Table Of Contents
Restrictions for ACL IP Options Selective Drop
Information About ACL IP Options Selective Drop
Using ACL IP Options Selective Drop
Benefits of Using ACL IP Options Selective Drop
How to Configure ACL IP Options Selective Drop
Configuring ACL IP Options Selective Drop
Configuration Example for ACL IP Options Selective Drop
Configuring ACL IP Options Selective Drop: Example
Verifying ACL IP Options Selective Drop: Example
ACL IP Options Selective Drop
First Published: July 22, 2002Last Updated: May 15, 2006The 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.
History for the ACL IP Options Selective Drop Feature
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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
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
This section contains the following configuration information:
•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
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 Creating an IP Access List to Filter IP Options, TCP Flags, Noncontiguous Ports, or TTL Values.
Configuration Example for ACL IP Options Selective Drop
This section provides the following configuration examples:
•Configuring ACL IP Options Selective Drop: Example
•Verifying ACL IP Options Selective Drop: Example
Configuring ACL IP Options Selective Drop: Example
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.endVerifying ACL IP Options Selective Drop: Example
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 trafficIP statistics:Rcvd: 15000 total, 0 local destination0 format errors, 0 checksum errors, 0 bad hop count0 unknown protocol, 0 not a gateway0 security failures, 0 bad options, 15000 with optionsOpts: 0 end, 0 nop, 0 basic security, 0 loose source route0 timestamp, 0 extended security, 0 record route0 stream ID, 0 strict source route, 0 alert, 0 cipso0 otherFrags: 0 reassembled, 0 timeouts, 0 couldn't reassemble0 fragmented, 0 couldn't fragmentBcast: 0 received, 0 sentMcast: 0 received, 0 sentSent: 0 generated, 0 forwardedDrop: 0 encapsulation failed, 0 unresolved, 0 no adjacency0 no route, 0 unicast RPF, 15000 forced dropAdditional References
The following sections provide references related to ACL IP Options Selective Drop.
Related Documents
Related Topic Document TitleConfiguring IP access lists
IP access list commands
Cisco IOS Security Command Reference
Using access lists for filtering IP options
Creating an IP Access List to Filter IP Options, TCP Flags, Noncontiguous Ports, or TTL Values
Standards
MIBs
MIBs MIBs LinkNone
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
Technical Assistance
Command Reference
The following commands are introduced or modified in the feature or features
•ip options
For information about these commands, see the Cisco IOS Security Command Reference at
http://www.cisco.com/en/US/docs/ios/security/command/reference/sec_book.html.
For information about all Cisco IOS commands, see the Command Lookup Tool at
http://tools.cisco.com/Support/CLILookup or the Master Command List.
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