Connectivity fault management (CFM) loopback support for loopback messages (LBMs) on Ethernet ports allows operators to run standards-based Layer 1 and Layer 2 OAM tests on ports receiving unlabeled packets.
The 7705 SAR supports CFM MEPs associated with different endpoints (that is, Up and Down SAP MEPs, Up and Down spoke SDP MEPs, Up and Down mesh SDP MEPs, and network interface facility Down MEPs). In addition, for traffic received from an uplink (network ingress), the 7705 SAR supports CFM LBM for both labeled and unlabeled packets. CFM loopbacks are applied to the Ethernet port.
Refer to the 7705 SAR OAM and Diagnostics Guide, ‟Ethernet OAM Capabilities”, for information on CFM MEPs.
Figure: CFM Loopback on Ethernet Ports shows an application where an operator leases facilities from a transport network provider in order to transport traffic from a cell site to their MTSO. The operator leases a certain amount of bandwidth between the two endpoints (the cell site and the MTSO) from the transport provider, who offers Ethernet Virtual Private Line (EVPL) or Ethernet Private Line (EPL) PTP service. Before the operator offers services on the leased bandwidth, the operator runs OAM tests to verify the SLA. Typically, the transport provider (MEN provider) requires that the OAM tests be run in the direction of (toward) the first Ethernet port that is connected to the transport network. This is done in order to eliminate the potential effect of queuing, delay, and jitter that may be introduced by an SDP or SAP.
Figure: CFM Loopback on Ethernet Ports shows an Ethernet verifier at the MTSO that is directly connected to the transport network (in front of the 7750 SR). Thus, the Ethernet OAM frames are not label-encapsulated. Given that Ethernet verifiers do not support label operations and the transport provider mandates that OAM tests be run between the two hand-off Ethernet ports, the verifier cannot be relocated behind the 7750 SR node at the MTSO. Therefore, CFM loopback frames received are not MPLS-encapsulated, but are simple Ethernet frames where the type is set to CFM (dot1ag or Y.1731).