OAM and BFD
LSPs and PWs are monitored through the use of OAM packets. The OAM is used for fault detection, raising alarms as well as in detecting when traffic should be switched to the pre-provisioned protection LSP(s).
OAM packets are co-routed with data packets (in-band) to detect data plane faults. The channel used for OAM, protection switching protocols and other maintenance traffic is the Generic Associated Channel (G-Ach).
To distinguish the OAM packets from other data packets, a standardized MPLS-TP label, GAL (G-Ach label) with value 13 is used. The terminating device uses the GAL to determine that the packet is an OAM packet. The G-Ach header is used to determine what sort of OAM traffic it is.
The OAM is available at LSP and PW levels. Current FOXMAN‑UN implementation does not supports OAM on PW. The data packet below illustrates an OAM at LSP level in details.
MPLS-TP introduces concept of maintenance domains to be managed and monitored. The main components are explained below:
− Maintenance Entity Group End Points (MEPs):
MEPs are edges of a maintenance domain. MEPs typically reside at the end points of LSPs and PWs.
− Maintenance Intermediate Points (MIPs):
MIPs define intermediate nodes to be monitored.
− Maintenance Entity Groups (MEGs):
Comprise all the MEPs and MIPs on an LSP or PW.
The NE supports OAM at the LSP level, that means the LERs are the MEPs, while the LSRs are the MIPs on this LSP.
BFD Session Establishment and Operation
Bidirectional Forwarding Detection (BFD) is an OAM mechanism that provides bidirectional continuity check to detect faults between two MEP endpoints connected by a link.
The BFD establishes a session between these endpoints over a particular link by exchanging control packets. The local MEP sends Continuity Check (CC) packets at a defined “CC Interval” and the remote MEP monitors the arrival of the CC packets and detects loss of Continuity (LOC) if the packets fail to appear over a certain time period.
If the link or path is protected, separate BFD sessions ( BFD CC Supervision) are automatically established in the working and protection LSP to monitor both links.