Cisco 3560G 48TS Software Configuration Guide - Page 503

Chapter 27 Configuring Network Security with ACLs Configuring IP ACLs Hardware and Software Treatment of IP ACLs ACL processing is primarily accomplished in hardware, but requires forwarding of some traffic flows to the CPU for software processing. If the hardware reaches its capacity to store ACL configurations, 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 packet handling capacity between hardware and software, 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 in a VLAN that must be routed are routed in software, but are bridged in hardware. If ACLs cause large numbers of packets to be sent 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. IP ACL Configuration Examples This section provides examples of configuring and applying IP ACLs. For detailed information about compiling ACLs, refer to the Security Configuration Guide and the "IP Services" chapter of the Cisco IOS IP and IP Routing Configuration Guide for IOS Release 12.1. Figure 27-3 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. 78-16156-01 Catalyst 3560 Switch Software Configuration Guide 27-21