Mpls Basic Configuration Guide, Cisco Ios Xe Everest 16.5.1 (Cisco Asr 900 Series - Cisco ASR 900 Series Manual

MPLS LSP Ping, Traceroute, and AToM VCCV
Table 5: MPLS LSP Traceroute Example
Step
1
1
1
1
1
Device
LSR1
LSR2
LSR1
LSR2
LSR3

MPLS Basic Configuration Guide, Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)

MPLS Packet Type and
Description
MPLS echo request—With a
target FEC pointing to LSR4
and to a downstream mapping.
MPLS echo reply.
MPLS echo request—With the
same target FEC and the
downstream mapping received
in the echo reply from LSR2.
MPLS echo request.
MPLS reply packet.
MPLS LSP Traceroute Operation
Device Action
• Sets the TTL of the label
stack to 1.
• Sends the request to
LSR2.
Receives packet with TTL = 1.
• Processes the UDP packet
as an MPLS echo request.
• Finds a downstream
mapping, replies to LSR1
with its own downstream
mapping based on the
incoming label, and sends
a reply.
• Sets the TTL of the label
stack to 2.
• Sends the request to
LSR2.
Receives packet with TTL = 2.
• Decrements the TTL.
• Forwards the echo request
to LSR3.
Receives packet with TTL = 1.
• Processes the UDP packet
as an MPLS echo request.
• Finds a downstream
mapping and replies to
LSR1 with its own
downstream mapping
based on the incoming
label.
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