Downloads |
 Feedback
|
Table Of Contents
Bidirectional Forwarding Detection
Prerequisites for Bidirectional Forwarding Detection
Restrictions for Bidirectional Forwarding Detection
Information About Bidirectional Forwarding Detection
BFD Support for Non-Broadcast Media Interfaces
BFD Support for VPN Routing and Forwarding Interfaces
BFD Support for Nonstop Forwarding with Stateful Switchover
BFD Support for Static Routing
Benefits of Using BFD for Failure Detection
How to Configure Bidirectional Forwarding Detection
Configuring BFD Session Parameters on the Interface
Configuring BFD Support for Dynamic Routing Protocols
Configuring BFD Support for BGP
Configuring BFD Support for EIGRP
Configuring BFD Support for IS-IS
Configuring BFD Support for OSPF
Configuring BFD Support for Static Routing
Configuring the BFD Slow Timer
Disabling BFD Echo Mode Without Asymmetry
Configuration Examples for Bidirectional Forwarding Detection
Example: Configuring BFD Support for Static Routing
Feature Information for Bidirectional Forwarding Detection
Bidirectional Forwarding Detection
First Published: January 14, 2008Last Updated: July 15, 2010This document describes how to enable the Bidirectional Forwarding Detection (BFD) protocol. BFD is a detection protocol that is designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators. Because the network administrator can use BFD to detect forwarding path failures at a uniform rate, rather than the variable rates for different routing protocol hello mechanisms, network profiling and planning will be easier, and reconvergence time will be consistent and predictable.
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 Bidirectional Forwarding Detection" 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
•
Prerequisites for Bidirectional Forwarding Detection
•
•
•
•
•
Prerequisites for Bidirectional Forwarding Detection
•
•
Restrictions for Bidirectional Forwarding Detection
•
•
Information About Bidirectional Forwarding Detection
•
BFD Operation
BFD provides a low-overhead, short-duration method of detecting failures in the forwarding path between two adjacent routers, including the interfaces, data links, and forwarding planes. BFD is a detection protocol that you enable at the interface and routing protocol levels. Cisco supports the BFD asynchronous mode, which depends on the sending of BFD control packets between two systems to activate and maintain BFD neighbor sessions between routers. Therefore, in order for a BFD session to be created, you must configure BFD on both systems (or BFD peers). Once BFD has been enabled on the interfaces and at the router level for the appropriate routing protocols, a BFD session is created, BFD timers are negotiated, and the BFD peers will begin to send BFD control packets to each other at the negotiated interval.
BFD provides fast BFD peer failure detection times independently of all media types, encapsulations, topologies, and routing protocols BGP, EIGRP, IS-IS, and OSPF. By sending rapid failure detection notices to the routing protocols in the local router to initiate the routing table recalculation process, BFD contributes to greatly reduced overall network convergence time. Figure 1 shows a simple network with two routers running OSPF and BFD. When OSPF discovers a neighbor (1) it sends a request to the local BFD process to initiate a BFD neighbor session with the OSPF neighbor router (2). The BFD neighbor session with the OSPF neighbor router is established (3).
Figure 1 Establishing a BFD Neighbor Relationship
Figure 2 shows what happens when a failure occurs in the network (1). The BFD neighbor session with the OSPF neighbor router is torn down (2). BFD notifies the local OSPF process that the BFD neighbor is no longer reachable (3). The local OSPF process tears down the OSPF neighbor relationship (4). If an alternative path is available the routers will immediately start converging on it.
Figure 2 Tearing Down an OSPF Neighbor Relationship
BFD Detection of Failures
Once a BFD session has been established and timer negations are complete, BFD peers send BFD control packets that act in the same manner as an IGP hello protocol to detect liveliness, except at a more accelerated rate. The following information should be noted:
•
•
BFD Version Interoperability
All BFD sessions appear as Version 1 by default and are interoperable with Version 0. The system automatically performs BFD version detection, and BFD sessions between neighbors will run in the highest common BFD version between neighbors. For example, of one BFD neighbor is running BFD Version 0 and the other BFD neighbor is running Version 1, the session will run BFD Version 0. The output from the show bfd neighbors [details] command will verify which BFD version a BFD neighbor is running.
BFD Session
The maximum number of BFD sessions that can be created is 500. The maximum number of BFD sessions for a stateful switchover is 128.
BFD Support for Non-Broadcast Media Interfaces
The BFD feature is supported on non-broadcast media interfaces including ATM, POS, serial, and VLAN interfaces. BFD support extends to ATM, FR, POS, and serial subinterfaces as well.
The bfd interval command must be configured on the interface to initiate BFD monitoring.
BFD Support for VPN Routing and Forwarding Interfaces
The BFD feature is VPN Routing and Forwarding (VRF) aware to provide fast detection of routing protocol failures between provider edge (PE) and customer edge (CE) routers.
BFD Support for Nonstop Forwarding with Stateful Switchover
Cisco IOS XE software supports BFD stateful switchover (SSO) for both planned and unplanned switchovers. Typically, when a networking device restarts, all routing peers of that device detect that the device went down and then came back up. This transition results in a routing flap, which could spread across multiple routing domains. Routing flaps caused by router restarts create routing instabilities, which are detrimental to the overall network performance. Nonstop forwarding (NSF) helps to suppress routing flaps in devices that are enabled with SSO, thereby reducing network instability.
NSF allows for the forwarding of data packets to continue along known routes while the routing protocol information is being restored after a switchover. With NSF, peer networking devices do not experience routing flaps. Data traffic is forwarded through intelligent line cards or dual forwarding processors while the standby RP assumes control from the failed active RP during a switchover. The ability of line cards and forwarding processors to remain up through a switchover and to be kept current with the Forwarding Information Base (FIB) on the active RP is key to NSF operation.
In devices that support dual RPs, SSO establishes one of the RPs as the active processor while the other RP is designated as the standby processor, and then synchronizes information between them. A switchover from the active to the standby processor occurs when the active RP fails, when it is removed from the networking device, or when it is manually taken down for maintenance.
BFD Support for Static Routing
Unlike dynamic routing protocols, such as OSPF and BGP, static routing has no method of peer discovery. Therefore, when BFD is configured, the reachability of the gateway is completely dependent on the state of the BFD session to the specified neighbor. Unless the BFD session is up, the gateway for the static route is considered unreachable, and therefore the affected routes will not be installed in the appropriate RIB.
For a BFD session to be successfully established, BFD must be configured on the interface on the peer and there must be a BFD client registered on the peer for the address of the BFD neighbor. When an interface is used by dynamic routing protocols, the latter requirement is usually met by configuring the routing protocol instances on each neighbor for BFD. When an interface is used exclusively for static routing, this requirement must be met by configuring static routes on the peers.
If a BFD configuration is removed from the remote peer while the BFD session is in the up state, the updated state of the BFD session is not signaled to IPv4 static. This will cause the static route to remain in the RIB. The only workaround is to remove the IPv4 static BFD neighbor configuration so that the static route no longer tracks BFD session state. Also, if you change the encapsulation type on a serial interface to one that is unsupported by BFD, BFD will be in a down state on that interface. The workaround is to shut down the interface, change to a supported encapsulation type, and then reconfigure BFD.
Benefits of Using BFD for Failure Detection
When you deploy any feature, it is important to consider all the alternatives and be aware of any trade-offs being made.
The closest alternative to BFD in conventional EIGRP, IS-IS, and OSPF deployments is the use of modified failure detection mechanisms for EIGRP, IS-IS, and OSPF routing protocols.
If you set EIGRP hello and hold timers to their absolute minimums, the failure detection rate for EIGRP falls to within a one- to two-second range.
If you use fast hellos for either IS-IS or OSPF, these Interior Gateway Protocol (IGP) protocols reduce their failure detection mechanisms to a minimum of one second.
There are several advantages to implementing BFD over reduced timer mechanisms for routing protocols:
•
•
•
How to Configure Bidirectional Forwarding Detection
You start a BFD process by configuring BFD on the interface. When the BFD process is started, no entries are created in the adjacency database; in other words, no BFD control packets are sent or received. BFD echo mode is enabled by default. BFD echo packets are sent and received in addition to BFD control packets. The adjacency creation takes places once you have configured BFD support for the applicable routing protocols. This section contains the following procedures:
•
•
•
•
Configuring BFD Session Parameters on the Interface
The steps in this procedure show how to configure BFD on the interface by setting the baseline BFD session parameters on an interface. Repeat the steps in this procedure for each interface over which you want to run BFD sessions to BFD neighbors.
SUMMARY STEPS
1.
2.
3.
4.
5.
DETAILED STEPS
Configuring BFD Support for Dynamic Routing Protocols
You can enable BFD support for dynamic routing protocols at the router level to enable BFD support globally for all interfaces or you can configure BFD on a per-interface basis at the interface level.
For Cisco IOS XE Release 2.1, the Cisco implementation of BFD supports the following routing protocols: BGP, EIGRP, IS-IS, and OSPF.
This section describes the following procedures:
•
•
•
•
Configuring BFD Support for BGP
This section describes the procedure for configuring BFD support for BGP, so that BGP is a registered protocol with BFD and will receive forwarding path detection failure messages from BFD.
Prerequisites
BGP must be running on all participating routers.
You must configure the baseline parameters for BFD sessions on the interfaces over which you want to run BFD sessions. See the "Configuring BFD Session Parameters on the Interface" section for more information.
SUMMARY STEPS
1.
2.
3.
4.
5.
6.
7.
DETAILED STEPS
What to Do Next
If you want to configure BFD support for another routing protocol, see the following sections:
•
•
•
Configuring BFD Support for EIGRP
This section describes the procedure for configuring BFD support for EIGRP, so that EIGRP is a registered protocol with BFD and will receive forwarding path detection failure messages from BFD. There are two methods for enabling BFD support for EIGRP:
•
•
Prerequisites
EIGRP must be running on all participating routers.
The baseline parameters for BFD sessions on the interfaces over which you want to run BFD sessions to BFD neighbors must be configured. See the "Configuring BFD Session Parameters on the Interface" section for more information.
SUMMARY STEPS
1.
2.
3.
4.
or
bfd interface type number5.
6.
7.
DETAILED STEPS
What to Do Next
If you want to configure BFD support for another routing protocol, see the following sections:
•
•
•
Configuring BFD Support for IS-IS
This section describes the procedures for configuring BFD support for IS-IS, so that IS-IS is a registered protocol with BFD and will receive forwarding path detection failure messages from BFD. There are two methods for enabling BFD support for IS-IS:
•
•
To configure BFD support for IS-IS, perform the steps in one of the following sections:
•
•
Prerequisites
IS-IS must be running on all participating routers.
The baseline parameters for BFD sessions on the interfaces that you want to run BFD sessions to BFD neighbors over must be configured. See the "Configuring BFD Session Parameters on the Interface" section for more information.
Configuring BFD Support for IS-IS for All Interfaces
To configure BFD on all IS-IS interfaces that support IPv4 routing, perform the steps in this section.
SUMMARY STEPS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
DETAILED STEPS
What to Do Next
If you want to configure only for a specific subset of interfaces, perform the tasks in the "Configuring BFD Support for IS-IS for One or More Interfaces" section.
Configuring BFD Support for IS-IS for One or More Interfaces
To configure BFD for only one or more IS-IS interfaces, perform the steps in this section.
SUMMARY STEPS
1.
2.
3.
4.
5.
6.
7.
8.
DETAILED STEPS
What to Do Next
If you want to configure BFD support for another routing protocol, see one of the following sections:
•
•
•
Configuring BFD Support for OSPF
This section describes the procedures for configuring BFD support for OSPF, so that OSPF is a registered protocol with BFD and will receive forwarding path detection failure messages from BFD. You can either configure BFD support for OSPF globally on all interfaces or configure it selectively on one or more interfaces.
There are two methods for enabling BFD support for OSPF:
•
•
See the following sections for tasks for configuring BFD support for OSPF:
•
•
Configuring BFD Support for OSPF for All Interfaces
To configure BFD for all OSPF interfaces, perform the steps in this section.
If you do not want to configure BFD on all OSPF interfaces and would rather configure BFD support specifically for one or more interfaces, see the "Configuring BFD Support for OSPF for One or More Interfaces" section.
Prerequisites
OSPF must be running on all participating routers.
The baseline parameters for BFD sessions on the interfaces over which you want to run BFD sessions to BFD neighbors must be configured. See the "Configuring BFD Session Parameters on the Interface" section for more information.
SUMMARY STEPS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
DETAILED STEPS
What to Do Next
If you want to configure BFD support for another routing protocol, see the following sections:
•
•
•
Configuring BFD Support for OSPF for One or More Interfaces
To configure BFD on one or more OSPF interfaces, perform the steps in this section.
Prerequisites
OSPF must be running on all participating routers.
The baseline parameters for BFD sessions on the interfaces over which you want to run BFD sessions to BFD neighbors must be configured. See the "Configuring BFD Session Parameters on the Interface" section for more information.
SUMMARY STEPS
1.
2.
3.
4.
5.
6.
7.
DETAILED STEPS
What to Do Next
If you want to configure BFD support for another routing protocol, see the following sections:
•
•
•
Configuring BFD Support for Static Routing
Perform this task to configure BFD support for static routing.
Repeat the steps in this procedure on each BFD neighbor. For more information, see the "Example: Configuring BFD Support for Static Routing" section.
SUMMARY STEPS
1.
2.
3.
4.
5.
6.
7.
8.
9.
DETAILED STEPS
Configuring BFD Echo Mode
This section contains the following configuration tasks for BFD echo mode:
•
•
BFD echo mode is enabled by default, but you can disable it such that it can run independently in each direction. Before you configure echo mode, you should be familiar with the following concepts:
BFD Echo Mode
Benefits of Running BFD Echo Mode
BFD echo mode works with asynchronous BFD. Echo packets are sent by the forwarding engine and forwarded back along the same path in order to perform detection—the BFD session at the other end does not participate in the actual forwarding of the echo packets. The echo function and the forwarding engine are responsible for the detection process, therefore the number of BFD control packets that are sent out between two BFD neighbors is reduced. And since the forwarding engine is testing the forwarding path on the remote (neighbor) system without involving the remote system, there is an opportunity to improve the interpacket delay variance, thereby achieving quicker failure detection times than when using BFD Version 0 with BFD control packets for the BFD session.
Echo Mode Without Asymmetry
Echo mode is described as without asymmetry when it is running on both sides (both BFD neighbors are running echo mode).
Prerequisites
BFD must be running on all participating routers.
Before using BFD echo mode, you must disable the sending of Internet Control Message Protocol (ICMP) redirect messages by entering the no ip redirects command, in order to avoid high CPU utilization.
The baseline parameters for BFD sessions on the interfaces over which you want to run BFD sessions to BFD neighbors must be configured. See the "Configuring BFD Session Parameters on the Interface" section for more information.
Configuring the BFD Slow Timer
The steps in this procedure show how to change the value of the BFD slow timer. Repeat the steps in this procedure for each BFD router.
SUMMARY STEPS
1.
2.
3.
4.
DETAILED STEPS
Disabling BFD Echo Mode Without Asymmetry
The steps in this procedure show how to disable BFD echo mode without asymmetry—no echo packets will be sent by the router, and the router will not forward BFD echo packets that are received from any neighbor routers.
Repeat the steps in this procedure for each BFD router.
SUMMARY STEPS
1.
2.
3.
4.
5.
DETAILED STEPS
Configuration Examples for Bidirectional Forwarding Detection
This section provides the following configuration example:
•
Example: Configuring BFD Support for Static Routing
In the following example, the network consists of Router A and Router B. Serial interface 2/0/0 on Router A is connected to the same network as serial interface 2/0 on Router B. In order for the BFD session to come up, Router B must be configured.
Router A
configure terminalip route static bfd Serial 2/0/0 10.201.201.2ip route 10.0.0.0 255.0.0.0 Serial 2/0/0 10.201.201.2interface Serial 2/0/0ip address 10.201.201.1 255.255.255.0bfd interval 500 min_rx 500 multiplier 5Router B
configure terminalip route static bfd Serial 2/0/0 10.201.201.1ip route 10.1.1.1 255.255.255.255 Serial 2/0/0 10.201.201.1interface Serial 2/0/0ip address 10.201.201.2 255.255.255.0bfd interval 500 min_rx 500 multiplier 5Note that the static route on Router B exists solely to enable the BFD session between 10.201.201.1 and 10.201.201.2. If there is no useful static route that needs to be configured, select a prefix that will not affect packet forwarding; for example, the address of a locally configured loopback interface.
Additional References
Related Documents
Cisco IOS commands
•
•
"Implementing Bidirectional Forwarding Detection for IPv6" module of the Cisco IOS XE IPv6 Configuration Guide, Release 2
Configuring and monitoring BGP
"Cisco BGP Overview" module of the Cisco IOS XE IP Routing Protocols Configuration Guide, Release 2
Configuring and monitoring EIGRP
"Configuring EIGRP" module of the Cisco IOS XE IP Routing Protocols Configuration Guide, Release 2
Configuring and monitoring IS-IS
"Integrated IS-IS Routing Protocol Overview" module of the Cisco IOS XE IP Routing Protocols Configuration Guide, Release 2
Configuring and monitoring OSPF
"Configuring OSPF" module of the Cisco IOS XE IP Routing Protocols Configuration Guide, Release 2
BFD commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples
Cisco IOS IP Routing: Protocol-Independent Command Reference
BGP commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples
EIGRP commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples
IS-IS commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples
OSPF commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples
Standards
MIBs
RFCs
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.
—
Technical Assistance
Feature Information for Bidirectional Forwarding Detection
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 Feature Information for Bidirectional Forwarding Detection
Bidirectional Forwarding Detection (standard implementation, Version 1)
Cisco IOS XE
Release 2.1This document describes how to enable the BFD protocol. BFD is a detection protocol that is designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators. Because the network administrator can use BFD to detect forwarding path failures at a uniform rate, rather than the variable rates for different routing protocol hello mechanisms, network profiling and planning will be easier, and reconvergence time will be consistent and predictable.
This feature was introduced on the Cisco ASR 1000 Series Aggregation Services Routers.
BFD Echo Mode
Cisco IOS XE
Release 2.1BFD echo mode works with asynchronous BFD. Echo packets are sent by the forwarding engine and forwarded back along the same path in order to perform detection—the BFD session at the other end does not participate in the actual forwarding of the echo packets. The echo function and the forwarding engine are responsible for the detection process, therefore the number of BFD control packets that are sent out between two BFD neighbors is reduced. And since the forwarding engine is testing the forwarding path on the remote (neighbor) system without involving the remote system, there is an opportunity to improve the interpacket delay variance, thereby achieving quicker failure detection times than when using BFD Version 0 with BFD control packets for the BFD session.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
BFD—EIGRP Support
Cisco IOS XE
Release 2.1BFD support for EIGRP can be configured so that EIGRP is a registered protocol with BFD and will receive forwarding path detection failure messages from BFD.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following section provides information about this feature:
•
BFD—VRF Support
Cisco IOS XE
Release 2.1The BFD feature support is extended to be VRF aware to provide fast detection of routing protocol failures between PE and CE routers.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
BFD—WAN Interface Support
Cisco IOS XE
Release 2.1BFD feature is supported on non-broadcast media interfaces including ATM, POS, serial, and VLAN interfaces. BFD support extends to ATM, FR, POS, and serial subinterfaces as well.
The bfd interval command must be configured on the interface to initiate BFD monitoring.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
IS-IS Support for BFD over IPv4
Cisco IOS XE
Release 2.1BFD support for OSPF can be configured globally on all interfaces or configured selectively on one or more interfaces. When BFD support is configured with IS-IS as a registered protocol with BFD, IS-IS receives forwarding path detection failure messages from BFD.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
•
OSPF Support for BFD over IPv4
Cisco IOS XE
Release 2.1BFD support for OSPF can be configured globally on all interfaces or configured selectively on one or more interfaces. When BFD support is configured with OSPF as a registered protocol with BFD, OSPF receives forwarding path detection failure messages from BFD.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
•
SSO—BFD
Cisco IOS XE
Release 2.1This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
•
Static Routes for BFD
Cisco IOS XE
Release 2.1Unlike dynamic routing protocols, such as OSPF and BGP, static routing has no method of peer discovery. Therefore, when BFD is configured, the reachability of the gateway is completely dependent on the state of the BFD session to the specified neighbor. Unless the BFD session is up, the gateway for the static route is considered unreachable, and therefore the affected routes will not be installed in the appropriate RIB.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
•
•
Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
© 2008-2010 Cisco Systems, Inc. All rights reserved.
