A loopback port is called a loopback port and is a virtual interface. If the router is not turned off, the loopback port still exists even if all the physical ports are turned off. Typically, loopback ports are used to detect and manage routers. At the same time, we can also use it to simulate the host, because it is very stable under normal circumstances.
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Router identification plays an important role in OSPF, and different router identification indicates the identities of different routers in OSPF. Generally, if it is not specified manually, the loopback port will be used as the router-id by default, as I said before, because the loopback port is very stable and will not be affected by the link up/down;
If the loopback port has no address, it will use the largest IP address on the physical interface as the router id. If there is no physical address, the router will prompt to specify the router id manually.
Baidu Encyclopedia-Router Loopback Address