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.)
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 Enables bfd on a system level | ||
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>] Disables bfd sessions but doesn't remove the configuration. | ||
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 |
| |
bfd interval
bfd interval [vrf <vrf-name>] [transmit-rate] [min-rx] [multiplier] Sets the interval rates between BFD messages. | ||
Syntax Description | transmit-rate | Transfer time between two consecutive BFD messages, the actual time is negotiated between two systems |
min_rx | Minimum time between neighbor messages, the actual time is negotiated between two systems | |
multiplier | Defines a time period to detect BFD failure | |
Default | transmit-rate – 300 | |
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 Enables BFD on the given interface for all OSPF neighbors on a number of active sessions. | ||
Syntax Description | N/A | |
Default | N/A | |
Configuration Mode | config interface ethernet | |
History | 3.6.4070 | |
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 Configures static route with BFD enabled on a specified VRF. | ||
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 | |
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 |
vrf | VRF name | |
Default | Default vrf | |
Configuration Mode | Any command mode | |
History | 3.6.4070 | |
| 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 | ||
