34-15
Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter34 Configuring Network Security with ACLs
Configuring IPv4 ACLs
After an ACL is created, any additions (possibly entered from the terminal) are placed at the end of the
list. You cannot selectively add or remove access list entries from a numbered access list.
Note When you are creating an ACL, remember that, by default, the end of the access list contains an implicit
deny statement for all packets if it did not find a match before reaching the end.
After creating a numbered extended ACL, you can apply it to terminal lines (see the “Applying an IPv4
ACL to a Terminal Line” section on page34-20), to interfaces (see the “Applying an IPv4 ACL to an
Interface” section on page 34-20), or to VLANs (see the “Configuring VLAN Maps” section on
page 34-30).
Resequencing ACEs in an ACL
Sequence numbers for the entries in an access list are automatically generated when you create a new
ACL. You can use the ip access-list resequence global configuration command to edit the sequence
numbers in an ACL and change the order in which ACEs are applied. For example, if you add a new ACE
to an ACL, it is placed at the bottom of the list. By changing the sequence number, you can move the
ACE to a different position in the ACL.
For information about the ip access-list resequence command:
http://www.cisco.com/en/US/docs/ios/12_2s/feature/guide/fsaclseq.html#wp1027188
Creating Named Standard and Extended ACLs
You can identify IPv4 ACLs with an alphanumeric string (a name) rather than a number. You can use
named ACLs to configure more IPv4 access lists in a router than if you were to use numbered access
lists. If you identify your access list with a name rather than a number, the mode and command syntax
are slightly different. However, not all commands that use IP access lists accept a named access list.
Note The name you give to a standard or extended ACL can also be a number in the supported range of access
list numbers. That is, the name of a standard IP ACL can be 1 to 99; the name of an extended IP ACL
can be 100 to 199. The advantage of using named ACLs instead of numbered lists is that you can delete
individual entries from a named list.
Consider these guidelines and limitations before configuring named ACLs:
•Not all commands that accept a numbered ACL accept a named ACL. ACLs for packet filters and
route filters on interfaces can use a name. VLAN maps also accept a name.
•A standard ACL and an extended ACL cannot have the same name.
•Numbered ACLs are also available, as described in the “Creating Standard and Extended IPv4
ACLs” section on page34-8.
•You can use standard and extended ACLs (named or numbered) in VLAN maps.