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

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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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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.

What do TTL values 255, 254, and 253 mean in a BFD session
The TTL values 255, 254, and 253 indicate that the lifetime of a BFD packet is 255, 254, and 253 respectively. From V200R001C01, you can configure a global TTL for a BFD session. By default, the lifetime of a static single-hop or dynamic single-hop BFD packet is 255, the lifetime of a static multi-hop BFD packet is 254, and the lifetime of a dynamic multi-hop BFD packet is 253.

Can the TLL value in returned ping packets be modified on the AR router
The TTL value in response ping packets cannot be modified, and the TTL in sent ping packets can be changed by running the ping -h < ttl-value > command in any view.

Which interfaces support BFD sessions on the AR router
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: 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, Ethernet sub-interfaces (including Dot1q sub-interfaces and QinQ 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 and Layer 3 Eth-Trunks. Note: Multicast BFD sessions cannot be established on Layer 2 Ethernet interfaces, Layer 2 Eth-Trunks, or Layer 3 Eth-Trunks of AR120, AR150, AR160, AR200 and 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.

How to distinguish a single-hop BFD session and a multi-hop one on the AR router
For static BFD sessions, when the session is created for the first time, a single-hop BFD session is bound to a local interface, while a multi-hop session is not. For dynamic BFD sessions, query the routes to determine whether a BFD session is a single-hop session or a multi-hop session. BFP for Internal Border Gateway Protocol (IBGP) sessions are multi-hop sessions.

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