The previous tip provided an overview of Layer 2 VPN's, the progression of standards in the Layer 2 MPLS VPN space and the differences in a Layer 3 versus Layer 2 VPNs. This tip will focus on some issues that arise when migrating to an MPLS VPN.
Many companies are in the process of deploying MPLS VPNs or are evaluating MPLS VPNs as a viable solution. Migration to an MPLS environment requires close coordination between the service provider and the company during the migration and extensive planning before the migration.
Most service provider's MPLS PE routers are not the same routers that terminate a customer's current access circuits. In most cases, ATM or Frame circuits are used to provide virtual circuit connectivity between remote sites across the service providers ATM and Frame backbones. These virtual circuits interconnect across a WAN cloud to form a traditional VPN. Once MPLS services are purchased, migration from the traditional VPN to the network based VPN is required. The site may still use Frame or ATM access circuits (and many other access types), but the circuit terminates on the provider's IP backbone and the virtual connectivity is established via MPLS .
Customer migration from the ATM and Frame backbones requires that the customer interface to both backbones during the migration. Most customers CPE interconnect to the WAN backbone via a single local access circuit. Two circuits (or potentially two virtual circuits) are required to interface with both the MPLS and legacy backbones. One circuit so that the site still has access to the rest of the network and another to the MPLS network. The customer could deploy a secondary router at each site and interconnect those to the MPLS VPN while maintaining the original connection to the traditional VPN. This is an expensive solution as it requires dual circuits and dual routers for the length of the migration. If the customer is executing a technology refresh at the same time as the MPLS migration this option will work and is the right direction to go in.
Proper planning is required in order to ensure a seamless transition to the MPLS VPN. The first factor to consider is whether or not you are going to build a parallel network or utilize dual connections on the same CPE. Once that decision is made you can begin the detailed plans surrounding routing architecture, circuit provisioning and decommissioning, deployment schedule and all of the many other tasks associated with these types of cutovers. These elements are internal to you and you may get assistance from the provider, but they primarily focus on the edge and the access circuits, not on how to mitigate downtime caused by inefficient routing, poor scheduling and poorly planned cutovers. There are many, many factors to consider and the next tip will discuss how to mitigate some of these factors.
Robbie Harrell (CCIE#3873) is the National Practice Lead for Advanced Infrastructure Solutions for SBC Communications. He has over 10 years of experience providing strategic, business, and technical consulting services to clients. Robbie resides in Atlanta, and is a graduate of Clemson University. His background includes positions as a Principal Architect at International Network Services, Lucent, Frontway and Callisma.
Four steps to take when migrating from MPLS to SD-WAN