IP Routing: OSPF Configuration Guide, Cisco IOS XE Release 3S
Configuring OSPF TTL Security Check and OSPF Graceful Shutdown
Downloads: This chapterpdf (PDF - 1.28MB) The complete bookPDF (PDF - 5.95MB) | The complete bookePub (ePub - 1.29MB) | Feedback

Configuring OSPF TTL Security Check and OSPF Graceful Shutdown

Contents

Configuring OSPF TTL Security Check and OSPF Graceful Shutdown

This module describes configuration tasks to configure various options involving Open Shortest Path First (OSPF). This module contains tasks that use commands to configure a lightweight security mechanism to protect OSPF sessions from CPU-utilization-based attacks and to configure a router to shut down a protocol temporarily without losing the protocol configuration.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and 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 table at the end of this module.

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

Information About OSPF TTL Security Check and OSPF Graceful Shutdown

TTL Security Check for OSPF

When the TTL Security Check feature is enabled, OSPF sends outgoing packets with an IP header Time to Live (TTL) value of 255 and discards incoming packets that have TTL values less than a configurable threshold. Since each device that forwards an IP packet decrements the TTL, packets received via a direct (one-hop) connection will have a value of 255. Packets that cross two hops will have a value of 254, and so on. The receive threshold is configured in terms of the maximum number of hops that a packet may have traveled. The value for this hop-count argument is a number from 1 to 254, with a default of 1.

The TTL Security Check feature may be configured under the OSPF router submode, in which case it applies to all the interfaces on which OSPF runs, or it may be configured on a per-interface basis.

Transitioning Existing Networks to Use TTL Security Check

If you currently have OSPF running in your network and want to implement TTL security on an interface-by-interface basis without any network interruptions, use the ip ospf ttl-security command and set the hop-count argument to 254. This setting causes outgoing packets to be sent with a TTL value of 255, but allows any value for input packets. Later, once the device at the other end of the link has had TTL security enabled you can start enforcing the hop limit for the incoming packets by using the same ip ospf ttl-security command with no hop count specified. This process ensures that OSPF packets will not be dropped because of a temporary mismatch in TTL security.

TTL Security Check for OSPF Virtual and Sham Links

In OSPF, all areas must be connected to a backbone area. If there is a break in backbone continuity, or the backbone is purposefully partitioned, you can establish a virtual link. The virtual link must be configured in both devices. The configuration information in each device consists of the other virtual endpoint (the other area border router [ABR]) and the nonbackbone area that the two devices have in common (called the transit area.) Note that virtual links cannot be configured through stub areas. Sham links are similar to virtual links in many ways, but sham links are used in Layer 3 Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) networks to connect Provider Edge (PE) routers across the MPLS backbone.

To establish a virtual link or a sham link, use the area virtual-link or area sham-link cost commands, respectively, in router configuration mode. To configure the TTL Security Check feature on a virtual link or a sham link, configure the ttl-security keyword and the hop-count argument in either command. Note that the hop-count argument value is mandatory in this case.

Benefits of the OSPF Support for TTL Security Check

The OSPF Support for TTL Security Check feature provides an effective and easy-to-deploy solution to protect OSPF neighbor sessions from CPU utilization-based attacks. When this feature is enabled, a host cannot attack an OSPF session if the host is not a member of the local or remote OSPF network, or if the host is not directly connected to a network segment between the local and remote OSPF networks. This solution greatly reduces the effectiveness of Denial of Service (DoS) attacks against an OSPF autonomous system.

OSPF Graceful Shutdown

The OSPF Graceful Shutdown feature provides the ability to temporarily shut down the OSPF protocol in the least disruptive manner and notify its neighbors that it is going away. All traffic that has another path through the network will be directed to that alternate path. A graceful shutdown of the OSPF protocol can be initiated using the shutdown command in router configuration mode.

This feature also provides the ability to shut down OSPF on a specific interface. In this case, OSPF will not advertise the interface or form adjacencies over it; however, all of the OSPF interface configuration will be retained. To initiate a graceful shutdown of an interface, use the ip ospf shutdown command in interface configuration mode.

How to Configure OSPF TTL Security Check and OSPF Graceful Shutdown

Configuring TTL Security Check on All OSPF Interfaces

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router ospf process-id

    4.    ttl security all-interfaces [ hops hop-count ]

    5.    end


DETAILED STEPS
     Command or ActionPurpose
    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 router ospf process-id


    Example:
    Router(config)# router ospf 109
     

    Enables OSPF routing, which places the device in router configuration mode.

     
    Step 4 ttl security all-interfaces [ hops hop-count ]


    Example:
    Router(config-router)# ttl security all-interfaces
     

    Configures TTL security check on all OSPF interfaces.

    Note   

    This configuration step applies only to normal OSPF interfaces. This step does not apply to virtual links or sham links that require TTL security protection. Virtual links and sham links must be configured independently.

     
    Step 5 end


    Example:
    Router(config-router)# end
     

    Returns to privileged EXEC mode.

     

    Configuring TTL Security Check on a Per-Interface Basis

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    interface type number

      4.    ip ospf ttl-security [hops hop-count | disable]

      5.    end

      6.    show ip ospf [process-id] interface [interface type interface-number] [brief] [multicast] [topology topology-name | base}]

      7.    show ip ospf neighbor interface-type interface-number [neighbor-id][detail]

      8.    show ip ospf [process-id] traffic [interface-type interface-number]

      9.    debug ip ospf adj


    DETAILED STEPS
       Command or ActionPurpose
      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 interface type number


      Example:
      Router(config)# interface GigabitEthernet 0/0/0
       

      Configures an interface type and enters interface configuration mode.

       
      Step 4 ip ospf ttl-security [hops hop-count | disable]


      Example:
      Router(config-if)# ip ospf ttl-security 
       

      Configures TTL security check feature on a specific interface.

      • The hop-countargument range is from 1 to 254.

      • The disable keyword can be used to disable TTL security on an interface. It is useful only if the ttl-security all-interfaces comand initially enabled TTL security on all OSPF interfaces, in which case disable can be used as an override or to turn off TTL security on a specific interface.

      • In the example, TTL security is being disabled on GigabitEthernet interface 0/0/0.

       
      Step 5 end


      Example:
      Router(config-if)# end
       

      Returns to privileged EXEC mode.

       
      Step 6 show ip ospf [process-id] interface [interface type interface-number] [brief] [multicast] [topology topology-name | base}]


      Example:
      Router# show ip ospf interface gigabitethernet 0/0/0
       

      (Optional) Displays OSPF-related interface information.

       
      Step 7 show ip ospf neighbor interface-type interface-number [neighbor-id][detail]


      Example:
      Router# show ip ospf neighbor 10.199.199.137
       

      (Optional) Displays OSPF neighbor information on a per-interface basis.

      • If one side of the connection has TTL security enabled, the other side shows the neighbor in the INIT state.

       
      Step 8 show ip ospf [process-id] traffic [interface-type interface-number]


      Example:
      Router# show ip ospf traffic
       

      (Optional) Displays OSPF traffic statistics.

      • The number of times a TTL security check failed is included in the output.

       
      Step 9 debug ip ospf adj


      Example:
      Router# debug ip ospf adj
       

      (Optional) Initiates debugging of OSPF adjacency events.

      • Information about dropped packets, including interface type and number, neighbor IP address, and TTL value, is included in the command output.

       

      Configuring OSPF Graceful Shutdown on a Per-Interface Basis

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    interface type number

        4.    ip ospf shutdown

        5.    end

        6.    show ip ospf [ process-id ] interface [ interface type interface-number ] [ brief ] [multicast] [topology topology-name | base}]

        7.    show ip ospf [ process-id ]


      DETAILED STEPS
         Command or ActionPurpose
        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 interface type number


        Example:
        Router(config)# interface GigabitEthernet 0/1/0
         

        Configures an interface type and number and enters interface configuration mode.

         
        Step 4 ip ospf shutdown


        Example:
        Router(config-if)# ip ospf shutdown
         

        Initiates an OSPF protocol graceful shutdown at the interface level.

        • When the ip ospf shutdown interface command is entered, the interface on which it is configured sends a link-state update advising its neighbors that is going down, which allows those neighbors to begin routing OSPF traffic around this router.

         
        Step 5 end


        Example:
        Router(config-if)# end
         

        Returns to privileged EXEC mode.

         
        Step 6 show ip ospf [ process-id ] interface [ interface type interface-number ] [ brief ] [multicast] [topology topology-name | base}]


        Example:
        Router# show ip ospf interface GigabitEthernet 0/1/0
         

        (Optional) Displays OSPF-related interface information.

         
        Step 7 show ip ospf [ process-id ]


        Example:
        Router# show ip ospf
         

        (Optional) Displays general information about OSPF routing processes.

         

        Configuration Examples for OSPF TTL Security Check and OSPF Graceful Shutdown

        Example: Transitioning an Existing Network to Use TTL Security Check

        The following example shows how to enable TTL security in an existing OSPF network on a per-interface basis.

        Configuring TTL security in an existing network is a three-step process:

        1. Configure TTL security with a hop count of 254 on the OSPF interface on the sending side device.

        2. Configure TTL security with no hop count on the OSPF interface on the receiving side device.

        3. Reconfigure the sending side OSPF interface with no hop count.

        configure terminal
         ! Configure the following command on the sending side router.
        	interface gigabitethernet 0/1/0
        	 ip ospf ttl-security hops 254
        	! Configure the next command on the receiving side router.
        	interface gigabitethernet 0/1/0
        	 ip ospf ttl-security 
        	! Reconfigure the sending side with no hop count.
        	 ip ospf ttl-security 
        	 end

        Additional References

        The following sections provide references related to the OSPF TTL Security Check and OSPF Graceful Shutdown features.

        Related Documents

        Related Topic

        Document Title

        Configuring OSPF

        "Configuring OSPF"

        OSPF commands

        Cisco IOS IP Routing: OSPF Command Reference

        Cisco IOS master command list, all releases

        Cisco IOS Master Command List, All Releases

        Standards

        Standard

        Title

        No new or modified standards are supported and support for existing standards has not been modified.

        --

        MIBs

        MIB

        MIBs Link

        No new or modified MIBs are supported and support for existing MIBs has not been modified.

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

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

        RFCs

        RFC

        Title

        No new or modified RFCs are supported and support for existing RFCs has not been modified.

        --

        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 Configuring OSPF TTL Security Check and OSPF Graceful Shutdown

        The following table provides release information about the feature or features described in this module. This table 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.

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

        Table 1 Feature Information for Configuring OSPF TTL Security Check and OSPF Graceful Shutdown

        Feature Name

        Releases

        Feature Information

        OSPF Graceful Shutdown

        Cisco IOS XE Release 2.1

        This feature provides the ability to temporarily shut down a protocol in the least disruptive manner and to notify its neighbors that it is going away.

        A graceful shutdown of a protocol can be initiated on all OSPF interfaces or on a specific interface.

        The following commands were introduced or modified:

        • ip ospf shutdown

        • show ip ospf

        • show ip ospf interface

        • shutdown (router OSPF)

        OSPF TTL Security Check

        Cisco IOS XE Release 2.1

        This feature increases protection against OSPF denial of service attacks, enables checking of TTL values on OSPF packets from neighbors, and allows users to set TTL values sent to neighbors.

        The following commands were introduced or modified:

        • area sham-link cost

        • area virtual-link

        • debug ip ospf adj

        • ip ospf ttl-security

        • show ip ospf interface

        • show ip ospf neighbor

        • show ip ospf traffic

        • ttl-security all-interfaces