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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter28 Configuring Network Securi ty with ACLs Configuring IPv4 ACLs
Hardware and Software Treatment of IP ACLs
ACL processing is primarily accomplished in hardware, but requires forwarding of some traf f ic flo ws to
the CPU for software processing. If the hardware reaches its ca pac ity to stor e ACL configurati ons,
packets are sent to the CPU for forwarding. The forwarding rate for software-forwarded traffic is
substantially less than for hardware-forwarded traffic.
Note If an ACL configuration cannot be implemented in hardware due to an out-of-resource condition on a
switch, then only the traffic in that VLAN arriving on that switch is affected (forwarded in software).
Software forwarding of packets might adversely impact the performance of the switch, depending on the
number of CPU cycles that this consumes.
For router ACLs, other factors can cause packets to be sent to the CPU:
•Using the log keyword
•Generating ICMP unreachable messages
When traffic flows are both logged and forwarded, forwarding is done by hardware, but logging must be
done by software. Because of the difference in pack et handlin g capacity between har dware and softwa re,
if the sum of all flows being logged (both permitted flows and denied flows) is of great enough
bandwidth, not all of the packets that are forwarded can be logged.
If router ACL configuration cannot be applied in hardware, packets arriving i n a V LAN th at must be
routed are routed in software. If ACLs cause large numbers of packets to be se nt 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 and
routed packets.
Router ACLs function as follows:
•The hardware controls permit and deny actions of standard and extended ACLs (input and output)
for security access control.
•If log has not been specified, the flows that match a deny statement in a security ACL are dropped
by the hardware if ip unreachables is disabled. The flows matching a permit statement are switched
in hardware.
•Adding the log keyword to an ACE in a router ACL causes a copy of the packet to be sent to the
CPU for logging only. If the ACE is a permit statement, the packet is still switched and routed
in hardware.
IPv4 ACL Configuration Examples
This section provides examples of configuring and applying IPv4 ACLs. For detailed information about
compiling ACLs, see the Cisco IOS Security Configuration Guide, Release 12.2 and to th e Configuring
IP Services” section in the “IP Addressing and Services” chapter of the Cisco IOS IP Configuration
Guide, Release 12.2.
Figure 28-3 shows a small networked office environment with routed Port 2 connected to Server A,
containing benefits and other information that all employees can access, and routed Port 1 connected to
Server B, containing confidential payroll data. All users can access Server A, but Server B has restricted
access.