Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 256 Access Control Lists (ACLs) Section 22 - Access Control Lists - Access control lists (ACLs) can be used for two purposes on Cisco devices: to filter traffic, and to identify traffic. Access lists are a set of rules, organized in a rule table. Each rule or line in an access-list provides a condition, either permit or deny: • When using an access-list to filter traffic, a permit statement is used to “allow” traffic, while a deny statement is used to “block” traffic. • Similarly, when using an access list to identify traffic, a permit statement is used to “include” traffic, while a deny statement states that the traffic should “not” be included. It is thus interpreted as a true/false statement. Filtering traffic is the primary use of access lists. However, there are several instances when it is necessary to identify traffic using ACLs, including: • Identifying interesting traffic to bring up an ISDN link or VPN tunnel • Identifying routes to filter or allow in routing updates • Identifying traffic for QoS purposes When filtering traffic, access lists are applied on interfaces. As a packet passes through a router, the top line of the rule list is checked first, and the router continues to go down the list until a match is made. Once a match is made, the packet is either permitted or denied. There is an implicit ‘deny all’ at the end of all access lists. You don’t create it, and you can’t delete it. Thus, access lists that contain only deny statements will prevent all traffic. Access lists are applied either inbound (packets received on an interface, before routing), or outbound (packets leaving an interface, after routing). Only one access list per interface, per protocol, per direction is allowed. More specific and frequently used rules should be at the top of your access list, to optimize CPU usage. New entries to an access list are added to the bottom. You cannot remove individual lines from a numbered access list. You must delete and recreate the access to truly make changes. Best practice is to use a text editor to manage your access-lists. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 257 Types of Access Lists There are two categories of access lists: numbered and named. Numbered access lists are broken down into several ranges, each dedicated to a specific protocol: 1–99 IP standard access list 100-199 IP extended access list 200-299 Protocol type-code access list 300-399 DECnet access list 400-499 XNS standard access list 500-599 XNS extended access list 600-699 Appletalk access list 700-799 48-bit MAC address access list 800-899 IPX standard access list 900-999 IPX extended access list 1000-1099 IPX SAP access list 1100-1199 Extended 48-bit MAC address access list 1200-1299 IPX summary address access list 1300-1999 IP standard access list (expanded range) 2000-2699 IP extended access list (expanded range Remember, individual lines cannot be removed from a numbered access list. The entire access list must be deleted and recreated. All new entries to a numbered access list are added to the bottom. Named access lists provide a bit more flexibility. Descriptive names can be used to identify your access-lists. Additionally, individual lines can be removed from a named access-list. However, like numbered lists, all new entries are still added to the bottom of the access list. There are two common types of named access lists: • IP standard named access lists • IP extended named access lists Configuration of both numbered and named access-lists is covered later in this section. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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Cisco CCNA Study Guide - Router Alley

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