Access control lists (ACLs) perform packet filtering to control the movement of packets through a network. Packet filtering provides security by limiting the access of traffic into a network, restricting user and device access to a network, and preventing traffic from leaving a network. IP access lists reduce the chance of spoofing and denial-of-service attacks, and allow dynamic, temporary user-access through a firewall.

IP access lists can also be used for purposes other than security, such as to control bandwidth, restrict the content of routing updates, redistribute routes, trigger dial-on-demand (DDR) calls, limit debug output, and identify or classify traffic for quality of service (QoS) features.

An access list is a sequential list that consists of at least one permit statement and possibly one or more deny statements. In the case of IP access lists, these statements can apply to IP addresses, upper-layer IP protocols, or other fields in IP packets.

Access lists are identified and referenced by a name or a number. Access lists act as packet filters, filtering packets based on the criteria defined in each access list.

After you configure an access list, for the access list to take effect, you must either apply the access list to an interface (by using the ip access-group command), a vty (by using the access-class command), or reference the access list by any command that accepts an access list. Multiple commands can reference the same access list.

In the following configuration, an IP access list named branchoffices is configured on Fast Ethernet interface 0/1/0 and applied to incoming packets. Networks other than the ones specified by the source address and mask pair cannot access Fast Ethernet interface 0/1/0. The destinations for packets coming from sources on network 172.16.7.0 are unrestricted. The destination for packets coming from sources on network 172.16.2.0 must be 172.31.5.4.

ip access-list extended branchoffices
 10 permit 172.16.7.0 0.0.0.3 any 
 20 permit 172.16.2.0 0.0.0.255 host 172.31.5.4
!
interface fastethernet 0/1/0
 ip access-group branchoffices in

All access lists must be identified by a name or a number. Named access lists are more convenient than numbered access lists because you can specify a meaningful name that is easier to remember and associate with a task. You can reorder statements in or add statements to a named access list.

Named access lists support the following features that are not supported by numbered access lists:

• IP options filtering

• Noncontiguous ports

• TCP flag filtering

• Deleting of entries with the no permit or no deny command

Note	Not all commands that accept a numbered access list will accept a named access list. For example, vty uses only numbered access lists.

The following rules apply to access lists:

• Only one access list per interface, per protocol, and per direction is allowed.

• An access list must contain at least one permit statement or all packets are denied entry into the network.

• The order in which access list conditions or match criteria are configured is important. While deciding whether to forward or block a packet, Cisco software tests the packet against each criteria statement in the order in which these statements are created. After a match is found, no more criteria statements are checked. The same permit or deny statements specified in a different order can result in a packet being passed under one circumstance and denied in another circumstance.

• If an access list is referenced by a name, but the access list does not exist, all packets pass. An interface or command with an empty access list applied to it permits all traffic into the network.

• Standard access lists and extended access lists cannot have the same name.

• Inbound access lists process packets before the packets are routed to an outbound interface. Inbound access lists that have filtering criteria that deny packet access to a network saves the overhead of routing lookup. Packets that are permitted access to a network based on the configured filtering criteria are processed for routing. For inbound access lists, when you configure a permit statement, packets are processed after they are received, and when you configure a deny statement, packets are discarded.

• Outbound access lists process packets before they leave the device. Incoming packets are routed to the outbound interface and then processed by the outbound access list. For outbound access lists, when you configure a permit statement, packets are sent to the output buffer, and when you configure a deny statement, packets are discarded.

• An access list can control traffic arriving at a device or leaving a device, but not traffic originating at a device.

The following tips will help you avoid unintended consequences and help you create more efficient, useful access lists.

• Create the access list before applying it to an interface (or elsewhere), because if you apply a nonexistent access list to an interface and then proceed to configure the access list, the first statement is put into effect, and the implicit deny statement that follows could cause you immediate access problems.

• Another reason to configure an access list before applying it is because an interface with an empty access list applied to it permits all traffic.

• All access lists need at least one permit statement; otherwise, all packets are denied and no traffic passes.

• Because the software stops testing conditions after it encounters the first match (to either a permit or deny statement), you will reduce processing time and resources if you put the statements that packets are most likely to match at the beginning of the access list. Place more frequently occurring conditions before less frequent conditions.

• Organize your access list so that more specific references in a network or subnet appear before more general ones.

• Use the statement permit any any if you want to allow all other packets not already denied. Using the statement permit any any in effect avoids denying all other packets with the implicit deny statement at the end of an access list. Do not make your first access list entry permit any any because all traffic will get through; no packets will reach the subsequent testing. In fact, once you specify permit any any , all traffic not already denied will get through.

• Although all access lists end with an implicit deny statement, we recommend use of an explicit deny statement (for example, deny ip any any ). On most platforms, you can display the count of packets denied by issuing the show access-list command, thus finding out more information about who your access list is disallowing. Only packets denied by explicit deny statements are counted, which is why the explicit deny statement will yield more complete data for you.

• While you are creating an access list or after it is created, you might want to delete an entry.

  • You cannot delete an entry from a numbered access list; trying to do so will delete the entire access list. If you need to delete an entry, you need to delete the entire access list and start over.
  • You can delete an entry from a named access list. Use the no permit or no deny command to delete the appropriate entry.

• In order to make the purpose of individual statements more scannable and easily understood at a glance, you can write a helpful remark before or after any statement by using the remark command.

• If you want to deny access to a particular host or network and find out if someone from that network or host is attempting to gain access, include the log keyword with the corresponding deny statement so that the packets denied from that source are logged for you.

• This hint applies to the placement of your access list. When trying to save resources, remember that an inbound access list applies the filter conditions before the routing table lookup. An outbound access list applies the filter conditions after the routing table lookup.

• Before you add new ACL statements, provide time to the parser to clean up the deletion.

You can apply access lists to the inbound or outbound interfaces of a device. Applying an access list to an inbound interface controls the traffic that enters the interface and applying an access list to an outbound interface controls the traffic that exits the interface.

When software receives a packet at the inbound interface, the software checks the packet against the statements that are configured for the access list. If the access list permits packets, the software processes the packet. Applying access lists to filter incoming packets can save device resources because filtered packets are discarded before entering the device.

Access lists on outbound interfaces filter packets that are transmitted (sent) out of the interface. You can use the TCP Access Control List (ACL) Splitting feature of the Rate-Based Satellite Control Protocol (RBSCP) on the outbound interface to control the type of packets that are subject to TCP acknowledgment (ACK) splitting on an outbound interface.

You can reference an access list by using a debug command to limit the amount of debug logs. For example, based on the filtering or matching criteria of the access list, debug logs can be limited to source or destination addresses or protocols.

You can use access lists to control routing updates, dial-on-demand (DDR), and quality of service (QoS) features.