• Ping and traceroute are unsupported with SR-TE static auto tunnel, BGP Dynamic TE, and on-demand next hop auto tunnels.

• Strict-SID option is not supported by the path installed by OSPF.

• MPLS traceroute does not support popping of two explicit null labels in one node.

• Rerouting the path to IP over MPLS segment without using Layer3 VPN is not supported due to IP routing destination not being a MPLS FEC.

In the case of segment routing, each segment nodal label and adjacent label along the routing path is put into the label stack of an echo request message from initiator Label Switch Router (LSR); MPLS data plane forward this packet to the label stack target, and the label stack target reply the echo message back.

• The feature enables the MPLS OAM functionality in the Segment Routing Network where the traffic is engineering via SR-TE tunnels or native SR forwarding.

• In traditional MPLS networks, source node chooses the path based on hop by hop signaling protocols such as LDP or RSVP-TE. In Segment Routing Networks, the path is specified by set of segments which are advertised by the IGP protocols (currently OSPF and ISIS).

• As the volume of services offered using SR increase, it is important that the operator essentially is able to do the connectivity verification and the fault isolation in the SR architecture.

• The segment assignment is not based on hop by hop protocols as in traditional MPLS network, any broken transit node could lead in traffic blackholing, which could lead to undesired behavior.

• Both SR and SR-TE supports load balancing, it is important to trace all the ECMP paths available between source and target routers. The features offers the multipath traceroute support for both TE and native SR paths.

• The following are the main benefits of Segment Routing-OAM Support:

  • Operations: Network monitoring and fault management.

  • Administration: Network discovery and planning.

  • Maintenance: Corrective and preventive activities, minimize occurrences and impact of failures.

MPLS ping and traceroute are extendable by design. You can add SR support by defining new FECs and/or additional verification procedures. MPLS ping verifies MPLS data path and performs the following:

• Encapsulates echo request packet in MPLS labels.

• Measures coarse round trip time.

• Measures coarse round trip delay.

MPLS ping and traceroute are extendable by design. You can add SR support by defining new forwarding equivalence classes (FECs) and/or additional verification procedures. MPLS traceroute verifies forwarding and control plane at each hop of the LSP to isolate faults. Traceroute sends MPLS echo requests with monotonically increasing time-to-live (TTL), starting with TTL of 1. Upon TTL expiry, transit node processes the request in software and verifies if it has an LSP to the target FEC and intended transit node. The transit node sends echo reply containing return code specifying the result of above verification and label stack to reach the next-hop, as well as ID of the next-hop towards destination, if verification is successful. Originator processes echo reply to build the next echo request containing TTL+1. Process is repeated until the destination replies that it is the egress for the FEC.

The MPLS OAM mechanisms help with fault detection and isolation for a MPLS data-plane path by the use of various target FEC stack sub-TLVs that are carried in MPLS echo request packets and used by the responder for FEC validation. While it is obvious that new sub-TLVs need to be assigned for segment routing, the unique nature of the segment routing architecture raises the need for additional operational considerations for path validation.

The forwarding semantic of Adjacency Segment ID is to pop the Segment ID and send the packet to a specific neighbor over a specific link. A malfunctioning node may forward packets using Adjacency Segment ID to an incorrect neighbor or over an incorrect link. The exposed Segment ID (of an incorrectly forwarded Adjacency Segment ID) might still allow such packet to reach the intended destination, although the intended strict traversal has been broken. MPLS traceroute may help with detecting such a deviation.

The format of the following Segment ID sub-TLVs follows the philosophy of Target FEC Stack TLV carrying FECs corresponding to each label in the label stack. This allows LSP ping/traceroute operations to function when Target FEC Stack TLV contains more FECs than received label stack at responder nodes. Three new sub-TLVs are defined for Target FEC Stack TLVs (Type 1), Reverse-Path Target FEC Stack TLV (Type 16) and Reply Path TLV (Type 21).

 sub-Type    Value Field
 --------  ---------------
    34      IPv4 IGP-Prefix Segment ID
    35      IPv6 IGP-Prefix Segment ID
    36      IGP-Adjacency Segment ID

ping mpls ipv4 <prefix/prefix_length> [fec-type [ldp | bgp | generic | isis | ospf]] [sr-path-type [ip | sid | strict-sid]] 
[destination <addr_start> [<addr_end> [<addr_incr_mask> | <addr_incr>]]]
[source <addr>]
[exp <exp-value>]
[pad <pattern>]
[ttl <ttl>]
[reply [mode [ipv4 | router-alert | no-reply]]
[dscp <dscp-bits>]
[pad-tlv]]
[verbose]
[output {interface <tx-interface>} [nexthop <nexthop ip addr>]]
[{dsmap | ddmap [l2ecmp]} [hashkey {none | {ipv4 | ipv4-label-set {bitmap <bitmap_size>}}]

traceroute mpls [multipath] ipv4 <prefix/prefix_length> [fec-type [ldp | bgp | generic | isis | ospf]] [sr-path-type [ip | sid | strict-sid]] 
[timeout <seconds>]
[destination <addr_start> [<addr_end> [<addr_incr_mask> | <addr_incr>]]]
[source <addr> ]
[exp <exp-value>]
[ttl <ttl-max>]
[reply [mode [ipv4 | router-alert | no-reply]]
[pad-tlv]]
[output {interface <tx-interface>} [nexthop <nexthop ip addr>]]
[flags {fec | ttl}]
[hashkey ipv4 | ipv4-label-set {bitmap <bitmap_size>}]

• fec-type: IPv4 Target FEC type, use head end auto detected FEC type by default.

• sr-path-type: Segment routing path type selection algorithm. Use IP imposition path, when option is specified.

This section provides a summary on the main Command Line Interfaces (CLIs) that are needed to verify segment routing OAM feature(s). Ping and traceroute commands illustrate the operation and output over IGP (OSPF SR), ISIS SR, and SR-TE. Change the actual tunnel numbers and IP addresses based on the actual values needed and enabled in the configurations.