Bidirectional Forwarding Detection (BFD) Infrastructure

Many protocols uses slow Hello mechanisms and failure is detection usually seconds after the problem occurs. The BFD goal is to provide low overhead short duration detection of failures between adjacent nodes and single mechanism that can be used for liveness detection over any media.

BFD session is established by the application that uses it. There is no discovery mechanism. E.g. in OSPF BFD session is established to neighbors that were discovered by OSPF hello protocol.

BFD supports multiple modes: one of them is Asynchronous.

In Asynchronous mode a system periodically sends BFD packets to verify connectivity. If a number of packets in a row are not received – the session is declared down.

A system can be passive or active. Active system initiates BDF sessions. Both systems can be active. (Only active mode is supported.)

Note

BFD is not yet supported for BGP unnumbered. Adjusting keepalive/hold timers may need to be considered to achieve faster convergence.

Session Establishment

A session begins with exchange of control packets. When bidirectional communication is achieved – a session becomes Up.

After session becomes up – control packet rate can be incremented.

Each side informs the neighbor in what intervals it is going to send BFD packets and what minimum interval it can receive BFD packets is.

Detection time is different in both directions and depends on negotiated parameters.

In Asynchronous mode—agreed transmit interval or remote system—max between local minimum rx time and last received min transmit time.

Detection time is equal to agreed transmit interval of remote system multiplied to multiplier received from remote system.

Interaction with Protocols

BFD session can be single-hop or multi-hop:

Single hop session traverse between two adjacent IP neighbors. BFD control packet should be encapsulated in UDP with DPORT = 3784. SPORT should be in range 49152 to 65535. Same SPORT must be used for all control BFD packets for given session and is unique between different sessions. TTL value is 255.
Multi-hop sessions traverse between to remote ip neighbors. Control packets are encapsulated in UDP with DPORT = 4784.

If different protocols want to establish a BFD session with the same remote system for same data plane – they should share BFD session.

IPv4 and IPv6 data protocols have different BFD sessions.

In OSPF Protocol neighbor discovery protocol establishes single hop BFD sessions. For OSPF when session fails – it tears down OSPF neighbor.

BFD session is established to BGP neighbor (single hop or multiple hop).

Single hop BFD session can be established for static route next hop.

BFD Commands

protocol bfd

	protocol bfd no protocol bfd Enables bfd on a system level The no form of the command removes bfd configuration.
Syntax Description	N/A
Default	N/A
Configuration Mode	config router bgp
History	3.6.4070
Example	switch (config router bgp)# protocol bfd
Related Commands
Notes	The command returns an error if BFD is enabled in clients already running on the system (static routes or BGP of OSPF)

bfd shutdown

	bfd shutdown [vrf <vrf-name>] no bfd shutdown [vrf <vrf-name>] Disables bfd sessions but doesn't remove the configuration. if VRF is not given the command will be executed in active VRF.
Syntax Description	N/A
Default	N/A
Configuration Mode	config router bgp
History	3.6.4070
Example	switch (config) # ip bfd shutdown
Related Commands
Notes	The command “no ip bfd shutdown” or BFD interval parameters modification are affect traffic for all protocols; OSPF, BGP, static routes. The dynamic protocols (OSPF and BGP) restore the connection based on Hello protocol. For static routes, please execute “no ip route static bfd <ip address>”

bfd interval

	bfd interval [vrf <vrf-name>] [transmit-rate] [min-rx] [multiplier] no bfd interval Sets the interval rates between BFD messages. The no form of the command removes bfd interval rates.
Syntax Description	transmit-rate	Transfer time between two consecutive BFD messages, the actual time is negotiated between two systems Range: 50-60000 (msec)
	min_rx	Minimum time between neighbor messages, the actual time is negotiated between two systems Range: 50-60000 (msec)
	multiplier	Defines a time period to detect BFD failure Range: 3-50
Default	transmit-rate – 300 min-rx – 150 multiplier – 3
Configuration Mode	config
History	3.6.4070
Example	switch (config) # ip bfd interval transmit-rate 300 multiplier 3 min-rx 300 force
Related Commands
Notes	The command is executed in the active VRF if a VRF is not specified

ip ospf bfd

	ip ospf bfd no ip ospf bfd Enables BFD on the given interface for all OSPF neighbors on a number of active sessions. The no form of the command disables BFD on all OSPF neighbors.
Syntax Description	N/A
Default	N/A
Configuration Mode	config interface ethernet
History	3.6.4070
History	3.6.4110	Added “no” form of the command
Example	switch (config interface ethernet 1/2)# ip ospf bfd
Related Commands
Notes	The command “ip ospf bfd” affects traffic, OSPF restores the connection based on Hello protocol

ip route bfd

	ip route [vrf <vrf_name>] <prefix> <next_hop> bfd no ip route [vrf <vrf_name>] <prefix> <next_hop> bfd Configures static route with BFD enabled on a specified VRF. The no form of the commands removes the route.
Syntax Description	vrf-name	VRF session name
	prefix	Subnet IP address
	next_hop	IP address of next hop
Default	N/A
Configuration Mode	config
History	3.6.4070
History	3.7.1100	Updated command syntax and Example
Example	switch (config) # ip route vrf default 1.1.1.0/24 3.3.3.3 bfd
Related Commands
Notes	When a session fails, all static routes pointing to the specified gateway are removed from the routing decision

show ip route static

	show ip route [vrf [<vrf-name> \| all]] static Displays static routing table of VRF instance.
Syntax Description	all	Displays routing tables for all VRF instances
Syntax Description	vrf	VRF name
Default	Default vrf
Configuration Mode	Any command mode
History	3.6.4070
History	3.7.1100	Update command syntax
Example	switch (config) # show ip route vrf default static
Related Commands	ip route
Notes	If no routing-context is specified, the “routing-context” VRF is automatically displayed

show ip bfd neighbors

	show ip bfd [vrf <name> \| all] neighbors [brief \| <ip>] Displays BFD table of neighbor VRF instances.
Syntax Description	all	Displays tables for all VRF instances
Default	N/A
Configuration Mode	Any command mode
History	3.6.4110
Example
switch (config) # show ip bgp neighbors 1000::1040 BGP neighbor: 1000::1040, remote AS: 100, link: external BGP version: 4, remote router ID: 2.1.1.1 BGP State: ESTABLISHED Last read: 0:00:09:28, last write: 0:00:09:28, hold time is: 180, keepalive interval in seconds: 60 BFD State: Up Configured hold time in seconds: 180, keepalive interval in seconds: 60 Minimum holdtime from neighbor in seconds: 180 Neighbor capabilities: Route refresh: advertise and received Graceful Restart Capability: advertise and received Address family IPv4 Unicast: advertise and received Address family IPv6 Unicast: n/a Message statistics: InQ depth is: 0 OutQ depth is: 0 ---- ----- Sent Rcvd ---- ----- Opens: 1 1 Notifications: 0 0 Updates: 4 4 Keepalives: 1587 1593 Route Refresh: 0 0 Total: 1592 1598 Default minimum time between advertisement runs in seconds: 30 For address family: IPv4 Unicast BGP table version: 7 Output queue size : 0 ---- ---- Sent Rcvd Prefix activity: ---- ---- Prefixes Current: 4 2 Prefixes Total: 4 2 Implicit Withdraw: 0 0 Explicit Withdraw: 0 0 Used as bestpath: n/a 2 Used as multipath: n/a n/a -------- ------- Outbound Inbound Local Policy Denied Prefixes: -------- ------- Total: 0 0 Connections established: 1; dropped: 1 Last reset: 0:23:01:17, due to: 0 (0) External BGP neighbor possible distance in hops: 1 Connection state is: ESTABLISHED Local host: 1.1.1.1, Local port: 49616 Foreign host: 1000::1040, Foreign port: 179
Related Commands
Notes

On This Page