Specifications

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7-2
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
OL-25782-02
Chapter 7 Configuring Bidirectional Forwarding Detection for BGP
Information About BFD
Asynchronous Mode
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 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.
BFD Detection of Failures
Once 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.
When a failure occurs in the network:
1. The BFD neighbor session with the BFD neighbor router is torn down.
2. BFD notifies the local BFD process that the BFD neighbor is no longer reachable.
3. The local BFD process tears down the BFD 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.
BFD Echo Function
The BFD echo function sends echo packets from the forwarding engine to the remote BFD neighbor. The
BFD neighbor forwards the echo packet back along the same path in order to perform detection; the BFD
neighbor does not participate in the actual forwarding of the echo packets. The echo function and the
forwarding engine are responsible for the detection process. BFD can use the slow timer to slow down
the asycnhronous session when the echo function is enabled and reduce the number of BFD control
packets that are sent between two BFD neighbors. Also, the forwarding engine tests the forwarding path
on the remote (neighbor) system without involving the remote system, so there is less interpacket delay
variability and faster failure detection times.
The echo function is without asymmetry when both BFD neighbors are running echo function.