Bidirectional Forwarding Detection
Encyclopedia
Bidirectional Forwarding Detection (BFD) is a network protocol used to detect faults between two forwarding engines connected by a link
. It provides low-overhead detection of faults even on physical media that don't support failure detection of any kind, such as Ethernet
, virtual circuit
s, tunnels
and MPLS
Label Switched Path
s.
BFD establishes a session between two endpoints over a particular link. If more than one link exists between two systems, multiple BFD sessions may be established to monitor each one of them. The session is established with a three-way handshake, and is torn down the same way. Authentication may be enabled on the session. A choice of simple password, MD5 or SHA1 authentication is available.
BFD does not have a discovery mechanism; sessions must be explicitly configured between endpoints. BFD may be used on many different underlying transport mechanisms and layers, and operates independently of all of these. Therefore, it needs to be encapsulated by whatever transport it uses. For example, monitoring MPLS LSPs involves piggybacking session establishment on LSP-Ping packets. Protocols that support some form of adjacency setup, such as OSPF
or IS-IS
, may also be used to bootstrap a BFD session. These protocols may then use BFD to receive faster notification of failing links than would normally be possible using the protocol's own keepalive
mechanism.
A session may operate in one of two modes: asynchronous mode and demand mode. In asynchronous mode, both endpoints periodically send Hello packets to each other. If a number of those packets are not received, the session is considered down.
In demand mode, no Hello packets are exchanged after the session is established; it is assumed that the endpoints have another way to verify connectivity to each other, perhaps on the underlying physical layer. However, either host may still send Hello packets if needed.
Regardless of which mode is in use, either endpoint may also initiate an Echo function. When this function is active, a stream of Echo packets is sent, and the other endpoint then sends these back to the sender via its forwarding plane. This is used to test the forwarding path on the remote system.
and IPv6
. BFD's operation in conjunction with Open Shortest Path First
(OSPF) and IS-IS
protocols has also been outlined in RFC 5881.
Link (telecommunications)
In telecommunications a link is the communications channel that connects two or more communicating devices. This link may be an actual physical link or it may be a logical link that uses one or more actual physical links...
. It provides low-overhead detection of faults even on physical media that don't support failure detection of any kind, such as Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, virtual circuit
Virtual circuit
In telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication...
s, tunnels
Tunneling protocol
Computer networks use a tunneling protocol when one network protocol encapsulates a different payload protocol...
and MPLS
Multiprotocol Label Switching
Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
Label Switched Path
Label Switched Path
In MPLS networking, a Label Switched Path is a path through an MPLS network, set up by a signaling protocol such as LDP, RSVP-TE, BGP or CR-LDP. The path is set up based on criteria in the forwarding equivalence class ....
s.
BFD establishes a session between two endpoints over a particular link. If more than one link exists between two systems, multiple BFD sessions may be established to monitor each one of them. The session is established with a three-way handshake, and is torn down the same way. Authentication may be enabled on the session. A choice of simple password, MD5 or SHA1 authentication is available.
BFD does not have a discovery mechanism; sessions must be explicitly configured between endpoints. BFD may be used on many different underlying transport mechanisms and layers, and operates independently of all of these. Therefore, it needs to be encapsulated by whatever transport it uses. For example, monitoring MPLS LSPs involves piggybacking session establishment on LSP-Ping packets. Protocols that support some form of adjacency setup, such as OSPF
Open Shortest Path First
Open Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
or IS-IS
IS-IS
Intermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
, may also be used to bootstrap a BFD session. These protocols may then use BFD to receive faster notification of failing links than would normally be possible using the protocol's own keepalive
Keepalive
A keepalive is a message sent by one device to another to check that the link between the two is operating, or to prevent this link from being broken.-Description:...
mechanism.
A session may operate in one of two modes: asynchronous mode and demand mode. In asynchronous mode, both endpoints periodically send Hello packets to each other. If a number of those packets are not received, the session is considered down.
In demand mode, no Hello packets are exchanged after the session is established; it is assumed that the endpoints have another way to verify connectivity to each other, perhaps on the underlying physical layer. However, either host may still send Hello packets if needed.
Regardless of which mode is in use, either endpoint may also initiate an Echo function. When this function is active, a stream of Echo packets is sent, and the other endpoint then sends these back to the sender via its forwarding plane. This is used to test the forwarding path on the remote system.
Standardization
In June 2010, the BFD protocol standardization process is the RFC stage. RFC 5880 defines the BFD protocol, detecting MPLS LSP failure, using BFD to monitor connectivity across multiple network hops, and using BFD for IPv4IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
and IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
. BFD's operation in conjunction with Open Shortest Path First
Open Shortest Path First
Open Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
(OSPF) and IS-IS
IS-IS
Intermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
protocols has also been outlined in RFC 5881.
External links
- IETF BFD Working Group
- BFD presentation by Juniper Networks
- RFC 5880
- BFD white paper from IP Infusion
- NetworkWorld article: Reducing Link Failure Detection Time with BFD