Refining an IP Access List

The ability to apply sequence numbers to IP access list entries simplifies access list changes. Prior to the IP Access List Entry Sequence Numbering feature, there was no way to specify the position of an entry within an access list. If you wanted to insert an entry in the middle of an existing list, all of the entries after the desired position had to be removed, then the new entry was added, and then all the removed entries had to be reentered. This method was cumbersome and error prone.

Sequence numbers allow users to add access list entries and resequence them. When you add a new entry, you specify the sequence number so that it is in a desired position in the access list. If necessary, entries currently in the access list can be resequenced to create room to insert the new entry.

An access list sequence number is a number at the beginning of a permit or deny command in an access list. The sequence number determines the order that the entry appears in the access list. The ability to apply sequence numbers to IP access list entries simplifies access list changes.

Prior to having sequence numbers, users could only add access list entries to the end of an access list; therefore, needing to add statements anywhere except the end of the list required reconfiguring the entire access list. There was no way to specify the position of an entry within an access list. If a user wanted to insert an entry (statement) in the middle of an existing list, all of the entries after the desired position had to be removed, then the new entry was added, and then all the removed entries had to be reentered. This method was cumbersome and error prone.

This feature allows users to add sequence numbers to access list entries and resequence them. When a user adds a new entry, the user chooses the sequence number so that it is in a desired position in the access list. If necessary, entries currently in the access list can be resequenced to create room to insert the new entry. Sequence numbers make revising an access list much easier.

• For backward compatibility with previous releases, if entries with no sequence numbers are applied, the first entry is assigned a sequence number of 10, and successive entries are incremented by 10. The maximum sequence number is 2147483647. If the generated sequence number exceeds this maximum number, the following message is displayed:

	Exceeded maximum sequence number.

• If the user enters an entry without a sequence number, it is assigned a sequence number that is 10 greater than the last sequence number in that access list and is placed at the end of the list.
• If the user enters an entry that matches an already existing entry (except for the sequence number), then no changes are made.
• If the user enters a sequence number that is already present, the following error message is generated:

	Duplicate sequence number.

• If a new access list is entered from global configuration mode, then sequence numbers for that access list are generated automatically.
• Sequence numbers are not nvgened. That is, the sequence numbers themselves are not saved. In the event that the system is reloaded, the configured sequence numbers revert to the default sequence starting number and increment. The function is provided for backward compatibility with software releases that do not support sequence numbering.
• This feature works with named and numbered, standard and extended IP access lists.

Benefits and possible uses of time ranges include the following:

• The network administrator has more control over permitting or denying a user access to resources. These resources could be an application (identified by an IP address/mask pair and a port number), policy routing, or an on-demand link (identified as interesting traffic to the dialer).
• Network administrators can set time-based security policy, including the following:
  • Perimeter security using access lists
  • Data confidentiality with IP Security Protocol (IPsec)
• When provider access rates vary by time of day, it is possible to automatically reroute traffic cost effectively.
• Network administrators can control logging messages. Access list entries can log traffic at certain times of the day, but not constantly. Therefore, administrators can simply deny access without needing to analyze many logs generated during peak hours.

Filter noninitial fragments of packets with an extended access list if you want to block more of the traffic you intended to block, not just the initial fragment of such packets. You should first understand the following concepts.

If the fragmentskeyword is used in additional IP access list entries that deny fragments, the fragment control feature provides the following benefits:

The behavior of access list entries regarding the use or lack of use of the fragments keyword can be summarized as follows:

Be aware that you should not add the fragments keyword to every access list entry because the first fragment of the IP packet is considered a nonfragment and is treated independently of the subsequent fragments. An initial fragment will not match an access list permit or deny entry that contains the fragments keyword. The packet is compared to the next access list entry, and so on, until it is either permitted or denied by an access list entry that does not contain the fragments keyword. Therefore, you may need two access list entries for every deny entry. The first deny entry of the pair will not include the fragments keyword and applies to the initial fragment. The second deny entry of the pair will include the fragments keyword and applies to the subsequent fragments. In the cases in which there are multiple deny entries for the same host but with different Layer 4 ports, a single deny access list entry with the fragments keyword for that host is all that needs to be added. Thus all the fragments of a packet are handled in the same manner by the access list.

Packet fragments of IP datagrams are considered individual packets, and each counts individually as a packet in access list accounting and access list violation counts.

Perform this task if you want to add entries to an existing access list, change the order of entries, or simply number the entries in an access list to accommodate future changes.

Note	Remember that if you want to delete an entry from an access list, you can simply use the no deny or no permit form of the command, or the no sequence-number command if the statement already has a sequence number.

Note	Access list sequence numbers do not support dynamic, reflexive, or firewall access lists.

1.    enable

2.    configure terminal

3.    ip access-list resequence access-list-name starting-sequence-number increment

4.    ip access-list {standard| extended} access-list-name

5.   Do one of the following:

6.   Do one of the following:

7.    Repeat Step 5 and Step 6 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry.

8.    end

9.    show ip access-lists access-list-name

Router# show ip access-lists xyz123
Standard IP access list xyz123
100 permit 10.4.4.0, wildcard bits 0.0.0.255
105 permit 10.5.5.5, wildcard bits 0.0.0.255
115 permit 10.0.0.0, wildcard bits 0.0.0.255
130 permit 10.5.5.0, wildcard bits 0.0.0.255
145 permit 10.0.0.0, wildcard bits 0.0.0.255

1.    enable

2.    configure terminal

3.    ip access-list extended name

4.    [sequence-number] deny protocol source[source-wildcard] [operator port[port]] destination[destination-wildcard] [operator port[port]]

5.    [sequence-number] deny protocol source[source-wildcard][operator port[port]] destination[destination-wildcard] [operator port[port]] fragments

6.    [sequence-number] permit protocol source[source-wildcard] [operator port[port]] destination[destination-wildcard] [operator port[port]]

7.    Repeat some combination of Steps 4 through 6 until you have specified the values on which you want to base your access list.

8.    end

9.    show ip access-list

• What to Do Next