Command Line Interface
4-94
4
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.
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
10.1.1.1 are 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 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)#
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)#