Cisco Cisco CRS-X Multishelf System White Paper
© 2013 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information.
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Although nominally less costly to deploy because of their less-sophisticated NPUs, the lean core architecture and
its associated label switch routers (LSRs) introduce problems that may ultimately erode potential savings. All IP
services will be shifted to the edge or provider edge routers outside the backbone, effectively turning the backbone
into an inner-core. Implementing a lean core architecture can also be a complex and disruptive undertaking that
requires operators to re-architect an existing network.
Finally, lean core architectures do not address the other factors affecting backbone network costs and
inefficiencies. This model still manages packet, optical TDM, and DWDM layers separately, and control-plane
integration remains limited as in full IP core architectures. As with conventional architectures, topology information
is effectively isolated within the packet, optical TDM, and DWDM layers, limiting operational efficiency.
Multi-Layer Control Plane: Generalized MPLS
Although new architectural approaches such as hollow core and lean core attempt to address the initial cost factors
for evolving a network, they ignore the challenges associated with isolated packet, optical TDM, and DWDM layer
control planes. This isolation leads to inefficiencies and difficulties in provisioning, monitoring, troubleshooting,
service velocity, and more, when laborious coordination is required between network layers and disparate
administrative organizations. To address layer isolation, the industry is developing a consolidated, multilayer
control plane that would work across layers, based on MPLS and known as Generalized MPLS (GMPLS).
GMPLS defines two models of operation, peering and overlay. In the peering model (Figure 1), operators create a
single routing domain that encompasses both the IP and transport layers. Layer 3 and Layer 1 devices share
topology information, providing Layer 3 routers with visibility into Layer 1 transport paths, wavelengths, loads, risk
groups, and so on. This communication enables Layer 3 routers to calculate best paths, request circuits that meet
their requirements, and move or recover and restore circuits based on path conditions.
Figure 1. Peering Model