What do TTL values 255, 254, and 253 mean in a BFD session

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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.

Other related questions:
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.

How do I distinguish a single-hop BFD session and a multi-hop BFD session
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. Dynamic BFD sessions: Query the routes to determine whether a BFD session is a single-hop session or a multi-hop session. BFD for Internal Border Gateway Protocol (IBGP) sessions are multi-hop sessions.

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.

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.

Meaning of content in a session table
A session table example is displayed as follows: ID: a48f3648905d02c0553591da1 //Indicates the session ID. zone: dmz -> trust //The first packet of the session is from the DMZ (source zone) to the Trust zone (destination zone). ttl: 00:20:00 left: 00:19:43 //ttl indicates the aging time of the session table, and left indicates the remaining time for aging. Interface: E1 Nexthop: 10.0.0.145 Mac: 00-00-5e-00-01-0f //Indicates the outbound interface, next-hop IP address, and MAC address of the first packet of the session. <-- packets:686 bytes:50264 --> packets:500 bytes:40828 //<-- indicates the number of bytes and packets in the inbound direction of the session. --> indicates the number of bytes and packets in the outbound direction or within the zone. 121.14.74.21:14000<--10.252.204.111:16503 //<-- indicates that the first packet of the session is in inbound direction. --> indicates that the first packet of the session is in outbound direction or the same zone. In session packet statistics, the arrow points the packet direction. 1. The following 10 packets are collected in the direction from 172.16.10.1 to 172.16.0.96. udp VPN: public -> public Zone: trust -> untrust TTL: 00:02:00 Left: 00:01:59 Interface: G2/0/1 Nexthop: 172.16.0.96 MAC: 00-00-00-00-00-00 <-- packets:0 bytes:0 --> packets:10 bytes:5636 172.16.10.1:1517-->172.16.0.96:1231 2. The following five packets are collected in the direction from 172.16.1.26 to 172.16.10.22. udp VPN: public -> public Zone: untrust -> trust TTL: 00:02:00 Left: 00:02:00 Interface: G2/0/0 Nexthop: 172.16.10.22 MAC: 00-00-00-00-00-00 <-- packets:5 bytes:7930 --> packets:0 bytes:0 172.16.10.22:1517<--172.16.1.26:48988

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