Hardware And Software Treatment Of Ip Acls; Troubleshooting Acls - Cisco Catalyst 2960 Software Configuration Manual

Configuring IPv4 ACLs
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.
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 packets.

Troubleshooting ACLs

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:
•
•
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:
permit tcp source source-wildcard destination destination-wildcard range 5 60
permit tcp source source-wildcard destination destination-wildcard range 15 160
permit tcp source source-wildcard destination destination-wildcard range 115 1660
permit tcp source source-wildcard destination destination-wildcard
And if this message appears:
ACLMGR-2-NOVMR: Cannot generate hardware representation of access list [chars]
The flag-related operators are not available. To avoid this issue,
•
Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE
31-20
Modify the ACL configuration to use fewer resources.
Rename the ACL with a name or number that alphanumerically precedes the ACL names or
numbers.
Move the fourth ACE before the first ACE by using ip access-list resequence global configuration
command:
permit tcp source source-wildcard destination destination-wildcard
Chapter 31 Configuring Network Security with ACLs
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