Cisco Cisco CRS-X Multishelf System White Paper

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Adding backbone capacity is costly. Until recently, service providers chose this option, because they were

confident about getting a return on their investments. By 2015, however, adding capacity will carry more risk than

reward. A breaking point will be reached where the costs of expanding backbone networks will exceed the revenue

potential present in the traffic carried.

There are many factors moving service providers toward this breaking point,

some technical, some architectural, and some organizational. They all result from the manner in which backbone

architectures have traditionally been built. Continued reliance on conventional linear scaling solutions, adding

components to increase capacity, as a way to address nonlinear issues will eventually result in decreasing returns

to scale. To pull back from this breaking point, a new method for growing backbone networks must be found.

This method must address multiple dimensions at once and eliminate architectural boundaries, isolated

administrative domains, and the inefficiencies affecting the packet, optical TDM, and DWDM layers.

Existing solutions focus on solving a single dimension of a multidimensional problem. Alternative architectures for

the IP core, such as the lean core and hollow core, focus on reducing capital costs. Separately, new multilayer

control-plane protocols focus on increasing efficiencies between the transport and IP layers. If implemented

independently, neither an alternative core architecture nor a new protocol will yield justifiable returns on

investment. Therefore, to thrive, service providers must find ways to increase revenue and reduce capital and

operational costs from their backbone networks, while increasing bandwidth, flexibility, and efficiency.

Alternative Backbone Architectures

Alternative backbone architectures attempt to eliminate the need for a traditional full IP core architecture, with the

goal of decreasing costs. New developments in platforms and protocols offer alternatives to full IP core

architectures, but sacrifices are made in flexibility and efficiency in favor of cost cutting. The two predominant

alternative architectures, hollow core and lean core offer initial savings, risking the long-term investment protection

inherent in full IP core architectures.

Hollow core architectures attempt to eliminate the costs associated with core backbone routers by replacing them

with a transport switching function (most often assumed to be an optical transport (OTN) switching layer) that offers

a lower overall cost per bit for a given interface speed.

In the hollow core model, these switches create a dense mesh of circuits between each of the edge and peering

nodes. As with the full IP core, the packet, optical TDM, and DWDM layers are managed separately, and there is

little control-plane integration. In many cases even this level of integration will not be possible because the

interface between the packet and TDM layers will be via Ethernet VLANs multiplexed onto circuits by specialized

transponders that preclude G.709-based inter-layer communication. This lack of control-plane integration prevents

topology information from being shared between the packet, optical TDM, and DWDM layers. Routers know only

the routing topology, and the DWDM and TDM switches know only the optical topology.

Lean core architectures are an adaptation of full IP architectures in which backbone routers have reduced network

processing unit (NPU) functionality or memory. With reduced NPU memory, the router can only learn internal

routes, which forces operators to use Multiprotocol Label Switching (MPLS) to forward subscriber traffic instead of

IP. Another option for cutting the cost of NPUs would be reducing their functionality to a minimum so that they are

only capable of providing transport functionality and forwarding via MPLS.

“Service Provider Capex, Revenue, and Capex by Equipment Type,” Infonetics, 2012; “Core and Edge Routers 5-Year

Forecast,” Dell’Oro Group, January 2012; and “Evolving Backbone Networks Using and MPLS SuperCore, Juniper, June 2011.