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Catalyst 2960 and 2960-S Switch Software Configuration Guide
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Chapter 31 Configuring Network Security with ACLs
Configuring IPv4 ACLs
This example shows how to apply access list 3 to filter packets going to the CPU:
Switch(config)# interface vlan 1
Switch(config-if)# ip access-group 3 in
Note When you apply the ip access-group interface configuration command to a Layer 3 SVI, the inter face
must have 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 sending 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.
When you apply an undefined ACL to an interface, the switch acts as if the ACL has not be en 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 perfo rmance 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 ne eded 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.