Figure 15-13 Lsp Trace Work Flow - Zte ZXR10 ZSR V2 Configuration Manual (System Management

ZXR10 ZSR V2 Configuration Guide (System Management)

Figure 15-13 LSP Trace Work Flow

The MPLS LSP trace procedure between LSR1 and LSR4 is described below:
1. LSR1: LSR1 sends an MPLS Echo Request packet to LSR2. The destination
address of the packet is the FEC on LSR4.
In the Echo Request packet, the TTL value in the MPLS header is 1, the
destination address in the IP header is 127.0.0.1, and both the source port
number and destination port number in the UDP header are 3503.
2. LSR2: When receiving the request packet whose TTL value is 1, LSR2 processes
the packet. It finds that itself is not the destination. Therefore, LSR2 responds to
LSR1 with an MPLS Echo Reply packet.
In the Echo Reply packet, LSR2 fills in a corresponding return code. If the return
code is 3, the node is the destination. If the return code is 6, the node is an
intermediate node. LSR1 determines whether the packet reaches the destination
according to the return code.
3. LSR1: After receiving the Echo Reply packet from LSR2, LSR1 knows the
address and label information on LSR2. According to the return code, LSR1
knows that the packet did not reach the destination. LSR1 sends an MPLS Echo
Request packet to LSR2 again. The destination of the packet is the FEC on
LSR4.
In the Echo Request packet, the TTL value in the MPLS header is 2, the
destination address in the IP header is 127.0.0.1, and both the source port
number and destination port number in the UDP header are 3503.
4. LSR2: After receiving the Echo Request packet whose TTL value is 2, LSR2
searches for label information and then forwards the packet to LSR3. The TTL
value decrements by one.
5. LSR3: After receiving the packet whose TTL value 1, LSR3 finds that itself is not
the destination either. Therefore, LSR3 responds to LSR1 with an MPLS Echo
Reply packet.
SJ-20140504150128-007|2014-05-10 (R1.0)
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