Alcatel-Lucent 6850-48 ネットワークガイド
Configuring BFD
BFD Overview
OmniSwitch AOS Release 6 Network Configuration Guide
September 2009
page 30-9
BFD Overview
Detecting communication failures as soon as possible is the first step in any network recovery process;
until a failure is detected, network convergence can’t begin. By rapidly detecting failures, BFD enables
faster convergence of routing protocols particularly on shared media such as ethernet.
until a failure is detected, network convergence can’t begin. By rapidly detecting failures, BFD enables
faster convergence of routing protocols particularly on shared media such as ethernet.
The BFD protocol is very similar to the widely-used Hello mechanisms prevalent in a majority of routing
protocols, with the exception that BFD tests bidirectional communication links, has smaller packets, and is
focused exclusively on path-failure detection. BFD can also be less CPU-intensive in routers with distrib-
uted architecture because unlike routing protocol Hello packets, BFD packets can be processed on the
interface modules rather than the control plane.
protocols, with the exception that BFD tests bidirectional communication links, has smaller packets, and is
focused exclusively on path-failure detection. BFD can also be less CPU-intensive in routers with distrib-
uted architecture because unlike routing protocol Hello packets, BFD packets can be processed on the
interface modules rather than the control plane.
BFD protocol is a fairly simple Hello protocol designed to provide fast forwarding path failure detection
that can be enabled at the interface and routing protocol levels. It helps in the verification of forwarding
plane-to-forwarding plane connectivity (including links, interfaces, tunnels). It allows semantic separation
of forwarding plane connectivity and control plane connectivity. BFD is a single mechanism that works
independently of underlying media, data, and network protocols. It can be encapsulated within any rout-
ing protocol i.e. it can run on top of any routing protocol being forwarded between two systems. More-
over, it requires no changes to the existing protocols. This implementation of BFD supports BGP, OSPF,
VRRP tracking, and static route protocols.
that can be enabled at the interface and routing protocol levels. It helps in the verification of forwarding
plane-to-forwarding plane connectivity (including links, interfaces, tunnels). It allows semantic separation
of forwarding plane connectivity and control plane connectivity. BFD is a single mechanism that works
independently of underlying media, data, and network protocols. It can be encapsulated within any rout-
ing protocol i.e. it can run on top of any routing protocol being forwarded between two systems. More-
over, it requires no changes to the existing protocols. This implementation of BFD supports BGP, OSPF,
VRRP tracking, and static route protocols.
Common BFD Applications include:
• Control plane liveliness detection
• Tunnel endpoint liveliness detection
Benefits of Using BFD For Failure Detection
It is more advantageous to implement BFD rather than reduce timer mechanisms for routing protocols due
to the following reasons:
to the following reasons:
• BFD can detect failures in milliseconds without having to fine-tune routing protocol Hello timers.
• BFD is not tied to any particular routing protocol. As a result, BFD provides a generic and consistent
failure detection mechanism for OSPF, BFP, VRRP Remote Tracking, and static routes.
• BFD is less CPU-intensive than reduced timer mechanisms for routing protocols.
How the BFD Protocol Works
A BFD session must be explicitly configured between two adjacent systems. Once BFD has been enabled
on the interfaces and at the appropriate Layer 3 routing protocol level, a BFD session is created for the
adjacent systems and BFD timers are negotiated between these systems.
on the interfaces and at the appropriate Layer 3 routing protocol level, a BFD session is created for the
adjacent systems and BFD timers are negotiated between these systems.
The BFD protocol does not have a neighbor discovery mechanism to detect neighboring systems; proto-
cols that BFD services notify BFD of devices to which it needs to establish sessions. For example, an
OSPF implementation may request BFD to establish a session with a neighbor discovered using the OSPF
Hello protocol.
cols that BFD services notify BFD of devices to which it needs to establish sessions. For example, an
OSPF implementation may request BFD to establish a session with a neighbor discovered using the OSPF
Hello protocol.
Once a session is established, BFD peers - neighboring systems sharing a BFD session - begin sending
BFD control packets to each other over the bidirectional forwarding path. The packets are transmitted
periodically at the negotiated rate. The BFD control packets function in a similar manner to that of an IGP
Hello protocol, except at a more accelerated rate.
BFD control packets to each other over the bidirectional forwarding path. The packets are transmitted
periodically at the negotiated rate. The BFD control packets function in a similar manner to that of an IGP
Hello protocol, except at a more accelerated rate.