Operational Mode And Echo Function; Bfd Packet Formats - Alcatel-Lucent OmniSwitch 9900 Series Network Configuration Manual

Configuring BFD
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.
In order to agree with its peers about how rapidly failure detection takes 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.

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 an Asynchronous control packet mode or an Asynchronous Echo
function.
•
When the Asynchronous control packet mode is activated, 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.
•
The Asynchronous Echo function is used to verify the forwarding path between neighboring BFD
systems. When active, a BFD system transmits Echo packets only (no control packets are sent) 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 considered down.
VRRP and Static Routes only use the Asynchronous Echo function, so BFD sends only Echo packets.
Other protocols (OSPF, IS-IS, BGP) use the Asynchronous control packet mode, so BFD initiates and
maintains sessions by sending control packets.
Consider the following regarding how BFD operates between peers:
•
Transmitting Echo packets is only allowed over a single hop; transmitting BFD control packets is
allowed over multiple hops.
•
The Echo function does not require a BFD session to run; instead, the function is activated when BFD
is enabled for the switch that is going to send the Echo packets. The peer switches that are going to
receive the Echo packets do not require a BFD configuration since this function is not a BFD session.

BFD Packet Formats

The detection packets BFD sends are UDP packets which are of two types: BFD control packets and Echo
packets.
BFD Control Packets
There is no specific encapsulation type for BFD control packets; instead, the BFD IETF RFC-5880
recommends an encapsulation type that is "appropriate to the medium and network" used. This
OmniSwitch AOS Release 8 Network Configuration Guide December 2017
BFD Overview
page 20-11