IPv4 Access Control Lists (ACLs)

Planning an ACL Application

■Generally, you should list ACEs from the most specific (individual hosts) to the most general (subnets or groups of subnets) unless doing so permits traffic that you want dropped. For example, an ACE allowing a small group of workstations to use a specialized printer should occur earlier in an ACL than an entry used to block widespread access to the same printer.

IPv4 ACL Configuration and Operating Rules

■Static Port ACLs: A static port ACL filters traffic entering the switch on the port(s) or trunk(s) to which it is assigned.

■Per Switch ACL Limits for All ACL Types. At a minimum an ACL must have one, explicit “permit” or “deny” Access Control Entry. You can configure up to 512 ACLs. For IPv4 ACLs, the maximums are as follows:

•Named (Extended or Standard) ACLs: Up to 512 (minus any numeric standard or extended ACL assignments)

•Numeric Standard ACLs: Up to 99; numeric range: 1 - 99

•Numeric Extended ACLs: Up to 100; numeric range: 100 - 199

•The maximum number of ACEs supported by the switch is up to 1024 for IPv4 ACEs. The maximum number of ACEs allowed on a port depends on the concurrent resource usage by multiple configured features. For more information, use the show < qos access-list > resources command and/or refer to “Monitoring Shared Resources” on page 9-100.

■Implicit Deny: In any static IPv4 ACL, the switch automatically applies an implicit deny ip any that does not appear in show listings. This means that the ACL denies any IPv4 packet it encounters that does not have a match with an entry in the ACL. Thus, if you want an ACL to permit any packets that you have not expressly denied, you must enter a permit any or permit ip any any as the last ACE in an ACL. Because, for a given packet the switch sequentially applies the ACEs in an ACL until it finds a match, any packet that reaches the permit any or permit ip any any entry will be permitted, and will not encounter the deny ip any ACE the switch automatically includes at the end of the ACL. For an example, refer to figure 9-4on page 9-23.For Implicit Deny operation in dynamic ACLs, refer to chapter 6, “Configuring RADIUS Server Support for Switch Services”

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