What are meanings of the TTL values of 255, 254, 253 in BFD session on the AR router

TTL values 255, 254, and 253 indicate that the lifetime of the BFD packets are 255, 254, and 253 respectively.
Starting from V200R001C01, the AR supports the global TTL for BFD sessions.
By default, the TTL of BFD packets of a static or dynamic single-hop session is 255; the TTL of BFD packets of a static multi-hop session is 254; the TTL of BFD packets of a dynamic multi-hop session is 253.

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