Despite the high cost and lengthy deployment times, MPLS has continued to be the enterprise-standard for networking largely because of its guaranteed performance levels as evidenced by MPLS service level agreements (SLAs). When evaluating an MPLS carrier, it’s important to know what services or guarantees are available.
Availability, packet loss, latency, and jitter are common attributes included in an SLA agreement. These categories and other performance attributes generally can be negotiated upon and tweaked according to the size of the contract, the network locations, and other factors.
Availability of the network is a top concern for network managers. It’s important to remember that every MPLS link is composed of the carrier’s MPLS network and the last mile on each end of the connection. The last mile is provided and managed by the local carrier in that particular location. The most common cause of MPLS network outages occurs on the local carrier’s last mile circuit. Therefore it is important to ensure the MPLS provider has an SLA with the local carrier, which is a separate SLA from the one being negotiated between the MPLS customer and MPLS provider.
An SLA of four hours to restore connectivity is common. The provider should clarify when the clock to restore connectivity starts. Some providers begin the timer from when the second ping fails, while others don’t start the timer until the customer has opened a trouble ticket. If a provider experiences frequent brief outages, an availability SLA of four hours doesn’t provide any recourse. That’s why it’s crucial to consider the additional SLA components separately.
Brief outages and “dirty lines” among other issues, will result in lost packets. The type of MPLS service being delivered makes a difference with this SLA. For instance, an organization with a private MPLS network should expect very little to no packet loss. Again, the SLA for packet loss only applies to the MPLS backbone network and not the last mile circuits. The percent of packet loss for an SLA is based on whether the circuit is intra-region or inter-region and will vary depending on the location of the two end-points.For example, the SLA for packet loss within North America may only be .1% whereas Latin America to North America could be .7%.
Latency refers to the time it takes for a packet to traverse a circuit round-trip. In general, a carrier will want the latency SLA threshold to be 15-20% higher than the actual performance.. As latency varies from region to region, the provider may offer a quote on this particular SLA that differs from other locations for the same organization. It’s good to note that the latency being quoted is typically an average of backbone latency for that region not the specific network paths under consideration. Because it is a region-based SLA, if you experience an outage but the region overall is fine, you will not receive a credit under this SLA (credit is a rebate against the monthly recurring cost for the month the service did not fulfill the guaranteed SL). For large contracts, an organization may be able to work with the carrier on a custom latency SLA based on actual tests of that portion of the network.
If real-time traffic such as voice and video will be utilizing the MPLS network, the jitter SLA should be carefully considered. Jitter is variations in delay caused by network congestion, route changes, and timing drift (if clocking is used). The number quoted will most likely be based on a monthly average and is a good measuring stick as to whether the circuit can support voice and video jitter thresholds. Providers may offer this SLA end-to-end for certain regions at a higher cost and may require redundant configuration, but inter-region connections are more likely to cover the backbone only.
An installation SLA provides a guarantee for the installation schedule. If the carrier does not meet this SLA, they offer a credit of the monthly recurring charge or the non-recurring charge for installation. Many providers will waive the non-recurring fee so that part of the SLA offers no significant benefit.
Additional Performance Attributes
Other less common performance attributes can be negotiated with the carrier. Some carriers will offer a proactive notification of outages, which means the provider will will inform the customer via email or phone call of any outage when it occurs. If a secondary network is configured, a fast-failover SLA can be included that measures how quickly traffic will cut-over from the primary circuit to the secondary circuit during a failure.