Bidirectional forwarding detection (BFD) provides a general-purpose, standard, medium- and protocol-independent fast failure detection mechanism. It can detect and monitor the connectivity of forwarding paths to detect communication failures quickly so that measures can be taken to ensure service continuity and enhance network availability.

BFD can uniformly and quickly detect the failures of the bidirectional forwarding paths between two devices for upper-layer protocols such as routing protocols. The hello mechanism used by upper-layer protocols needs seconds to detect a link failure, while BFD can provide detection measured in milliseconds.

Single-hop detection and multihop detection

BFD can be used for single-hop and multihop detections.

· Single-hop detection—Detects the IP connectivity between two directly connected systems.

· Multihop detection—Detects any of the paths between two systems. These paths have multiple hops, and might overlap.

BFD session modes

BFD session modes include echo mode (using echo packets) and control mode (using control packets).

Echo mode

Echo packets are encapsulated into UDP packets with port number 3785.

The local end of the link sends echo packets to establish BFD sessions and monitor link status. The peer end does not establish BFD sessions and only forwards the packets back to the originating end. If the local end does not receive echo packets from the peer end within the detection time, it considers the session to be down.

An echo-mode BFD session supports only single-hop detection and is independent of the operating mode.

Control mode

Control packets are encapsulated into UDP packets with port number 3784 for single-hop detection or port number 4784 for multihop detection.

The two ends of the link negotiate the establishment of BFD sessions by using the session parameters carried in control packets. Session parameters include session discriminators, desired minimum packet sending and receiving intervals, and local BFD session state.

Before a BFD session is established, BFD has two operating modes—active and passive.

· Active mode—BFD actively sends BFD control packets regardless of whether any BFD control packet is received from the peer.

· Passive mode—BFD does not send control packets until a BFD control packet is received from the peer.

At least one end must operate in active mode for a BFD session to be established.

After a BFD session is established, the two ends can operate in the following BFD operating modes:

· Asynchronous mode—The device periodically sends BFD control packets. The device considers that the session is down if it does not receive any BFD control packets within a specific interval.

· Demand mode—The device periodically sends BFD control packets with the D bit set. If the peer end is operating in Asynchronous mode (default), the peer end stops sending BFD control packets after receiving control packets with the D bit set. In this case, BFD detects only the connectivity from the local end to the peer end. If the peer end does not receive control packets within the detection time, the session is declared to be down. If the peer end is operating in Demand mode, both ends stop sending BFD control packets. The system uses other mechanisms such as Hello mechanism and hardware detection to detect links. The Demand mode can be used to reduce the overhead when a large number of BFD sessions exist.

Supported features

Features	Reference
Static routing IS-IS OSPF RIP BGP IP fast reroute (FRR)	Layer 3—IP Routing Configuration Guide
IPv6 static routing OSPFv3	Layer 3—IP Routing Configuration Guide
MPLS L3VPN MPLS OAM	MPLS Configuration Guide
Track	"Configuring Track"
Ethernet link aggregation	Layer 2—LAN Switching Configuration Guide

Protocols and standards

· RFC 5880, Bidirectional Forwarding Detection (BFD)

· RFC 5881, Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)

· RFC 5882, Generic Application of Bidirectional Forwarding Detection (BFD)

· RFC 5883, Bidirectional Forwarding Detection (BFD) for Multihop Paths

· RFC 5884, Bidirectional Forwarding Detection (BFD) for MPLS Label Switched Paths (LSPs)

· RFC 5885, Bidirectional Forwarding Detection (BFD) for the Pseudowire Virtual Circuit Connectivity Verification (VCCV)

· RFC 7130, Bidirectional Forwarding Detection (BFD) on Link Aggregation Group (LAG) Interfaces

Restrictions and guidelines: BFD configuration

· By default, the device runs BFD version 1 and is compatible with BFD version 0. You cannot change the BFD version to 0 through commands. When the peer device runs BFD version 0, the local device automatically switches to BFD version 0.

· After a BFD session is established, the two ends negotiate BFD parameters, including minimum sending interval, minimum receiving interval, initialization mode, and packet authentication, by exchanging negotiation packets. They use the negotiated parameters without affecting the session status.

Configuring echo-mode BFD sessions

Restrictions and guidelines

As a best practice, do not configure the source IP address to be on the same network segment as any local interfaces. If you configure such a source IP address, a large number of ICMP redirect packets might be sent from the peer, resulting in link congestion.

Procedure

1. Enter system view.

system-view

2. Configure the source IP address of echo packets.

IPv4:

bfd echo-source-ip ip-address

IPv6:

bfd echo-source-ipv6 ipv6-address

The source IPv6 address of echo packets can only be a global unicast address. For information about configuring IPv6 addresses, see IPv6 basics in Layer 3—IP Services Configuration Guide.

By default, no source IP address is configured for echo packets.

3. (Optional.) Set the echo mode parameters.

a. Enter interface view.

interface interface-type interface-number

b. Set the minimum interval for receiving BFD echo packets.

bfd min-echo-receive-interval interval

The default setting is 400 milliseconds.

c. Set the detection time multiplier.

bfd detect-multiplier value

The default setting is 5.

Configuring control-mode BFD sessions

About BFD session creation methods

BFD sessions are distinguished by the local discriminator and remote discriminator in control packets. The local discriminator of a BFD session is assigned by the local device, and the remote discriminator is obtained during BFD session negotiation.

Restrictions and guidelines

BFD version 0 does not support the following commands:

· bfd authentication-mode

· bfd demand enable

· bfd echo enable

· bfd session init-mode

Configuring BFD session parameters for single-hop detection

1. Enter system view.

system-view

2. Specify the mode for establishing a BFD session.

bfd session init-mode { active | passive }

By default, active is specified.

3. Enter interface view.

interface interface-type interface-number

4. (Optional.) Configure the authentication mode for single-hop control packets.

bfd authentication-mode { m-md5 | m-sha1 | md5 | sha1 | simple } key-id { cipher cipher-string | plain plain-string }

By default, single-hop BFD packets are not authenticated.

5. Enable the Demand BFD session mode.

bfd demand enable

By default, the BFD session is in Asynchronous mode.

6. Set the minimum interval for transmitting single-hop BFD control packets.

bfd min-transmit-interval interval

The default setting is 400 milliseconds.

7. Set the minimum interval for receiving single-hop BFD control packets.

bfd min-receive-interval interval

The default setting is 400 milliseconds.

8. Set the single-hop detection time multiplier.

bfd detect-multiplier value

The default setting is 5.

Configuring BFD session parameters for multihop detection

1. Enter system view.

system-view

2. Specify the mode for establishing a BFD session.

bfd session init-mode { active | passive }

By default, active is specified.

3. (Optional.) Configure the authentication mode for multihop BFD control packets.

bfd multi-hop authentication-mode { m-md5 | m-sha1 | md5 | sha1 | simple } key-id { cipher cipher-string | plain plain-string }

By default, no authentication is performed.

4. Configure the destination port number for multihop BFD control packets.

bfd multi-hop destination-port port-number

The default setting is 4784.

5. Set the multihop detection time multiplier.

bfd multi-hop detect-multiplier value

The default setting is 5.

6. Set the minimum interval for receiving multihop BFD control packets.

bfd multi-hop min-receive-interval interval

The default setting is 400 milliseconds.

7. Set the minimum interval for transmitting multihop BFD control packets.

bfd multi-hop min-transmit-interval interval

The default setting is 400 milliseconds.

Enabling the echo function

About this task

This function enables the local system to periodically send echo packets to the remote system. The remote system loops back the echo packets to the local system without processing them. If the local system does not received the looped-back echo packets, it declares the BFD session down.

This function is supported only for single-hop detection.

Restrictions and guidelines

This function does not take effect on BFD sessions associated with interface states.

Procedure

1. Enter system view.

system-view

2. Enter interface view.

interface interface-type interface-number

3. Enable the echo function.

bfd echo [ receive | send ] enable

By default, the echo function is disabled.

Associating the interface state with BFD

About this task

By creating a BFD session for single-hop detection through exchange of BFD control packets, this feature implements fast link detection. When BFD detects a link fault, it sets the link layer protocol state to DOWN(BFD). This behavior helps applications relying on the link layer protocol state achieve fast convergence. The source IP address of control packets is specified manually, and the destination IP address is fixed at 224.0.0.184. As a best practice, specify the IP address of the interface as the source IP address. If the interface does not have an IP address, specify a unicast IP address other than 0.0.0.0 as the source IP address.

You can associate the state of the following interfaces with BFD:

· Layer 2 Ethernet interfaces.

· Member ports in a Layer 2 aggregation group.

· Layer 3 Ethernet interfaces.

· Member ports in a Layer 3 aggregation group.

· Layer 3 Ethernet subinterfaces.

· VLAN interfaces.

Restrictions and guidelines

The echo function does not take effect on BFD sessions associated with interface states.

Procedure

1. Enter system view.

system-view

2. Enter interface view or.

interface interface-type interface-number

3. Associate the interface state with BFD.

bfd detect-interface source-ip ip-address [ discriminator local local-value remote remote-value ]

If the peer device does not support obtaining BFD session discriminators through autonegotiation, you must specify the discriminators on both the local and peer devices. Without the discriminators, the BFD session cannot come up.

Configuring BFD session flapping suppression

About this task

When BFD detects a link failure, it tears down the BFD session and notifies the upper-layer protocol of the failure. When the upper-layer protocol re-establishes a neighbor relationship, the BFD session comes up again. BFD session flaps occur when a link fails and recovers repeatedly, which consumes significant system resources and causes network instability.

This feature allows you to suppress BFD session flapping by using the initial-interval, secondary-interval, and maximum-interval arguments.

· A BFD session is suppressed within the specified interval. The suppression time does not exceed the maximum-interval.

· After a BFD session goes down for the second time, it cannot be re-established within the initial-interval.

· After a BFD session goes down for the third time, it cannot be re-established within the secondary-interval.

· After a BFD session goes down for the fourth time and at any later time, the following rules apply:

¡ If secondary-interval × 2n-3 is smaller than or equal to the maximum-interval, the BFD session cannot be re-established within the secondary-interval × 2n-3.

¡ If secondary-interval × 2n-3 is greater than the maximum-interval, the BFD session cannot be re-established within the maximum-interval.

The letter n, starting from 4, is the number of times the BFD session flaps.

Procedure

1. Enter system view.

system-view

2. Configure BFD session flapping suppression.

bfd dampening [ maximum maximum-interval initial initial-interval secondary secondary-interval ]

By default, BFD sessions are not suppressed.

The value for the initial-interval or secondary-interval argument cannot be greater than the value for the maximum-interval argument.

Configuring a BFD template

About this task

Perform this task to specify BFD parameters in a template for sessions without next hops. You can configure BFD parameters for LSPs and PWs through a BFD template.

Procedure

1. Enter system view.

system-view

2. Create a BFD template and enter BFD template view.

bfd template template-name

3. Configure the authentication mode for BFD control packets.

bfd authentication-mode { m-md5 | m-sha1 | md5 | sha1 | simple } key-id { cipher cipher-string | plain plain-string }

By default, no authentication is performed.

BFD version 0 does not support this command.

4. Set the detection time multiplier.

bfd detect-multiplier value

The default setting is 5.

5. Set the minimum interval for receiving BFD echo packets.

bfd min-echo-receive-interval interval

The default setting is 400 milliseconds.

6. Set the minimum interval for receiving BFD control packets.

bfd min-receive-interval interval

The default setting is 400 milliseconds.

7. Set the minimum interval for transmitting BFD control packets.

bfd min-transmit-interval interval

The default setting is 400 milliseconds.

Enabling SNMP notifications for BFD

About this task

To report critical BFD events to an NMS, enable SNMP notifications for BFD. For BFD event notifications to be sent correctly, you must also configure SNMP as described in Network Management and Monitoring Configuration Guide.

Procedure

1. Enter system view.

system-view

2. Enable SNMP notifications for BFD.

snmp-agent trap enable bfd

By default, SNMP notifications are enabled for BFD.

Verifying and maintaining BFD

To display BFD session information, execute the following command in any view:

display bfd session [ discriminator value | verbose ]

To clear BFD session statistics, execute the following command in user view:

reset bfd session statistics

Configuring SBFD

About SBFD

Seamless BFD (SBFD) is a unidirectional failure detection mechanism that provides shorter detection time than BFD. SBFD is used in scenarios where only one end of a link requires failure detection, such as MPLS TE tunneling and LSP establishment through LDP. For information about SRLSP, see SR-MPLS configuration in Segment Routing Configuration Guide. For information about LSP establishment through LDP, see LDP configuration in MPLS Configuration Guide.

An SBFD session involves the following roles:

· Initiator—Periodically sends SBFD control packets to detect connectivity to remote entities. The initiator injects SBFD control packets into an MPLS TE tunnel or LDP LSP. The initiator initiates SBFD sessions and maintains SBFD session state.

· Reflector—Listens for incoming SBFD control packets on local entities and replies with response SBFD control packets.

As shown in Figure 1, in an SR-based MPLS TE tunnel, the SRLSP from Router A to Router E is considered available if Router A (initiator) can receive response SBFD control packets from Router E (reflector).

Figure 1 Initiator and reflector in SBFD

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Restrictions and guidelines: SBFD configuration

A node can act as the initiator of one session and the reflector of another session at the same time.

An SBFD session can only be established statically. You must manually specify the remote discriminator for an SBFD session.

SBFD tasks at a glance

To configure SBFD, perform the following tasks:

· Configuring the initiator

¡ Configuring the initiator for LDP LSP detection

¡ Configuring the initiator for MPLS TE tunnel detection

· Configuring the reflector

· Configuring a BFD template

Configuring the initiator

Restrictions and guidelines

The remote discriminator in SBFD control packets sent by the initiator must be specified in the sbfd local-discriminator command on the reflector. If the discriminator is not specified on the reflector, the reflector does not reply with response SBFD control packets.

Configuring the initiator for LDP LSP detection

1. Enter system view.

system-view

2. Enable BFD for MPLS.

mpls bfd enable

By default, BFD for MPLS is disabled.

For more information about this command, see MPLS OAM commands in MPLS Command Reference.

3. (Optional.) Set the detection time multiplier.

bfd multi-hop detect-multiplier value

The default setting is 5.

4. (Optional.) Set the minimum interval for sending SBFD control packets.

bfd multi-hop min-transmit-interval interval

The default setting is 400 milliseconds.

5. Configure SBFD to verify LSP connectivity for an FEC.

mpls sbfd dest-addr mask-length [ nexthop nexthop-address ] remote remote-id [ template template-name ]

By default, SBFD is not configured to verify LSP connectivity for an FEC.

For more information about this command, see MPLS OAM commands in MPLS Command Reference.

Configuring the initiator for MPLS TE tunnel detection

1. Enter system view.

system-view

2. Enable BFD for MPLS.

mpls bfd enable

By default, BFD for MPLS is disabled.

For more information about this command, see MPLS OAM commands in MPLS Command Reference.

3. (Optional.) Set the detection time multiplier.

bfd multi-hop detect-multiplier value

The default setting is 5.

4. (Optional.) Set the minimum interval for sending SBFD control packets.

bfd multi-hop min-transmit-interval interval

The default setting is 400 milliseconds.

5. Enter the view of an MPLS TE tunnel interface.

interface tunnel number [ mode mpls-te ]

6. Enable SBFD to verify MPLS TE tunnel connectivity.

mpls sbfd remote remote-id [ template template-name ]

By default, SBFD is not enabled for an MPLS TE tunnel.

For more information about this command, see MPLS OAM commands in MPLS Command Reference.

Configuring the reflector

Restrictions and guidelines

If you configure an IPv4 address as the local discriminator, the device automatically converts it to an integer. Configure an IPv4 address local discriminator only when it is required for interoperation with a third-party device.

The reflector replies with a response SBFD control packet only when the remote discriminator in the SBFD control packet sent from the initiator is specified in the sbfd local-discriminator command.

Procedure

1. Enter system view.

system-view

2. Configure a local discriminator.

sbfd local-discriminator { ipv4-address | integer-value }

By default, no local discriminator is configured.

You can execute this command multiple times to configure multiple local discriminators.

Configuring a BFD template

About this task

You can configure BFD parameters for LSPs and MPLS TE tunnels through a BFD template.

Procedure

1. Enter system view.

system-view

2. Create a BFD template and enter BFD template view.

bfd template template-name

3. Set the detection time multiplier.

bfd detect-multiplier value

The default setting is 5.

4. Set the minimum interval for sending SBFD control packets.

bfd min-transmit-interval interval

The default setting is 400 milliseconds.

Verifying and maintaining SBFD

To display SBFD session information, execute the following command in any view:

display sbfd session { initiator | reflector } [ discriminator value | verbose ]

15-High Availability Configuration Guide

Echo mode

Control mode

Restrictions and guidelines: BFD configuration

Configuring echo-mode BFD sessions

Restrictions and guidelines

Procedure

About this task

Restrictions and guidelines

Procedure

About this task

Restrictions and guidelines

Procedure

About this task

Procedure

About this task

Procedure

Enabling SNMP notifications for BFD

About this task

Procedure

Verifying and maintaining BFD

Restrictions and guidelines

Procedure

About this task

Procedure

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