An ACL is used to define packet filtering and forwarding rules for traffic traversing the switch. Each packet arriving on an ingress port and/or VLAN is compared to the access list applied to that interface and is either permitted or denied. Packets egressing an interface can also be filtered on the platforms listed for this feature in the Switch Engine 32.2 Feature License Requirements document. However, only a subset of the filtering conditions available for ingress filtering are available for egress filtering.
In addition to forwarding or dropping packets that match an ACL, the switch can also perform additional operations such as incrementing counters, logging packet headers, mirroring traffic to a monitor port, sending the packet to a QoS profile, and metering the packets matching the ACL to control bandwidth. (Metering is supported only on the platforms listed for this feature in the Switch Engine 32.2 Feature License Requirements document.) Using ACLs has no impact on switch performance (with the minor exception of the mirror-cpu action modifier).
ACLs are typically applied to traffic that crosses Layer 3 router boundaries, but it is possible to use access lists within a Layer 2 virtual LAN (VLAN).
NoteSome locally CPU-generated packets are not subject to egress ACL processing.
ACLs are created in two different ways. One method is to create an ACL policy file and apply that ACL policy file to a list of ports, a VLAN, or to all interfaces. The second method to create an ACL is to use the CLI to specify a single rule, called a dynamic ACL; this is the default.
NoteACLs applied to a VLAN are actually applied to all ports on the switch, without regard to VLAN membership. The result is that resources are consumed per chip.
An ACL policy file is a text file that contains one or more ACL rule entries. This first method creates ACLs that are persistent across switch reboots, can contain a large number of rule entries, and are all applied at the same time. See ACL Rule Syntax for information about creating ACL rule entries.
Policy files are also used to define routing policies. Routing policies are used to control the advertisement or recognition of routes communicated by routing protocols. ACL policy files and routing policy files are both handled by the policy manager, and the syntax for both types of files is checked by the policy manager.
NoteAlthough ExtremeXOS does not prohibit mixing ACL and routing type entries in a policy file, it is strongly recommended that you do not mix the entries and do use separate policy files for ACL and routing policies.
Dynamic ACLs persist across reboots; however, you can configure non-persistent dynamic ACLS that disappear when the switch reboots. Dynamic ACLs consist of only a single rule. Multiple dynamic ACLs can be applied to an interface. See Layer-2 Protocol Tunneling ACLs for information about creating dynamic ACLs. The precedence of ACLs can be configured by defining zones and configuring the priority of both the zones and the ACLs within a zone. See Configuring ACL Priority for more information.