How does bfd work

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. Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for Bidirectional Forwarding Detection" section.

An account on Cisco. PXF is enabled by default and is generally not turned off. You should implement fast convergence for the routing protocol that you are using.

See the IP routing documentation for your version of Cisco IOS software for information on configuring fast convergence. BFD support is not available for all platforms and interfaces. BFD neighbors must be no more than one IP hop away. Multihop configurations are not supported.

To confirm BFD support for a specific platform or interface and obtain the most accurate platform and hardware restrictions, see the Cisco IOS software release notes for your software version. When BFD tries to set up a connection between routing protocols and establish a th session between a Cisco Internet router and adjacent routers, the following error message is displayed:.

In this special case, the BFD session between the routing peers will not be established. The minimum hello interval is 50 ms with up to three Max retries for a BFD control packet to be received from a remote system before a session with a neighbor is declared down. Note For the most accurate platform and hardware restrictions, see the Cisco IOS software release notes for your software version. Before you configure BFD, you should become familiar with the information in the following sections:.

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. 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 2 shows what happens when a failure occurs in the network 1. If an alternative path is available the routers will immediately start converging on it. 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 detects a failure, but the routing protocol must take action to bypass a failed peer.

Cisco IOS Release BFD LC sessions will have no knowledge of sessions being added or deleted by the clients. This database will contain only the minimum required information. It will also update transmit and receive counters. BFD control packets are received and processed, as well as sent, from the LC itself. When you deploy any feature, it is important to consider all the alternatives and be aware of any trade-offs being made.

There are several advantages to implementing BFD over reduced timer mechanisms for routing protocols:. 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.

The adjacency creation takes places once you have configured BFD support for the applicable routing protocols. This section contains the following procedures:. The steps in this procedure show how to configure BFD on the interface by setting the baseline BFD session parameters on an interface. You can enable BFD support for 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.

Note In order to display the full output of the show bfd neighbors details command on a Cisco series router, you must enter the command on the line card. Enter the attach slot-number command to establish a CLI session with a line card. The registered protocols are not shown in the output of the show bfd neighbors details command when it is entered on a line card.

If you want to configure BFD support for another routing protocol, see the following sections:. Configures the router to send a system logging syslog message when an EIGRP neighbor goes up or down.

Note In order to see the full output of the show bfd neighbors details command on a Cisco series router, you must enter the command on the line card.

You can then disable BFD for one or more of those interfaces using the isis bfd disable command in interface configuration mode.

Optional Returns the router to global configuration mode. Enter this command only if you want to follow Step 6 and Step 7 to disable BFD for one or more interfaces. Note You should use the disable keyword only if you enabled BFD on all of the interfaces that IS-IS is associated with using the bfd all-interfaces command in router configuration mode.

Displays information that can be used to verify if the BFD neighbor is active and displays the routing protocols that BFD has registered. After a BFD session is established between two systems, BFD packets are periodically sent over the path between the two systems. If one system does not receive BFD packets within a specified period, a fault has occurred on the path.

After detecting the link fault through BFD, the upper-layer protocol can take measures to promptly rectify the fault. To minimize the impact of device faults on services and enhance network reliability, a network device must be able to quickly detect faults in communication with adjacent devices.

Measures can then be taken to promptly rectify the faults to ensure service continuity. In practice, hardware detection is used to detect link faults. However, not all media provides the hardware detection mechanism. In this case, applications use the Hello mechanism of the upper-layer protocol to detect faults, which takes usually seconds.

This long detection period causes severe packet loss when traffic is transmitted at gigabit rates. On a Layer 3 network, the Hello mechanism cannot detect faults for all routes, such as static routes. This means that a fault between interconnected systems is difficult to locate. BFD addresses these issues and provides fast fault detection independent of media and routing protocols.

BFD is useful because it can:. Generally, BFD is not used independently. This section describes two typical application scenarios of BFD. This allows interfaces to be more sensitive to link faults and minimizes the impact of faults on indirectly connected links. When detecting a link fault, a BFD session immediately sends a Down message to the corresponding interface. As shown in the figure, SwitchA and SwitchB are directly connected at Layer 3, with transit devices being deployed between them.

When the link between the transit devices is faulty, SwitchA and SwitchB will take a long time to detect the fault. This will slow down route convergence and increase service interruption time. The BFD session status is associated with the interface status. When a link fault is detected, the BFD session immediately sends a Down message to the corresponding interface and the interface enters the BFD Down state.

A link failure or topology change may lead to route recalculation. Therefore, the convergence time of routing protocols must be shortened as much as possible to improve network performance. The lower the BFD failure detection timer value, the faster the failure detection and vice versa. For example, the timers can adapt to a higher value if the adjacency fails that is, the timer detects failures more slowly.

Or a neighbor can negotiate a higher value for a timer than the configured value. The timers adapt to a higher value when a BFD session flap occurs more than three times in a span of 15 seconds milliseconds. A back-off algorithm increases the receive Rx interval by two if the local BFD instance is the reason for the session flap. The transmission Tx interval is increased by two if the remote BFD instance is the reason for the session flap. You can use the clear bfd adaptation command to return BFD interval timers to their configured values.

The clear bfd adaptation command is hitless, meaning that the command does not affect traffic flow on the routing device.