Hardware And Software Treatment Of Ip Acls - Cisco 3845 - Security Bundle Router Software Manual

Configuring IPv4 ACLs
To remove the specified access group, use the no ip access-group {access-list-number | name} {in | out}
interface configuration command.
This example shows how to apply access list 2 to a port to filter packets entering the port:
Switch(config)# interface gigabitethernet0/1
Router(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.
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.
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:
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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. If ACLs cause large numbers of packets to be sent to the CPU, the switch
performance can be negatively affected.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
26-20
Using the log keyword
Generating ICMP unreachable messages
Chapter 26 Configuring Network Security with ACLs
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