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Interexchange Carrier MPLS Network Design Study

Chapter Description

USCom is a fictitious nationwide data and long-distance voice service provider in the U.S. This chapter discusses the current USCom MPLS network design, its evolution, and how USCom characteristics and objectives influenced the corresponding design decisions that were made in order to illustrate how design decisions should stem from the characteristics of your company.

Design Lessons to Be Taken from USCom

A number of observations can be made from USCom's design decisions:

  • Straightforward engineering rules such as structured VRF naming conventions, route distinguisher/route target allocation schemes, and well-defined configuration templates allow for a simpler Layer 3 MPLS VPN service deployment.
  • Operation of the Internet service can be kept exactly as before deployment of the Layer 3 MPLS VPN service by separating forwarding of Internet and VPN traffic in the core. VPN traffic is carried over MPLS LSPs, while Internet traffic remains forwarded as IP traffic.
  • PE router protection techniques, such as limiting the number of routes within a VRF or restricting the number of prefixes received from a given client, should be a mandatory part of the Layer 3 MPLS VPN service deployment.
  • Simple tuning of certain router parameters, such as the input hold-queue and Selective Packet Discard (SPD), can considerably enhance convergence of the BGP control plane.
  • Route reflectors should be deployed to help scale the number of BGP TCP sessions required at the PE routers.
  • Enabling path MTU discovery at the PE routers and route reflectors allows the TCP protocol used by BGP to run more efficiently, thus providing better convergence times.
  • Where core bandwidth is plentiful/cheap/quick to provision, the core QoS design can rely on pure overengineering to maintain QoS during single failures and to achieve a good SLA that satisfies mission-critical and multimedia applications. This is a low operational expenses (opex) design because of simpler engineering, configuration, monitoring, troubleshooting, and fine-tuning. This is usually an attractive avenue for "facilities-owned" operators with an optical infrastructure.
  • Even when no QoS mechanism is supported in the core, and unmanaged CE routers are deployed, it is a good idea to offer an optional QoS mechanism on egress PE routers. Doing so provides added value for customers because it manages congestion on the last weak link in the chain (the first weak link, the CE-PE link, can be managed by the customer anyway) and does not add significant complexity to the design.
  • A network can follow a simple design to be able to offer a 50-ms convergence time upon link or SRLG failure by means of MPLS Traffic Engineering Fast Reroute, at a minimal cost in terms of opex and capital expenditure (capex). Such backup tunnels can be automatically configured and set up with minimal configuration.
  • Node failures may be covered by minimal IGP tuning to obtain a few seconds of rerouting time upon a router failure that affects data forwarding. USCom might consider more-aggressive IS-IS parameter settings if it has to increase its network availability in the future.