Alcatel-Lucent 6850-48 ネットワークガイド
BFD Overview
Configuring BFD
page 30-10
OmniSwitch AOS Release 6 Network Configuration Guide
September 2009
Each time a BFD system successfully receives a BFD control packet on a BFD session, the detect-timer
for that session is reset to zero. As long as the BFD peer systems receive the control packets from each
other within the negotiated time interval [(Detect Multiplier) * (Required Minimum Rx Interval)], the BFD
session remains up, and any routing protocol that encapsulates the BFD maintains its adjacencies, i.e. it
continues its periodic transmission of BFD control packets at the negotiated rate.
for that session is reset to zero. As long as the BFD peer systems receive the control packets from each
other within the negotiated time interval [(Detect Multiplier) * (Required Minimum Rx Interval)], the BFD
session remains up, and any routing protocol that encapsulates the BFD maintains its adjacencies, i.e. it
continues its periodic transmission of BFD control packets at the negotiated rate.
In case a system stops receiving the packets within the predetermined time frame, some component in the
bidirectional path to that particular system is assumed to have failed, and the BFD system simply informs
its client protocol that a failure has occurred. It does this by sending rapid failure detection notices to
respective registered routing protocols in the local router to initiate the router table recalculation process in
order to accelerate routing convergence and network uptime.
bidirectional path to that particular system is assumed to have failed, and the BFD system simply informs
its client protocol that a failure has occurred. It does this by sending rapid failure detection notices to
respective registered routing protocols in the local router to initiate the router table recalculation process in
order to accelerate routing convergence and network uptime.
In order to agree with its peers about how rapidly failure detection will take place, each system estimates
the rate at which it can send and receive BFD control packets. This design also enables fast systems on
shared medium with a slow system to detect failures more rapidly between fast systems while allowing the
slow system to participate to the best of its ability.
the rate at which it can send and receive BFD control packets. This design also enables fast systems on
shared medium with a slow system to detect failures more rapidly between fast systems while allowing the
slow system to participate to the best of its ability.
Operational Mode and Echo Function
The BFD protocol offers two different modes of operation:
• Asynchronous mode
• Demand mode (not supported)
This implementation of BFD supports the Asynchronous mode. In this mode, BFD neighbors periodically
send BFD control packets to each other. A time interval for transmitting and receiving such packets is
negotiated between the two BFD systems. If a neighboring system fails to receive a number of control
packets continuously over a specific period of time, the session is considered down and BFD informs the
appropriate routing protocol.
send BFD control packets to each other. A time interval for transmitting and receiving such packets is
negotiated between the two BFD systems. If a neighboring system fails to receive a number of control
packets continuously over a specific period of time, the session is considered down and BFD informs the
appropriate routing protocol.
In addition to the operational mode, an Echo function is available to verify the forwarding path between
neighboring BFD systems. When enabled, a BFD system transmits Echo packets to a BFD neighbor,
which then sends the packets back to the originating system along the forwarding path. If no Echo packets
are received back from the BFD neighbor within a configured Echo time interval, the session is consid-
ered down.
neighboring BFD systems. When enabled, a BFD system transmits Echo packets to a BFD neighbor,
which then sends the packets back to the originating system along the forwarding path. If no Echo packets
are received back from the BFD neighbor within a configured Echo time interval, the session is consid-
ered down.
The Echo function is a configurable option and can work on its own or simultaneously with the Asynchro-
nous mode. Note that using the Echo function with the Asynchronous mode lowers the rate at which
control packets are sent because Echo packets are then used to detect session liveliness. In addition, trans-
mitting Echo packets is only allowed over a single hop; transmitting BFD control packets is allowed over
multiple hops.
nous mode. Note that using the Echo function with the Asynchronous mode lowers the rate at which
control packets are sent because Echo packets are then used to detect session liveliness. In addition, trans-
mitting Echo packets is only allowed over a single hop; transmitting BFD control packets is allowed over
multiple hops.
Once a BFD session is started, the BFD peers can decide whether or not Echo packets are actually trans-
mitted. A session is considered down when the peers receive no BFD control packets from each other or if
sufficient Echo packets are missed within a specific period of time.
mitted. A session is considered down when the peers receive no BFD control packets from each other or if
sufficient Echo packets are missed within a specific period of time.
BFD Packet Formats
The detection packets BFD sends are UDP packets which are of two types: BFD control packets and Echo
packets.
packets.