Introduction; Vpls: The Basics; What Is H-Vpls - Juniper M10i Application Note

Introduction

Virtual private LAN service (VPLS) has become a very attractive technology over the past few years, as it allows service
providers to deploy carrier-class services over an Ethernet-based network in a reliable and flexible way. Starting
mainly with business services and now with broadband multiplay services, service providers have gotten deployment
experience with it, and have also found some of the challenges that this technology brings, especially in terms of
scalability. In many cases, the initial answer to those challenges has been hierarchical virtual private LAN service
(H-VPLS).
Sometimes technologies tend to take on a life of their own, becoming "the" answer to all problems in the network.
This is, to some extent, what is happening with H-VPLS.
There are cases where we could say H-VPLS is thought to be the breakthrough technology that will address all of the
network's problems. But in most cases, the reality is that it will either not address them or not address them in the
most effective way, and in the process, will often generate additional problems that should not be minimized.
The purpose of this document is to demystify the use of H-VPLS, explaining its value, identifying the problems it
really solves and those it does not, and discussing other approaches or technologies that could address the problems
that H-VPLS does not solve. For those not familiar with VPLS, let's start with some basics on its terminology and the
different implementation options.

VPLS: The Basics

VPLS is one of the key MPLS-based services that has developed in the industry over the past few years. As its name
implies, the purpose of VPLS is to provide a private multipoint LAN-type Ethernet connectivity service. For those
more familiar with technologies like ATM, we could say VPLS is the LAN emulation service for MPLS.
VPLS has special relevance in the service provider space as the way to deliver Layer 2 (L2) multipoint transparent
services over an Ethernet infrastructure using MPLS. But what is so special about this? The key point is MPLS. There
are different ways or approaches for a service provider to deliver services over an Ethernet infrastructure, but not
all of them fit into the requirements that a service provider has in terms of scalability, reliability, service flexibility,
and operational complexity. MPLS has become the catalyst that can turn an Ethernet infrastructure into carrier class
making it suitable for the service provider, as opposed to a VLAN-based or QinQ operation that has demonstrated
through multiple examples that it does not provide what is required in the carrier environment.
VPLS, as the main technology in use in the Metro Ethernet space, has two implementation options that the industry
has standardized:
• RFC4761 – BGP-based VPLS
• RFC4762 – LDP-based VPLS
Both of these implementation options are virtually identical from the forwarding plane perspective, but they differ in
the control plane, particularly in the protocol they use to signal and establish the pseudowires, BGP or LDP. It is not
the intention of this document to discuss the pros and cons of each approach. RFC4762 defines a hierarchical mode
of operation for LDP VPLS called H-VPLS.

What Is H-VPLS?

H-VPLS is the hierarchical version of LDP-based VPLS as described in RFC 4762 - Virtual Private LAN Service (VPLS)
Using Label Distribution Protocol (LDP) Signaling. As it is explained in the RFC:
The solution described above requires a full mesh of tunnel LSPs between all the provider edge (PE) routers that
participate in the VPLS service. For each VPLS service, n*(n-1)/2 pseudowires must be set up between the PE
routers. While this creates signaling overhead, the real detriment to large scale deployment is the packet replication
requirements for each provisioned pseudowires on a PE router. Hierarchical connectivity, described in this document,
reduces signaling and replication overhead to allow large-scale deployment.
Copyright © 2010, Juniper Networks, Inc.
APPLICATION NOTE - Demystifying H-VPLS
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