The potential benefits of MPLS VPNs are customer-specific, and the highly touted benefit of end-to-end quality-of-service may be just a myth if your service traverses more than one network.

Inter-service provider service-level agreement (SLA) guarantees may be a sticking point — just as they are with other technologies — because of a lack of willingness on the part of service providers to offer them.

“Low latency is interpreted differently by different providers,” says Bruce Davie, a Cisco Systems Inc. Fellow in the company’s IOS Technologies Division. “But the differences aren’t such that it can’t be done. One of the key reasons (enterprises are asking for MPLS) is the potential for inter-provider SLAs.”

The lack of inter-provider SLA assurance is one reason Coca-Cola Enterprises chose to manage its MPLS VPN service itself rather than opting for a fully managed service. The bottler owns, configures and manages it own routers, as well as the interconnection between AT&T Corp. and British Telecommunications PLC through two 10Gbps-capable Cisco 12000 series Internet routers in its data centre, says John Ridley, senior enterprise network architect.

“That’s where we marry the two networks,” Ridley says. “We do it; they don’t do it. There’s no carrier gateway involved.”

Nonetheless, the bottler did negotiate some “favourable” SLAs with the two carriers based on latency, Ridley says, so he does not foresee inter-service provider SLAs to be an issue.

“You’d have to be in Los Angeles and talking to a server in Brussels for that to be an issue,” he says. “We just aren’t set up that way.”