What is the function of the default multicast address of BFD on an AR

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When BFD is used to detect the physical status of a link, you may not specify the remote IP address. In some situations, the remote end such as the Eth-Trunk member link is not assigned an IP address. A BFD session needs to be bound to a multicast IP address and the AR needs to send BFD control packets to the multicast IP address.
In the following situations, you must change the default multicast IP address:
- On the network, other protocols use the multicast IP address.
- Multiple BFD sessions exist on the BFD detection path. For example, when Layer 3 interfaces are connected through Layer 2 switching devices that support BFD, you must configure different default multicast IP addresses for the devices where different BFD sessions are established.
You can run the default-ip-address ip-address command to configure the default multicast IP address for the BFD session.
[Huawei] bfd
[Huawei-bfd] default-ip-address 224.0.0.150 //Set the default multicast address bound to the BFD session to 224.0.0.150.

Other related questions:
What is the function of the default-ip-address command for BFD on S series switches
For S series switches (except S1700): When using BFD to detect the physical status of a link, you may not specify the peer IP address (In some situations, the peer such as an Eth-Trunk member interface is not assigned an IP address). In this case, you need to bind a BFD session to a multicast IP address so that BFD control packets can be sent to the peer device. In either of the following situations, change the default multicast IP address: - On the live network, other protocols except BFD use the multicast IP address. - Multiple BFD sessions exist on a path. For example, because Layer 3 interfaces are connected through Layer 2 switching devices that support BFD, configure different default multicast IP addresses for the devices where different BFD sessions are established. You can run the default-ip-address ip-address command to configure the default multicast IP address for BFD. [HUAWEI] bfd [HUAWEI-bfd] default-ip-address 224.0.0.150

What are the precautions for configuring multicast BFD
The BFD packets sent in multicast mode are untagged. When a port configured with multicast BFD receives tagged packets, it checks the tag of the packets. If the tag is different from the port's default VLAN ID, the port discards the packets. To configure multicast BFD on an interface, run the port default vlan command to specify the default VLAN of the interface.

Is the BFD Echo function applicable to multi-hop BFD sessions
The BFD Echo function is only applicable to single-hop BFD sessions. For two directly connected devices, one device supports BFD, and the other device supports only forwarding at the network layer and does not support BFD. To rapidly detect forwarding failures between the two devices, configure the BFD Echo function on the BFD-supporting device. The BFD-supporting device sends an Echo Request packet to the remote device, and the remote device sends the Echo Request packet back along the same path to detect the connectivity of the forwarding link.

Is multicast enabled on the AR router by default
By default, the multicast function of the AR router is disabled.

What is the BFD default detecting interval?
1000 milliseconds.

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