocol (ICMP).
•Ethernet ACLs filter non-IP traffic.
Supported ACLs
The switch supports two applications of ACLs to filter traffic:
•Router ACLs access-control routed traffic between VLANs. All Catalyst 3550 switches can create
router ACLs, but you must have the enhanced multilayer software image on your switch to filter
packets routed between VLANs.
•VLAN ACLs or VLAN maps access-control all packets (bridged and routed). You can use VLAN
maps to filter traffic between devices in the same VLAN. You do not need the enhanced image to
create or apply VLAN maps. VLAN maps are configured to provide access-cont rol based on Layer 3
addresses for IP. Unsupported protocols are access-controlled through MAC addresses using
Ethernet ACEs.
After a VLAN map is applied to a VLAN, all packets (routed or bridged) entering the VLAN are
checked against the VLAN map. Packets can either enter the V LAN through a switch port or through
a routed port after being routed.
This switch also supports Quality of Service (QoS) classification ACLs. For more information, see the
“Classification Based on QoS ACLs” section on page20-7.

Router ACLs

You can apply router ACLs on switch virtual interfaces (SVIs), which are Layer 3 interfaces to V LANs;
on physical Layer 3 interfaces; and on Layer 3 EtherChannel interfaces. Router ACLs ar e applie d on
interfaces for specific directions (inbound or outbound).