Why can association between BFD session status and interface status on S series switches be used only on multicast BFD sessions

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For S series switches, a multicast BFD session uses multicast packets and is not affected by the protocol status on an interface. When the BFD session goes Down and the protocol status on the interface changes to BFDSatusDown, multicast BFD packets can still be sent. After the link recovers, the multicast BFD session becomes Up after negotiation. As a result, the interface is unlocked and becomes Up. Unicast packets, however, cannot be sent or received after the protocol status on the interface goes Down, which causes deadlock.

Other related questions:
What are the functions and typical scenarios of the association between BFD session status and interface status
For S series switches, you can run the process-interface-status command in the BFD session view to enable the association between the BFD status and the interface status. When the BFD session status changes, run the display this interface command in the interface view. If the command output shows that Line protocol current state is UP (BFD Status Down), the routing management module withdraws the host routes and network segment routes on the interface. Application scenario: The association between the BFD status and the interface status are often used when a Layer 2 device or data transmission device exists between two Layer 3 devices, or when a single fiber failure occurs.

Precautions for the configuration of multicast BFD sessions on S series switches
For S series switches, the packets sent by multicast BFD are untagged. When a BFD interface receives a tagged BFD packet, the BFD module checks whether the VLAN ID of the BFD packet is the same as the default VLAN ID on the interface. If not, the BFD module discards the packet. To configure multicast BFD on an interface, run the port default vlan command to specify the default VLAN of the interface.

Which interfaces can be configured with BFD sessions
In V200R001C00, only Ethernet interfaces support BFD sessions. In V200R001C01 and later versions, the following interfaces support common BFD sessions: -Physical interfaces: Layer 3 Ethernet interfaces, serial interfaces (including synchronous SA interfaces, E1/T1 interfaces, CE1/CT1/PRI interfaces, CE3 interfaces, and CPOS interfaces), POS interfaces, ATM interfaces (including ADSL interfaces, VDSL interfaces, E1-IMA interfaces, and G.SHDSL interfaces). -Logical interfaces: Ethernet sub-interfaces (including Dot1q sub-interfaces and QinQ sub-interfaces), dialer interfaces, VLANIF interfaces, Layer 3 Eth-Trunk interfaces, IP-Trunk interfaces, MP-Group interfaces, MFR interfaces, IMA-group interfaces, serial sub-interfaces, channelized serial sub-interfaces, IMA-group sub-interfaces, POS sub-interfaces, ATM sub-interfaces, MFR sub-interfaces, and Eth-Trunk sub-interfaces (Dot1q sub-interfaces). NOTE: When a physical Ethernet interface consists of multiple sub-interfaces, a BFD session can be established on the physical Ethernet interface or each sub-interface. In V200R001C01 and later versions, the following interfaces support multicast BFD sessions: -Physical interfaces: Layer 2 and Layer 3 Ethernet interfaces -Logical interfaces: Layer 2 Eth-Trunk interfaces and Layer 3 Eth-Trunk interfaces NOTE: -Multicast BFD sessions cannot be established on Layer 2 Ethernet interfaces, Layer 2 Eth-Trunk interfaces, or Layer 3 Eth-Trunk interfaces of AR120&AR150&AR160&AR200 or AR1200's SRUs. -When a physical Ethernet interface consists of multiple sub-interfaces, a BFD session can be established on the physical Ethernet interface or each sub-interface.

What are the functions and typical scenarios of BFD and interface status association
Using the process-interface-status command in the BFD session view, you can enable interface status association. When the BFD session status changes, the Line protocol current state is Up (BFD Status Down) when you run the display this interface command on the interface. At the same time, the RM withdraws the host routes and network segment routes on the interface. Application scenario: BFD for IFNET is usually applied to the scenario where a Layer 2 device exists between two routers, or there are transmission devices, or a single fiber fault occurs.

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