About MPLS Segment Routing OAM
MPLS segment routing (SR) has been deployed on the Cisco Nexus 9000 Series switches. As MPLS segment routing (SR) is deployed, a few diagnostic tools are required to help resolve the misconfigurations or failures in the segment routing network. Segment Routing Operations, Administration, and Maintenance (OAM) helps service providers monitor label-switched paths (LSPs) and quickly isolate forwarding problems to assist with fault detection and troubleshooting in the network.
MPLS SR OAM provides two main functions for diagnostics purposes:
The segment routing OAM feature provides support for the following FEC types:
Ping and traceroute to SR-IGP IS-IS IPv4 prefixes. This allows validation of prefix SIDs distributed in an IS-IS SR underlay.
Ping and traceroute to BGP IPv4 prefixes. This allows validation of prefix SIDs distributed in a BGP SR underlay.
Ping and traceroute to Generic IPv4 prefixes. This allows validation of prefix SIDs distributed in an SR underlay agnostic to the protocol that performed the distribution. The validation is performed by checking the Unicast Routing Information Base (URIB) and Unicast Label Information Base (ULIB).
Ping and traceroute to Nil FEC prefixes. This allows a less comprehensive data-plane-only validation for any MPLS SR prefix, with finer-grained control over the path the ping or traceroute takes. The path may be specified using an SR-TE policy name or SR-TE policy color and endpoint.
To enable MPLS OAM on Cisco Nexus 9000 Series switches, use the feature mpls oam CLI command. Use the no feature mpls oam CLI command to disable MPLS OAM on Cisco Nexus 9000 Series switches.
Segment Routing Ping
Similar to how an IP ping validates connectivity to an IP host, MPLS ping is used to validate unidirectional continuity along an MPLS Label-Switched Path (LSP). By providing a FEC representing the LSP to be validated, MPLS ping performs the following:
Confirms that the echo requests for the FEC reach an endpoint for the LSP. Except for the Nil FEC, for all other FEC types it confirms that the endpoint is the correct egress for that FEC.
Measures coarse round trip time.
Measures coarse round trip delay.
The MPLS LSP ping feature is used to check the connectivity between ingress Label Switch Routers (LSRs) and egress LSRs along an LSP. MPLS LSP ping uses MPLS echo request and reply messages, similar to Internet Control Message Protocol (ICMP) echo request and reply messages, to validate an LSP. The destination IP address of the MPLS echo request packet is different from the address used to select the label stack. The destination IP address is defined as a 127.x.y.z/8 address and it prevents the IP packet from being IP switched to its destination, if the LSP is broken.
Segment Routing Traceroute
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. This process is repeated until the destination replies that it is the egress for the FEC.
The MPLS LSP traceroute feature is used to isolate the failure point of an LSP. It is used for hop-by-hop fault localization and path tracing. The MPLS LSP Traceroute feature relies on the expiration of the Time to Live (TTL) value of the packet that carries the echo request. When the MPLS echo request message hits a transit node, it checks the TTL value and if it is expired, the packet is passed to the control plane, else the message is forwarded. If the echo message is passed to the control plane, a reply message is generated based on the contents of the request message