Access lists filter network traffic by controlling whether routed packets are forwarded or blocked at the router's interfaces. Your router examines each packet to determine whether to forward or drop the packet, on the basis of the criteria you specified within the access lists.

Access list criteria could be the source address of the traffic, the destination address of the traffic, the upper-layer protocol, or other information. Note that sophisticated users can sometimes successfully evade or fool basic access lists because no authentication is required.

There are many reasons to configure access lists; for example, you can use access lists to restrict contents of routing updates or to provide traffic flow control. One of the most important reasons to configure access lists is to provide security for your network, which is the focus of this chapter.

You should use access lists to provide a basic level of security for accessing your network. If you do not configure access lists on your router, all packets passing through the router could be allowed onto all parts of your network.

Access lists can allow one host to access a part of your network and prevent another host from accessing the same area. In the figure below, host A is allowed to access the Human Resources network, and host B is prevented from accessing the Human Resources network.

Figure 1

Using Traffic Filters to Prevent Traffic from Being Routed to a Network

You can also use access lists to decide which types of traffic are forwarded or blocked at the router interfaces. For example, you can permit e-mail traffic to be routed, but at the same time block all Telnet traffic.

Access lists should be used in "firewall" routers, which are often positioned between your internal network and an external network such as the Internet. You can also use access lists on a router positioned between two parts of your network, to control traffic entering or exiting a specific part of your internal network.

To provide the security benefits of access lists, you should at a minimum configure access lists on border routers--routers situated at the edges of your networks. This provides a basic buffer from the outside network, or from a less controlled area of your own network into a more sensitive area of your network.

On these routers, you should configure access lists for each network protocol configured on the router interfaces. You can configure access lists so that inbound traffic or outbound traffic or both are filtered on an interface.

Access lists must be defined on a per-protocol basis. In other words, you should define access lists for every protocol enabled on an interface if you want to control traffic flow for that protocol.

Note	Some protocols refer to access lists as filters.

This chapter describes how to use standard and static extended access lists, which are the basic types of access lists. Some type of basic access list should be used with each routed protocol that you have configured for router interfaces.

Besides the basic types of access lists described in this chapter, there are also more advanced access lists available, which provide additional security features and give you greater control over packet transmission. These advanced access lists and features are described in the other chapters within the part "Traffic Filtering and Firewalls."

Create access lists for each protocol you wish to filter, per router interface. For some protocols, you create one access list to filter inbound traffic, and one access list to filter outbound traffic.

To create an access list, you specify the protocol to filter, you assign a unique name or number to the access list, and you define packet filtering criteria. A single access list can have multiple filtering criteria statements.

Cisco recommends that you create your access lists on a TFTP server and then download the access lists to your router. This approach can considerably simplify maintenance of your access lists. For details, see "Creating and Editing Access List Statements on a TFTP Server" section later in this chapter.

The protocols for which you can configure access lists are identified in the table below.

• Assigning a Unique Name or Number to Each Access List
  
• Defining Criteria for Forwarding or Blocking Packets
  
• Creating and Editing Access List Statements on a TFTP Server
  

For some protocols, you can apply up to two access lists to an interface: one inbound access list and one outbound access list. With other protocols, you apply only one access list which checks both inbound and outbound packets.

If the access list is inbound, when the router receives a packet, the Cisco IOS software checks the access list's criteria statements for a match. If the packet is permitted, the software continues to process the packet. If the packet is denied, the software discards the packet.

If the access list is outbound, after receiving and routing a packet to the outbound interface, the software checks the access list's criteria statements for a match. If the packet is permitted, the software transmits the packet. If the packet is denied, the software discards the packet.

Note	Access lists that are applied to interfaces do not filter traffic that originates from that router.