Cisco Systems 3560 사용자 설명서

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.

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.

If this ACL manager message appears and [chars] is the access-list name,

ACLMGR-2-NOVMR: Cannot generate hardware representation of access list [chars]

The switch has insufficient resources to create a hardware representation of the ACL. The resources 
include hardware memory and label space but not CPU memory. A lack of available logical operation 
units or specialized hardware resources causes this problem. Logical operation units are needed for a 
TCP flag match or a test other than eq (ne, gt, lt, or range) on TCP, UDP, or SCTP port numbers.

Use one of these workarounds:

Modify the ACL configuration to use fewer resources.

Rename the ACL with a name or number that alphanumerically precedes the ACL names or 
numbers. 

To determine the specialized hardware resources, enter the show platform layer4 acl map privileged 
EXEC command. If the switch does not have available resources, the output shows that index 0 to 
index 15 are not available.

For more information about configuring ACLs with insufficient resources, see CSCsq63926 in the Bug 
Toolkit.

For example, if you apply this ACL to an interface: