Fault isolation (Linktrace messages)

Linktrace mechanism is used to isolate faults at Ethernet MAC layer. Linktrace can be used to isolate a fault associated with a given Virtual Bridge LAN Service. It should be noted that fault isolation in a connectionless (multi-point) environment is more challenging than a connection oriented (point-to-point) environment. In case of Ethernet, fault isolation can be even more challenging since a MAC address can age out when a fault isolates the MAC address. Consequently a network-isolating fault results in erasure of information needed for locating the fault.

A Linktrace Message uses a set of reserved multicast MAC address. The Linktrace Message gets initiated by a MEP and traverses hop-by-hop and each Maintenance Point (a MEP or MIP) along the path intercepts this Linktrace Message and forwards it onto the next hop after processing it until it reaches the destination MEP. The processing includes looking at the destination MAC address contained in the Linktrace Message.

Each MP along the path returns a unicast Linktrace Reply back to the originating MEP. The MEP sends a single LTM to the next hop along the trace path; however, it can receive many Linktrace Responses from different MPs along the trace path and the destination MEP as the result of the message traversing hop by hop. As mentioned previously, the age-out of MAC addresses can lead to erasure of information at MIPs, where this information is used for the Linktrace mechanism. Possible ways to address this behavior include:

Linktrace may also be used when no faults are apparent in order to discover the routes normally taken by data through the network. In the rare instances during network malfunctions where Linktrace cannot provide the information needed to isolate a fault, issuing Loopback Messages to MPs along the normal data path may provide additional useful information.

The Linktrace message is used by one MEP to trace the path to another MEP or MIP in the same domain. It is needed for Loopback (Ping). All intermediate MIPs respond back with a Link trace reply to the originating MEP. After decreasing the TTL by one, intermediate MIPs forward the Link trace message until the destination MIP or MEP is reached. If the destination is a MEP, every MIP along a given MA responds to the originating MEP. The originating MEP can then determine the MAC address of all MIPs along the MA and their precise location with respect to the originating MEP.