How does ACL work in Cisco?

How does ACL work in Cisco?

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.

Where Can Cisco routers apply ACL logic to packets?

Cisco routers can apply ACL logic to packets at the point at which the IP packets enter an interface, or the point at which they exit an interface. In other words, the ACL becomes associated with an interface and for a direction of packet flow (either in or out).

What happens when a packet matches a rule in an access list?

The packets are matched only until it matches the rule. Once a rule is matched then no further comparison takes place and that rule will be performed. There is an implicit deny at the end of every ACL, i.e., if no condition or rule matches then the packet will be discarded.

How is ACL implemented?

Configuring Access Control Lists

1. Create a MAC ACL by specifying a name.
2. Create an IP ACL by specifying a number.
3. Add new rules to the ACL.
4. Configure the match criteria for the rules.
5. Apply the ACL to one or more interfaces.

What does a Cisco router do with a packet when it matches an ACL deny statement?

If the packet matches a rule, its outcome is determined by the conditions of the statement: If the first rule the packet matches is a permit statement, it’s permitted; if it’s a deny statement, it’s denied.

How does a Cisco ACL handle noninitial fragments?

The use of a deny statement for noninitial fragments at the beginning of the ACL denies all noninitial fragments from accessing the router. Under rare circumstances, a valid session might require fragmentation and therefore be filtered if a deny fragment statement exists in the ACL.

Where does the fragments keyword go in ACL?

The first line of the ACL contains Layer 3 information, which matches the packet, but the ACL entry also has the fragments keyword, which doesn’t match the packet because FO = 0, so the next ACL entry is processed. The second line of the ACL contains Layer 3 and Layer 4 information.

When to use non initial fragments in Cisco IOS?

It is important to note that, as of Cisco IOS Software Releases 12.1 (2) and 12.0 (11), the new ACL code drops fragments that do not match any other line in the ACL. Earlier releases allow non-initial fragments through if they do not match any other line of the ACL.

Is there an explicit deny statement in Cisco ACL?

Note: Although all ACLs contain an implicit deny statement, Cisco recommends use of an explicit deny statemen, for example, deny ip any any. On most platforms, such statements maintain a count of the number of denied packets that can be displayed using the show access-list command.