Hardware And Software Treatment Of Ip Acls - Cisco Catalyst 3750 Software Configuration Manual

Chapter 25 Configuring Network Security with ACLs
This example shows how to apply access list 2 on an interface to filter packets entering the interface:
Switch(config)# interface gigabitethernet1/0/2
Switch(config-if)# ip access-group 2 in
Note When you apply the ip access-group interface configuration command to a Layer 3 interface (an SVI, a
Layer 3 EtherChannel, or a routed port), the interface must have been configured with an IP address.
Layer 3 access groups filter packets that are routed or are received by Layer 3 processes on the CPU.
They do not affect packets bridged within a VLAN.
For inbound ACLs, after receiving a packet, the switch checks the packet against the ACL. If the ACL
permits the packet, the switch continues to process the packet. If the ACL rejects the packet, the switch
discards the packet.
For outbound ACLs, after receiving and routing a packet to a controlled interface, the switch checks the
packet against the ACL. If the ACL permits the packet, the switch sends the packet. If the ACL rejects
the packet, the switch discards the packet.
By default, the input interface sends ICMP Unreachable messages whenever a packet is discarded,
regardless of whether the packet was discarded because of an ACL on the input interface or because of
an ACL on the output interface. ICMP Unreachables are normally limited to no more than one every
one-half second per input interface, but this can be changed by using the ip icmp rate-limit unreachable
global configuration command.
When you apply an undefined ACL to an interface, the switch acts as if the ACL has not been applied to
the interface and permits all packets. Remember this behavior if you use undefined ACLs for network
security.

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.
For router ACLs, other factors can cause packets to be sent to the CPU:
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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.
78-15870-01
Using the log keyword
Generating ICMP unreachable messages
Catalyst 3750 Metro Switch Software Configuration Guide
Configuring IP ACLs
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