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The Case for the Metro Core

Metro networks are the next big opportunity for convergence. Within metro networks, the emerging IP metro core is critical to efficiently address evolving traffic demands in the metro. It also paves the way for a converged metro network that is optimized for cloud services ― with minimal impact to service operations or organizations.

Traffic growth drives metro network evolution

Metro networks are the new center of traffic growth. Today, metro network traffic is already growing faster than backbone network traffic. Looking ahead, consumer, cloud and business traffic will increase the strain. An Alcatel-Lucent Bell Labs study predicts that between now and 2017 traffic volumes in metro networks will continue to accelerate with accumulated growth of 560%.

  • Mobile broadband traffic will grow 9-fold
  • Consumer Internet and cloud traffic will grow 4-fold
  • Business and data center interconnect traffic will grow 3-fold
  • Video will become 65% of all consumer traffic

The predicted growth in metro network traffic is almost twice as high as that of backbone traffic, which is expected to grow by 320% over the same period[1].

Rapid traffic growth puts pressure on service providers to increase capacity so they can avoid traffic bottlenecks. Data center traffic is a specific challenge. Enterprises, web scale companies and network providers are all making huge investments in data center equipment. Metro networks provide much of the connectivity and bandwidth needed to interconnect all of these data centers and deliver the benefits of cloud computing and storage.

Network advances drive metro network evolution

In addition to traffic growth, two advances in network design drive the need to converge metro networks:

  • IP edge functionality, content storage and data center server capacity are being decentralized
  • Access and backhaul infrastructures are being consolidated and converged

Decentralizing services shortens the data path between subscribers and services. A shorter path reduces network delivery costs. It also reduces latency and jitter for cloud-based applications and streaming media to improve performance. Placing IP service edge functionality, content delivery networks (CDNs) and data center servers closer to subscribers helps to cost efficiently scale service capacity.

However, decentralization also creates substantial new traffic flows in the metro network (green arrow in Figure 1). These traffic flows add to conventional east-west traffic between subscribers and centralized service infrastructure and Internet peering points. As a result, far more traffic will circulate within the metro and the metro network architecture must evolve to efficiently accommodate this shift.

The challenge is that most metro networks were designed to send metro traffic to centralized content servers, data centers and Internet peering points connected by an IP/MPLS backbone. These designs typically have clusters of metro aggregation nodes feeding into backbone routers. However, aggregation routers have limited or no capabilities to efficiently switch intra-metro traffic themselves. That means the backbone nodes must switch intra-metro traffic in addition to inter-metro and backbone transit traffic (Figure 2, left).

Because a rapidly growing portion of metro traffic growth will remain local, the scaling requirement that backbone routers face over the next 5 years is compounded. Upgrading the installed base of core routers is a valid option, but introducing a metro core into the metro aggregation network brings very compelling benefits.

metro core Benefits

The metro core is a new architectural element that provides an aggregation and distribution hub for subscriber and service traffic in the metropolitan area network. The metro core:

  • Acts as a regional backbone providing connectivity between the various access and aggregation networks within a given metropolitan area
  • Switches intra-metro traffic
  • Connects into a national backbone network to enable connectivity with other metropolitan, regional and national networks

Introducing a dedicated metro core offloads the growing volumes of intra-metro traffic from the backbone nodes to address the backbone scaling challenge. The backbone nodes can then focus on switching inter-metro and backbone transit traffic. The majority of metro traffic stays within the metro and most backbone traffic is transit traffic. As a result, each core routing platform need only dedicate a limited amount of switching capacity to interworking the metro core with the backbone for inter-metro traffic (Figure 2, right).

As the main aggregation and distribution hub for subscriber and service traffic in the metro network, the emerging metro core is also central to metro network convergence. It acts as an:

  • Aggregation point for the purpose-built metro aggregation networks already in place
  • Attachment point for service provider data center infrastructure and IP edge service infrastructure
  • Access point for the metro network to connect to the IP/MPLS backbone and the Internet

Around the metro core, service providers can gradually consolidate the network service edge and cloud-based services hosted from data centers. Figure 3 shows the metro core within a converged metro network architecture.

Depending on operational preferences, a metro core node might perform regional and national core switching roles on a common platform, or it might divide roles across platforms.

Traditionally, service providers break out the roles and functionality into several different pieces of network equipment. However combining roles on a common, versatile metro core platform reduces costs and complexity in the metro network. It also improves reliability. Network nodes that act as metro core nodes need certain functionality:

  • Ethernet and IP/MPLS link aggregation services to interconnect aggregation networks
  • IP/MPLS routing in the metro network and potentially the national backbone(s)
  • IP service distribution, including multicast delivery for broadcast IPTV traffic

Additional capabilities include:

  • Layer 2 and Layer 3 virtual private network (VPN) services. This allows:
    • Wholesale and high-end business users to offload IP/MPLS multiservice edge nodes.
    • VPN infrastructure services to provide logical separation of subscriber and service domains in the metro.
  • Border Gateway Protocol (BGP) peering to provide a common Internet hand-off point.
  • Data center interconnect with compute and storage resources for cloud-based services.

A strategy for core success

Metro networks have become the key area for traffic growth, capacity investments and network innovation. To take advantage of the metro network opportunity, service providers should invest in a next-generation metro infrastructure that:

  • Is cost- and performance-optimized for emerging cloud services
  • Supports a non-disruptive migration of legacy services on a common, converged packet-based infrastructure

Evolving metro networks with a focus on the metro core helps service providers address their needs for:

  • More capacity
  • Better cost efficiencies
  • Greater service flexibility

Combining network innovation and redesign is crucial to success. A metro network strategy that is centered on the metro core helps service providers achieve their goals in a profitable and sustainable way.

More information:

Application note: Upscaling the Metro: Deploying the 7950 XRS in the metro core More on the metro: The Cloud-optimized Metro. Moving the Metro Forward

To contact the authors or request additional information, please send an e-mail to


  1. [1] According to Alcatel-Lucent Bell Labs research
Arnold Jansen

About Arnold Jansen

Arnold is a senior solution marketing manager in Nokia’s Network Infrastructure business division and responsible for promoting IP routing products and solutions. Arnold has held a number of roles in research and innovation, sales, product management, and marketing during his 25 years in the telecommunications industry. He holds a Bachelor degree in Computer Science from the Rotterdam University of Applied Sciences.

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