Cisco ASR 900 Series User Manual

an IP packet. This explains why the packet keeps getting processed by P2. MPLS echo request packets cannot
be forwarded by use of the destination address in the IP header because the address is set to a 127/8 address.
Device# trace mpls ipv4 10.131.159.252/32 ttl 5
Tracing MPLS Label Switched Path to 10.131.159.252/32, timeout is 2 seconds
Codes: '!' - success, 'Q' - request not transmitted,
'.' - timeout, 'U' - unreachable,
'R' - downstream router but not target
Type escape sequence to abort.
0 10.131.191.230 MRU 1500 [Labels: 22 Exp: 0]
R 1 10.131.159.226 MRU 1504 [implicit-null] 40 ms
R 2 10.131.159.225 40 ms
R 3 10.131.159.225 40 ms
R 4 10.131.159.225 40 ms
R 5 10.131.159.225 40 ms
MPLS LSP Traceroute Tracks Untagged Cases
This troubleshooting section contains examples of how to use MPLS LSP Traceroute to determine potential
issues with packets that are tagged as implicit null and packets that are untagged.
Untagged output interfaces at a penultimate hop do not impact the forwarding of IP packets through a label
switched path (LSP) because the forwarding decision is made at the penultimate hop through use of the
incoming label. The untagged case causes Any Transport over Multiprotocol Label Switching (AToM) and
MPLS virtual private network (VPN) traffic to be dropped at the penultimate hop.
Troubleshooting Implicit Null Cases
In the following example, Tunnel 1 is shut down, and only a label switched path (LSP) formed with Label
Distribution Protocol (LDP) labels is established. An implicit null is advertised between the P2 and PE2
devices. Entering an MPLS LSP Traceroute at the PE1 device results in the following display:
Device# trace mpls ipv4 10.131.159.252/32
Tracing MPLS Label Switched Path to 10.131.159.252/32, timeout is 2 seconds
Codes: '!' - success, 'Q' - request not transmitted,
'.' - timeout, 'U' - unreachable,
'R' - downstream router but not target
Type escape sequence to abort.
0 10.131.191.230 MRU 1500 [Labels: 20 Exp: 0]
R 1 10.131.159.226 MRU 1500 [Labels: 19 Exp: 0] 80 ms
R 2 10.131.159.229 MRU 1504 [implicit-null] 28 ms
! 3 10.131.159.230 40 ms
This output shows that packets are forwarded from P2 to PE2 with an implicit-null label. Address
10.131.159.229 is configured for the P2 Fast Ethernet 0/0/0 out interface for the PE2 device.
Troubleshooting Untagged Cases
Untagged cases are valid configurations for Interior Gateway Protocol (IGP) label switched paths (LSPs) that
could cause problems for Multiprotocol Label Switching (MPLS) virtual private networks (VPNs).
A show mpls forwarding-table command and a show mpls ldp discovery command issued at the P2 device
show that the Label Distribution Protocol (LDP) is properly set up:
Device# show mpls forwarding-table 10.131.159.252
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
19 Pop tag 10.131.159.252/32 0 Et0/0 10.131.159.230
Device# show mpls ldp discovery
MPLS Basic Configuration Guide, Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
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MPLS LSP Ping, Traceroute, and AToM VCCV
ICMP ping and trace Commands and Troubleshooting

Cisco ASR 900 Series Specifications

General

Brand	Cisco
Model	ASR 900 Series
Category	Network Router
Language	English