Basic CFD concepts

A maintenance domain (MD) defines the network or part of the network where CFD plays its role. An MD is identified by its MD name.

To accurately locate faults, CFD introduces eight levels (from 0 to 7) to MDs. The bigger the number, the higher the level and the larger the area covered. Domains can touch or nest (if the outer domain has a higher level than the nested one) but cannot intersect or overlap.

MD levels facilitate fault location and make fault location more accurate. As shown in Figure 2, MD_A in light blue nests MD_B in dark blue. If a connectivity fault is detected at the boundary of MD_A, any of the devices in MD_A, including Device A through Device E, might fail. If a connectivity fault is also detected at the boundary of MD_B, the failure points can be any of Device B through Device D. If the devices in MD_B can operate correctly, at least Device C is operational.

CFD exchanges messages and performs operations on a per-domain basis. By planning MDs correctly in a network, you can use CFD to rapidly locate failure points.

A maintenance association (MA) is a part of an MD. You can configure multiple MAs in an MD as needed. An MA is identified by the MD name + MA name.

An MA serves the specified VLAN or no VLAN. An MA that serves a VLAN is considered to be carrying VLAN attribute. An MA that serves no VLAN is considered to be carrying no VLAN attribute. An MP can receive packets sent by other MPs in the same MA. The level of an MA equals the level of the MD that the MA belongs to.

An MP is configured on a port and belongs to an MA. MPs include the following types: maintenance association end points (MEPs) and maintenance association intermediate points (MIPs).

• MEP

  MEPs define the boundary of the MA. Each MEP is identified by a MEP ID.

  The MA to which a MEP belongs defines the VLAN of packets sent by the MEP. The level of a MEP equals the level of the MD to which the MEP belongs. The level of packets sent by a MEP equals the level of the MEP.

  The level of a MEP determines the levels of packets that the MEP can process. A MEP forwards packets at a higher level and processes packet of its level or lower. The processing procedure is specific to packets in the same VLAN. Packets of different VLANs are independent.

  MEPs include inward-facing MEPs and outward-facing MEPs:

  • An outward-facing MEP sends packets to its host port.

  • An inward-facing MEP does not send packets to its host port. Rather, it sends packets to other ports on the device.

• MIP

  A MIP is internal to an MA. It cannot send CFD packets actively, but it can handle and respond to CFD packets. By cooperating with MEPs, a MIP can perform a function similar to ping and traceroute. A MIP forwards packets of a different level without any processing and only processes packet of its level.

  The MA to which a MIP belongs defines the VLAN of packets that the MIP can receive. The level of a MIP is defined by its generation rule and the MD to which the MIP belongs. MIPs are generated on each port automatically according to the following MIP generation rules:

  • Default rule—If no lower-level MIP exists on an interface, a MIP is created on the current level. A MIP can be created even if no MEP is configured on the interface.

  • Explicit rule—If no lower-level MIP exists and a lower-level MEP exists on an interface, a MIP is created on the current level. A MIP can be created only when a lower-level MEP is created on the interface.

  If a port has no MIP, the system will check the MAs in each MD (from low to high levels), and follow the procedure as described in Figure 3 to create or not to create MIPs at the current level.

  Figure 3: Procedure of creating MIPs

  

Figure 4 demonstrates a grading example of the CFD module. Four levels of MDs (0, 2, 3, and 5) are designed. The bigger the number, the higher the level and the larger the area covered. MPs are configured on the ports of Device A through Device F. Port 1 of Device B is configured with the following MPs:

• A level 5 MIP.

• A level 3 inward-facing MEP.

• A level 2 inward-facing MEP.

• A level 0 outward-facing MEP.

A MEP list is a collection of local MEPs allowed to be configured and the remote MEPs to be monitored in the same MA. It lists all the MEPs configured on different devices in the same MA. The MEPs all have unique MEP IDs. When a MEP receives from a remote device a continuity check message (CCM) carrying a MEP ID not in the MEP list of the MA, it drops the message.

The local device must send CCM messages carrying the Remote Defect Indication (RDI) flag bits. Otherwise, the peer device cannot sense certain failures. When a local MEP has not learned all remote MEPs in the MEP list, the MEPs in the MA do not carry the RDI flag bits in CCMs.