4 Command Line Interface

Command Mode

IP Mask

Command Usage

•Packets crossing a port are checked against all the rules in the ACL until a match is found. The order in which these packets are checked is determined by the mask, and not the order in which the ACL rules were entered.

•First create the required ACLs and ingress or egress masks before mapping an ACL to an interface.

•If you enter dscp, you cannot enter tos or precedence. You can enter both tos and precedence without dscp.

•Masks that include an entry for a Layer 4 protocol source port or destination port can only be applied to packets with a header length of exactly five bytes.

Example

This example creates an IP ingress mask with two rules. Each rule is checked in order of precedence to look for a match in the ACL entries. The first entry matching a mask is applied to the inbound packet.

Console(config)#access-list ip mask-precedence in

Console(config-ip-mask-acl)#mask host any

Console(config-ip-mask-acl)#mask 255.255.255.0 any

Console(config-ip-mask-acl)#

This shows that the entries in the mask override the precedence in which the rules are entered into the ACL. In the following example, packets with the source address

10.1.1.1are dropped because the “deny 10.1.1.1 255.255.255.255” rule has the higher precedence according the “mask host any” entry.

Console(config)#access-list ip standard A2

Console(config-std-acl)#permit 10.1.1.0 255.255.255.0

Console(config-std-acl)#deny 10.1.1.1 255.255.255.255

Console(config-std-acl)#exit

Console(config)#access-list ip mask-precedence in

Console(config-ip-mask-acl)#mask host any

Console(config-ip-mask-acl)#mask 255.255.255.0 any

Console(config-ip-mask-acl)#

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