- Configuring OSPF
- OSPF Stub Router Advertisement
- OSPF Update Packet-Pacing Configurable Timers
- OSPF Sham-Link Support for MPLS VPN
- OSPF Support for Multi-VRF on CE Routers
- OSPF Forwarding Address Suppression in Translated Type-5 LSAs
- OSPF Inbound Filtering Using Route Maps with a Distribute List
- OSPF Shortest Path First Throttling
- OSPF Support for Fast Hello Packets
- OSPF Incremental SPF
- OSPF Limit on Number of Redistributed Routes
- OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements
- OSPF Support for Unlimited Software VRFs per Provider Edge (PE) Router
- OSPF Area Transit Capability
- OSPF Per-Interface Link-Local Signaling
- OSPF Link-State Database Overload Protection
- OSPF MIB Support of RFC 1850 and Latest Extensions
- OSPF Enhanced Traffic Statistics for OSPFv2 and OSPFv3
- Configuring OSPF TTL Security Check and OSPF Graceful Shutdown
- OSPF Sham-Link MIB Support
- OSPF: SNMP ifIndex Value for Interface ID in OSPFv2 and OSPFv3 Data Fields
- OSPFv2 Local RIB
- OSPF Support for Forwarding Adjacencies over MPLS Traffic Engineered Tunnels
- Area Command in Interface Mode for OSPFv2
- Finding Feature Information
- Contents
- Information About Configuring OSPF TTL Security Check and OSPF Graceful Shutdown
- How to Configure OSPF TTL Security Check and OSPF Graceful Shutdown
- Configuration Examples for OSPF TTL Security Check and OSPF Graceful Shutdown
- Additional References
- Feature Information for Configuring OSPF TTL Security Check and OSPF Graceful Shutdown
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
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 Configuring OSPF TTL Security Check and OSPF Graceful Shutdown" section.
Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE 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
•
Information About Configuring OSPF TTL Security Check and OSPF Graceful Shutdown
•How to Configure OSPF TTL Security Check and OSPF Graceful Shutdown
•Configuration Examples for OSPF TTL Security Check and OSPF Graceful Shutdown
•Feature Information for Configuring OSPF TTL Security Check and OSPF Graceful Shutdown
Information About Configuring OSPF TTL Security Check and OSPF Graceful Shutdown
To configure the OSPF features in this module, you should understand the following concepts:
•Transitioning Existing Networks to Use TTL Security Check
•TTL Security Check for OSPF Virtual and Sham Links
•Benefits of the OSPF Support for TTL Security Check
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 router 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 router at the other end of the link has had TTL security enabled, you can start enforcing the hop limit for 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 routers. The configuration information in each router consists of the other virtual endpoint (the other area border router [ABR]) and the nonbackbone area that the two routers 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
This section contains the following tasks:
•Configuring TTL Security Check on All OSPF Interfaces
•Configuring TTL Security Check on a Per-Interface Basis
•Configuring OSPF Graceful Shutdown on a Per-Interface Basis
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
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
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
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 router.
2. Configure TTL security with no hop count on the OSPF interface on the receiving side router.
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
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Configuring OSPF |
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OSPF commands |
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Cisco IOS master command list, all releases |
Standards
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No new or modified standards are supported and support for existing standards has not been modified. |
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MIBs
RFCs
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No new or modified RFCs are supported and support for existing RFCs has not been modified. |
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Technical Assistance
Feature Information for Configuring OSPF TTL Security Check and OSPF Graceful Shutdown
Table 1 lists the release history for this feature.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE 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 Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.
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OSPF Graceful Shutdown |
Cisco IOS XE |
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 sections provide information about this feature: • The following commands were introduced or modified: • • • • |
OSPF TTL Security Check |
Cisco IOS XE |
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 sections provide information about this feature: • • The following commands were introduced or modified: • • • • • • • • |
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