1. Ethernet P2P or VPLS is implemented on an additional layer over the LSP; and that is referenced as a psuedowire or Martini tunnel.This additional layer has its own headers and ID. In addition, the psuedowire has its own signaling procedure that should take place over the LSP. The mapping between a pseudowire and a LSP is not necessarily one-to-one: multiple psuedowires can ride the same LSP. The LSP might be up and passing traffic for some psuedowires but not the others. Many reasons can lead to this, for example an MTU mismatch on the psuedowire end-points on the ports facing the customer. In such case the LSP ping will succeed while the psuedowire is down and hence the customer service frames cannot get through. For this reason, some vendors (such as Cisco) came up with an additional “ping” that operates on the psuedowire level.. In this ping a user can specify the individual psudowire ID on which the ping traffic rides. This is a good solution for the problem however it has its own limitations [described in 2 below]
2. Since the pseudowire (and its underlying LSP) are implemented over MPLS, then the scope/reach of both pseuedowire and LSP ping tools in troubleshooting an Ethernet service is limited to the provider edge devices (where MPLS terminates). This means it does not include the access tail circuits (the local loops) all the way to the customer CPE. These access tails are more likely to fail than a provider’s MPLS backbone.
3. Hence, Ethernet Service OAM (SOAM) can provide better troubleshooting techniques. that is achieved by the loopback request/reply LBM/LBR (AKA Ethernet or CFM ping) and LTM/LTR (CFM link trace request/reply). Ethernet loopback and link trace are very useful in troubleshooting Ethernet services since they can cover the end-to-end service and they are guaranteed to ride the exact tunnel carrying the customer service frames. a LBM or LTM that is sent on an UP MEP covers the provider backbone (whether MPLS or any other encapsulation); likewise a LBM or LTM that is sent on a DOWN MEP cover the access tail circuit all the way to the end customer CPE.
4. Another advantage for SOAM is that Y.1731 can provide service and SLA assurance through its Delay Measurement Message/Reply (DMM/DMR). frame delay, and frame delay variation measurements are provided. a DMM/DMR pair can verify not only the continuty of the service but also the compliance to the SLA.
5. One carrier that has excellent troubleshooting tools that it puts in its customers’ hands at no charge is Inteliquent. Their EtherVision web tools implement all the mentioned techniques: MPLS PW ping, SOAM LBM/LBR, SOAM LTM/LTR, and DMM/DMR. All these tools and techniques are available on their EtherVision web portal so that customers can continuously monitor the continuity and performance of their services.
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