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

MPLS LSP Traceroute Operation
Step
1
1
1
1
You can use MPLS LSP Traceroute to validate IPv4 Label Distribution Protocol (LDP) and IPv4 RSVP FECs
by using appropriate keywords and arguments with the trace mpls command:
trace mpls ipv4 {destination-address destination-mask | traffic-eng
tunnel-interface tunnel-number}
By default, the TTL is set to 30. Therefore, the traceroute output always contains 30 lines, even if an LSP
problem exists. This might mean duplicate entries in the output, should an LSP problem occur. The device
address of the last point that the trace reaches is repeated until the output is 30 lines. You can ignore the
duplicate entries. The following example shows that the trace encountered an LSP problem at the device that
has an IP address of 10.6.1.6:
Device# traceroute mpls ipv4 10.6.7.4/32
Tracing MPLS Label Switched Path to 10.6.7.4/32, timeout is 2 seconds
Codes: '!' - success, 'Q' - request not transmitted,

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

68
Device
LSR1
LSR2
LSR3
LSR4
'.' - timeout, 'U' - unreachable,
'R' - downstream router but not target
MPLS LSP Ping, Traceroute, and AToM VCCV
MPLS Packet Type and
Description
MPLS echo request—With the
same target FEC and the
downstream mapping received
in the echo reply from LSR3.
MPLS echo request.
MPLS echo request.
MPLS echo reply.
Device Action
• Sets the TTL of the packet
to 3.
• Sends the request to
LSR2.
Receives packet with TTL = 3.
• Decrements the TTL.
• Forwards the echo request
to LSR3.
Receives packet with TTL = 2
• Decrements the TTL.
• Forwards the echo request
to LSR4.
Receives packet with TTL = 1.
• Processes the UDP packet
as an MPLS echo request.
• Finds a downstream
mapping and also finds
that the device is the
egress device for the
target FEC.
• Replies to LSR1.