Ordering of ACL Rules

Chapter 10: Security Configuration Guide

•Type of Service (TOS)

For IPX ACLs, the following fields can be specified:

•Source network address

•Destination network address

•Source IPX socket

•Destination IPX socket

When defining an ACL rule, each field in the rule is position sensitive. For example, for TCP, the source address must be followed by the destination address, followed by the source socket and the destination socket and so on. For example, the following describes the syntax of a TCP ACL:

acl name permit tcp source-addr dest-addr source-port dest-port tos

Not all the fields are required. If a field is not specified, it is treated as don't care. However, since each field is position sensitive, it may be necessary to “skip” some fields in order to specify a value for another field. To skip a field, the keyword any is used. For example, the following rule denies SMTP traffic between any two hosts:

acl nosmtp deny tcp any any smtp smtp

Note that in the above example, the tos field (Type of Service) is not specified and is treated as don't care. The keyword any is needed only to skip a don't care field in order to explicitly specify another field that is further down in the rule. If there are no other fields to specify, the keyword any is not really needed. For example, the following ACL permits all IP traffic to go through:

acl yesip permit ip

For an ACL with multiple rules, the ordering of the rules is very important. When the router looks at an ACL to determine whether a packet should be forwarded or not, it goes through each rule in the ACL sequentially. When the router finds a rule that matches the packet, all subsequent rules are ignored. That is, a first match algorithm is used. The action defined by this ACL, to permit or deny, is used to forward or drop the packet. There are no hidden or implied ordering of these rules. Nor is there precedence attached to each field. The router simply goes down the list, one rule at a time until there is a match. Consequently, rules that are more specific (i.e. with more details) should always be listed

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Cabletron Systems 9032578-02 manual Ordering of ACL Rules

Models: 9032578-02

Ordering of ACL Rules

9032578-02 specifications