ACL IP Options Selective Drop

First Published: July 22, 2002
Last Updated: May 15, 2006

The ACL IP Options Selective Drop feature allows Cisco routers to filter packets containing IP options or to mitigate the effects of IP options on a router or downstream routers by dropping these packets or ignoring the processing of the IP options.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for ACL IP Options Selective Drop" section.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Restrictions for ACL IP Options Selective Drop

•Information About ACL IP Options Selective Drop

•How to Configure ACL IP Options Selective Drop

•Configuration Example for ACL IP Options Selective Drop

•Additional References

•Feature Information for ACL IP Options Selective Drop

Restrictions for ACL IP Options Selective Drop

•Resource Reservation Protocol (RSVP) (Multiprotocol Label Switching traffic engineering [MPLS TE]), Internet Group Management Protocol Version 2 (IGMPv2), and other protocols that use IP options packets may not function in drop or ignore modes.

•On the Cisco 10720 Internet router, the ip option ignore command is not supported. Only drop mode (the ip option drop command) is supported.

•The ip option ignore command (ignore mode) is supported only on the Cisco 12000 series router.

Information About ACL IP Options Selective Drop

Before you configure the ACL IP Options Selective Drop feature, you should understand the concepts in the following sections:

•Using ACL IP Options Selective Drop

•Benefits of Using ACL IP Options Selective Drop

Using ACL IP Options Selective Drop

The ACL IP Options Selective Drop feature allows a router to filter IP options packets, thereby mitigating the effects of these packets on a router and downstream routers, and perform the following actions:

•Drop all IP options packets that it receives and prevent options from going deeper into the network.

•Ignore IP options packets destined for the router and treat them as if they had no IP options.

For many users, dropping the packets is the best solution. However, in environments in which some IP options may be legitimate, reducing the load that the packets present on the routers is sufficient. Therefore, users may prefer to skip options processing on the router and forward the packet as though it were pure IP.

Benefits of Using ACL IP Options Selective Drop

•Drop mode filters packets from the network and relieves downstream routers and hosts of the load from options packets.

•Drop mode minimizes loads to the Route Processor (RP) for options that require RP processing on distributed systems. Previously, the packets were always routed to or processed by the RP CPU. Now, the ignore and drop forms prevent the packets from impacting the RP performance.

How to Configure ACL IP Options Selective Drop

•Configuring ACL IP Options Selective Drop

Configuring ACL IP Options Selective Drop

This section describes how to configure the ACL IP Options Selective Drop feature.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip options {drop | ignore}

4. exit

5. show ip traffic

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

ip options {drop | ignore}

Example:

Router(config)# ip options drop

Drops or ignores IP options packets that are sent to the router.

Note On the Cisco 10720 Internet router, the ip option ignore command is not supported. Only drop mode (the ip option drop command) is supported.

Step 4 

exit

Example:

Router(config)# exit

Returns to privileged EXEC mode.

Step 5 

show ip traffic

Example:

Router# show ip traffic

(Optional) Displays statistics about IP traffic.


What to Do Next

If you are running Cisco IOS Release 12.3(4)T or a later release, you can also use the ACL Support for Filtering IP Options feature to filter packets based on whether the packet contains specific IP options. For more information, refer to the document Creating an IP Access List to Filter IP Options, TCP Flags, Noncontiguous Ports, or TTL Values.

Configuration Example for ACL IP Options Selective Drop

•Example: Configuring ACL IP Options Selective Drop

•Example: Verifying ACL IP Options Selective Drop

Example: Configuring ACL IP Options Selective Drop

The following example shows how to configure the router (and downstream routers) to drop all options packets that enter the network: 

Router(config)# ip options drop


% Warning:RSVP and other protocols that use IP Options packets may not function in drop or 
ignore modes.

end

Example: Verifying ACL IP Options Selective Drop

The following sample output is displayed after 15,000 options packets are sent using the ip options drop command. Note that the "forced drop" counter increases. 

Router# show ip traffic


IP statistics:

  Rcvd:  15000 total, 0 local destination

         0 format errors, 0 checksum errors, 0 bad hop count

         0 unknown protocol, 0 not a gateway

         0 security failures, 0 bad options, 15000 with options

  Opts:  0 end, 0 nop, 0 basic security, 0 loose source route

         0 timestamp, 0 extended security, 0 record route

         0 stream ID, 0 strict source route, 0 alert, 0 cipso

         0 other

  Frags: 0 reassembled, 0 timeouts, 0 couldn't reassemble

         0 fragmented, 0 couldn't fragment

  Bcast: 0 received, 0 sent

  Mcast: 0 received, 0 sent

  Sent:  0 generated, 0 forwarded

  Drop:  0 encapsulation failed, 0 unresolved, 0 no adjacency

         0 no route, 0 unicast RPF, 15000 forced drop

Additional References

Related Documents

Related Topic
Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Configuring IP access lists

Creating an IP Access List and Applying It to an Interface

Using access lists for filtering IP options

Creating an IP Access List to Filter IP Options, TCP Flags, Noncontiguous Ports, or TTL Values


Standards

Standards
Title

None

—


MIBs

MIBs
MIBs Link

None

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFCs
Title

None

—


Technical Assistance

Description
Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html


Feature Information for ACL IP Options Selective Drop

Table 1 lists the features in this module and provides links to specific configuration information.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Table 1 Feature Information for ACL IP Options Selective Drop 

Feature Name
Releases
Feature Information

ACL IP Options Selective Drop

12.0(22)S
12.3(4)T
12.2(25)S
12.2(27)SBC
12.0(32)S
12.3(19)

The ACL IP Options Selective Drop feature allows Cisco routers to filter packets containing IP options or to mitigate the effects of IP options on a router or downstream routers by dropping these packets or ignoring the processing of the IP options.

The following commands were introduced or modified: ip options.


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