Multicast Bidirectional Forwarding Detection (BFD) can check the continuity of the link between interfaces that do not have Layer 3 attributes (such as IP addresses) to quickly detect link faults.
After multicast BFD is configured, multicast BFD packets are sent using the IP layer. If the link is reachable, the remote interface receives the multicast BFD packets and forwards them to the BFD module. In this manner, the BFD module detects that the link is normal. If multicast BFD packets are sent over a trunk member link, they are delivered to the data link layer for link continuity check. The remote IP address used in a multicast BFD session is the default known multicast IP address (18.104.22.168 to 22.214.171.124). Any packet with the default known multicast IP address is sent to the BFD module for IP forwarding.
As shown in Figure 1, multicast BFD is configured on both Device A and Device B. BFD sessions are bound to the outbound interface If1, and the default multicast address is used. After the configuration is complete, multicast BFD quickly checks the continuity of the link between interfaces.