Bidirectional Forwarding Detection (BFD) is a detection protocol designed to provide fast forwarding–path failure detection times for media types, encapsulations, topologies, and routing protocols. BFD provides subsecond failure detection between two adjacent devices and can be less CPU-intensive than protocol hello messages because some of the BFD load can be distributed onto the data plane on supported modules.
Cisco NX-OS supports the BFD asynchronous mode, which sends BFD control packets between two adjacent devices to activate and maintain BFD neighbor sessions between the devices. You configure BFD on both devices (or BFD neighbors). Once BFD has been enabled on the interfaces and on the appropriate protocols, Cisco NX-OS creates a BFD session, negotiates BFD session parameters, and begins to send BFD control packets to each BFD neighbor at the negotiated interval. The BFD session parameters include the following:
Desired minimum transmit interval: The interval at which this device wants to send BFD hello messages.
Required minimum receive interval: The minimum interval at which this device can accept BFD hello messages from another BFD device.
Detect multiplier: The number of missing BFD hello messages from another BFD device before this local device detects a fault in the forwarding path.
Figure 1-6 shows how a BFD session is established. The figure 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 now established (3).
Figure 1-6 Establishing a BFD Neighbor Relationship
After a BFD session has been established and timer negotiations are complete, BFD neighbors send BFD control packets that act in the same manner as an IGP hello protocol to detect liveliness, except at a more accelerated rate. BFD detects a failure, but the protocol must take action to bypass a failed peer.
BFD sends a failure detection notice to the BFD-enabled protocols when it detects a failure in the forwarding path. The local device can then initiate the protocol recalculation process and reduce the overall network convergence time.
Figure 1-7 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 immediately start converging on it.
Note
The BFD failure detection occurs in less than a second, which is much faster than OSPF hello messages could detect the same failure.
Figure 1-7 Tearing Down an OSPF Neighbor Relationship
Table 1-14 lists BFD default parameters; you can alter BFD parameters as necessary.
Table 1-14 Default Settings for BFD Parameters
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