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Interexchange Carrier MPLS Network Design Study

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jul 22, 2005.

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.

USCom is a fictitious nationwide data and long-distance voice service provider in the U.S. that provides connectivity between local exchanges in different geographic regions. It also facilitates inter-Local Access and Transport Area (LATA) services (as described in the Federal Communications Commission [FCC] Telecommunications Act of 1996), as well as a complete portfolio of data services. USCom may be classified as an Interexchange Carrier (IXC) that owns its fiber and transmission facilities as well as a Layer 2 switching infrastructure (ATM and Frame Relay) spanning its service footprint.

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.

USCom's Network Environment

USCom has been offering Internet access for many years to other service providers (wholesale), Enterprises, and small/medium business customers. It currently has an installed base of more than 35,000 Internet ports. These Internet ports are supported on 350 Internet edge routers (called Internet access provider edge [PE] routers) located in their 100 Points of Presence (POPs) that are situated across the country. Internet connectivity is obtained via transit providers, private peering sessions, and connections in major cities to various Network Access Points (NAPs).

USCom has also had great success with its Layer 3 Multiprotocol Label Switching (MPLS) VPN service (which is based on the architecture described in [2547bis]) since its inception in 2002. Acceptance of the service has grown throughout USCom's customer base. Currently some 12,500 VPN ports are installed across the country, and this number is growing considerably on a monthly basis. The customer-managed customer edge (CE) routers are connected via 255 Layer 3 MPLS VPN PE routers hosted in USCom's various POPs. Note that PE routers are dedicated to either the Internet or Layer 3 MPLS VPN access. Given the success of this offering, USCom plans to add 6000 customer access links per annum, although based on the current trend this figure is considered conservative. Total traffic volume, which includes both Internet and VPN, is expected to grow at approximately 30 percent per annum.

USCom owns fiber across the country and is running a long-distance optical core based on dense wavelength division multiplexing (DWDM) technology. This translates to availability of raw high-speed links (OC-48 (2.488 Gbps) and OC-192 (10 Gbps)) for provider router (P router) and PE router interconnection, at relatively low cost and provisioning time. USCom can activate additional capacity by enabling additional wavelengths (lambdas) in a relatively short time frame. USCom takes advantage of this to enforce an overengineering policy for core router links.

The high-speed core links are provided to routers as native lambdas straight from the DWDM equipment without any intermediate SONET Add/Drop Multiplexer (ADM). (Note that SONET framing is in use between the routers and the DWDM equipment.) These links do not benefit from any protection at the optical level. Some links interconnecting P routers and PE routers are provided through a SONET infrastructure overlaid over the optical infrastructure. The SONET links are protected by means of SONET protection provided by Bidirectional Line Switch Rings (BLSRs) with four fibers, also called BLSR/4. (See [NET-RECOV] for more details on SONET-SDH recovery mechanisms.)

Intra-POP connectivity is achieved via Packet over SONET (PoS) or switched Gigabit Ethernet. Because of the relatively low cost of switched Gigabit Ethernet technology and the negligible cost of fibers within a premises, USCom also maintains an overengineered intra-POP capacity.

Access from CE router to PE router for both Internet and Layer 3 MPLS VPN connectivity is provided via Frame Relay, ATM, leased line, or SONET. Each of these physical (or logical) links is dedicated to a single CE router. These links involve a significant cost that typically precludes simple overengineering and mandates tight dimensioning. Access speeds range from 64 kbps to OC-48.

The USCom nationwide backbone POP topology, interconnected through OC-48 and OC-192 links, is illustrated in Figure 3-1.

Figure 1

Figure 3-1 USCom Nationwide Topology

The USCom network is structured into three levels of POPs. Each POP is classified as either a backbone (Level 1), medium (Level 2), or small (Level 3) facility. The level depends on the density of the customer access and combined traffic throughput requirements. All routers are operated as a single autonomous system, with American Registry for Internet Numbers (ARIN) assigned AS number 32765. USCom has been assigned the 23/8 IP address space. The company uses this for its internal infrastructure as well as customer allocation.

Level 1 POPs are the backbone POPs (as shown in Figure 3-1) comprising the high-capacity backbone P routers dedicated to long-distance transit and interconnection of lower-level POPs to this long-distance transit backbone. PE routers providing Internet and Layer 3 MPLS VPN services from these major locations are also deployed, as well as some additional P routers acting as an aggregation layer inside the POP for these PE routers. Aggregation P routers reduce the number of IGP adjacencies that have to be maintained by the backbone P routers to two, because each core P router has to peer with only two aggregation P routers (in addition to the other core P routers in the backbone) instead of with all the PE routers in the POP (whose number can be fairly high, and growing, in a Level 1 POP).

Each Level 1 POP has two backbone P routers that interconnect via OC-48, dual OC-48, or OC-192 links to the rest of the backbone network. They also interconnect with lower-level POPs using either OC-3 (155.52 Mbps) or OC-48 links. Each backbone P router is connected to both local aggregation P routers via a point-to-point OC-48 link. Each PE router (and there may be several) is connected to both aggregation P routers via OC-3 PoS links. There are currently 15 Level 1 POPs, the structure of which is illustrated in Figure 3-2.

Figure 2

Figure 3-2 USCom Level 1 POP Design

The Level 2 POPs are composed of P routers that connect to the Level 1 POPs, or another Level 2 POP, via OC-3 or OC-48 links, and the PE routers in medium access locations. Each PE router is connected to both backbone P routers via redundant switched Gigabit Ethernet (using two separate Gigabit Ethernet switches). There are currently 25 Level 2 POPs, the structure of which is illustrated in Figure 3-3.

Figure 3

Figure 3-3 USCom Level 2 POP Design

The Level 3 POPs are composed of PE routers in remote locations and P routers that connect to Level 2 POPs via OC-3 links. There are currently 60 Level 3 POPs, the structure of which is illustrated in Figure 3-4.

Figure 4

Figure 3-4 USCom Level 3 POP Design

Several years ago, USCom deployed a SONET network providing OC-3 links. These links are protected at the SONET layer by the protection mechanisms provided by four-fiber BLSRs. These allow recovery from any link failure, with some special conditions specified by the SONET standard, within 60 ms. USCom satisfies all the conditions, including ring distance limited to 1200 km, less than 16 SONET stations, and ring in idle state before protection. Figure 3-5 shows the protected OC-3 links provided by the four-fiber BLSRs and used between Level 1 and Level 2/3 POPs. Because these links are protected and stable, USCom decided to use them in the core network without any changes.

Figure 5

Figure 3-5 Protected OC-3 Links Provided by Four-Fiber BLSRs

Figure 3-5 also shows that the USCom optical network uses DWDM technology, allowing the multiplexing of tens of light paths over a single fiber. Note that USCom has deployed Coarse Wave Division Multiplexing (CWDM) equipment in some metro areas, offering a lower degree (4) of multiplexing. The DWDM equipment lets the company provide 1+1 optical protection. Such a protection scheme relies on specialized optical equipment performing traffic bridging along the primary and secondary light paths, each of which follows diverse paths. Upon a link failure, such as a fiber cut or optical equipment failure, the receiving side quickly detects the failure and switches the traffic received from the primary light path to the secondary. This type of mechanism, usually qualified as "single-ended," is undoubtedly efficient because it does not require any extra signaling mechanisms or coordination between the sender and receiver (just the receiving side performs the switching function). Hence, the rerouting time is very fast (a few milliseconds). Moreover, a strictly equivalent quality of service (QoS) is guaranteed upon a network element failure because the secondary path is identical to the primary path (although it might be longer to be diverse from the primary path). On the other hand, this requires dedicating half of the fiber capacity for backup recovery. Furthermore, such a protection scheme implies that additional optical equipment needs to be purchased.

Hence, USCom decided to use all the network bandwidth to route the primary traffic and rely on some upper-layer protection mechanisms (see the section "Network Recovery Design") to offer equivalent rerouting time at significantly lower costs. All the light paths provided to the IP/MPLS layer for inter-Level 1 links and Level 1-to-Level 2 links therefore are unprotected. This is perfectly in line with the previously described core network overengineering strategy adopted by USCom.

Although DWDM offers the ability to provide high bandwidth in a very cost-effective fashion, it has a downside. Multiple links share some common resources and equipment whose failure may impact several links. This is called Shared Risk Link Group (SRLG), and the production design should take it into account.

Putting all this information together, you can see from Figure 3-6 how connectivity is typically achieved from a Level 3 to a Level 2 to a Level 1 POP.

Figure 6

Figure 3-6 Inter-POP Connectivity Within the USCom Network

Table 3-1 summarizes the various types of links used in the USCom network, along with their main characteristics and localization.

Table 3-1 Link Types and Characteristics in the USCom Backbone

Link Type





10 Gbps


Level 1 POP-Level 1 POP


2.5 Gbps


Level 1 POP-Level 1 POP

Level 1 POP-Level 2 POP


2.5 Gbps

SONET protection

Level 1 POP-Level 2 POP

Level 2 POP-Level 2 POP


155 Mbps

SONET protection

Level 2 POP-Level 3 POP

Gigabit Ethernet

1 Gbps


Intra-Level 2 POP

Intra-Level 3 POP

During the past several years, USCom has gathered various network failure statistics; they are summarized in Table 3-2. These statistics have been used to assess USCom's design requirements for its backbone network.

Table 3-2 Link Failure Statistics Within the USCom Network

Failure Type

Link/Router Type



Link failure

OC-3 SONET links

On average once a day in the network

From a few seconds to several days (fiber cut)

Link failure

OC-48 and OC-192 links



Router interface failure



A few hours

Router failure (such as power supply, router software failure with traffic impact)


Once every two months


Router reboot (planned failure)

Edge (IA and VPN PE routers)

Once every six months

10 minutes

Router reboot (planned failure)


Once a year

10 minutes

2. USCom's Network Design Objectives | Next Section

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