Metro Ethernet

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A metropolitan-area Ethernet, Ethernet MAN, or metro Ethernet is a metropolitan area network (MAN) that is based on Ethernet standards. It is commonly used to connect subscribers to a larger service network or the Internet. Businesses can also use metropolitan-area Ethernet to connect their own offices to each other.

An Ethernet interface is much less expensive than a SONET/SDH or PDH interface of the same bandwidth. Ethernet also supports high bandwidths with fine granularity,[clarification needed] which is not available with traditional SDH connections. Another distinct advantage of an Ethernet-based access network is that it can be easily connected to the customer network, due to the prevalent use of Ethernet in corporate and, more recently, residential networks.

A typical service provider's network is a collection of layer 2 or/and layer 3 switches or routers connected through optical fiber. The topology could be a ring, hub-and-spoke (star), or full or partial mesh. The network will also have a hierarchy: core, distribution (aggregation) and access. The core in most cases is an existing IP/MPLS backbone, but may migrate to newer forms of Ethernet transport in the form of 10Gbit/s, 40Gbit/s or 100Gbit/s speeds.

Ethernet on the MAN can be used as pure Ethernet, Ethernet over SDH, Ethernet over MPLS or Ethernet over DWDM. Pure Ethernet-based deployments are cheap but less reliable and scalable, and thus are usually limited to small scale or experimental deployments. SDH-based deployments are useful when there is an existing SDH infrastructure already in place, its main shortcoming being the loss of flexibility in bandwidth management due to the rigid hierarchy imposed by the SDH network. MPLS based deployments are costly but highly reliable and scalable, and are typically used by large service providers.

Metropolitan Area Network (MAN) Topology

Familiar network domains are likely to exist regardless of the transport technology chosen to implement Metropolitan area networks: Access, aggregation/distribution, and core.[1]

Much of the functionality of Ethernet MANs such as virtual private lines or virtual private networks is implemented by the use of Ethernet VLAN tags that allow differentiation of each part of the network. Logical differentiation of the physical network helps to identify the rights that the traffic has and to ease the management of hosts' access rights with respect to other users and networks.

Pure Ethernet MANs

A pure Ethernet MAN uses only layer 2 switches for all of its internal structure. This allows for a very simple and cheap design, and also for a relatively simple initial configuration. The original Ethernet technology was not well suited for service provider applications; as a shared-media network, it was impossible to keep traffic isolated, which made implementation of private circuits impossible. Ethernet MANs became feasible in the late 90s due to the development of new techniques to allow transparent tunneling of traffic through the use of Virtual LANs as "point to point" or "multipoint to multipoint" circuits. Combined with new features such as VLAN Stacking (also known as VLAN Tunneling), and VLAN Translation, it became possible to isolate the customers' traffic from each other and from the core network internal signaling traffic. However, Ethernet is constantly evolving and has now carrier class features with the recent addition of IEEE 802.1ad (Provider Bridges)(also known as QinQ or stacked VLANs) and IEEE 802.1ah (Provider Backbone Bridges) (also known as MAC in MAC or PBB) and IEEE 802.1Qay (Provider Backbone Transport) (also known as PBT or PBB-TE). Spanning-tree, broadcast packets and dynamic MAC learning are disabled and sub 50ms failover features are introduced.

There are three main shortcomings with a pure non PBT/PBB enabled Ethernet MAN approach:

Despite these shortcomings, non PBT/PBB enabled Ethernet-based MANs are used for two primary purposes:

Myths regarding pure Ethernet:

The biggest myth being propagated regarding pure Ethernet MANs or carrier Ethernets is that there are 4094 VLANs available network wide for a provider network. This is simply not true. There are 4094 VLANs available on each switched path. So the VID(vlan id) cannot be reused along the path from point a to point z, but can be reused anywhere else in the network as long as the paths are separated. Larger pure ethernet aggregation devices allow for traffic classification up to two tags deep. This allows for up to 16.7 million paths on a device of this nature, which should be used to aggregate devices that can only classify traffic based on 4094 VLAN ids. So in most networks VLAN exhaustion is not an issue if the network is designed appropriately. The network should be designed so that devices supporting large MAC tables and traffic classification of two tags are interconnected, and they act as an aggregator for less expensive, smaller mac table, one tag switches in attached rings and segments. Attaching these devices to interconnect larger areas provides for the theoretic possibility of up to 16.7 million unique paths between these devices, limited only by the device processing and memory capabilities. In a properly designed geographically significant modular network, more expensive services such as MPLS and PBT can be postponed or eliminated entirely. VLANs are locally significant only.

Another myth is that RSTP convergence takes many seconds. In certain situations and with some equipment this may be true. However, some vendors are offering devices that will converge RSTP in sub-50ms with little to no planning or effort. Advanced network planning may be required to achieve these speeds in certain situations, but it is possible with certain vendor's RSTP deployment. Problems with spanning tree in many instances arise from poor planning, design, and deployment. Spanning tree should be segmented and designed in small domains to be successful. A spanning tree domain is an area in which BPDUs will propagate. While advanced features of MSTP can be utilized, so can building manual spanning tree domains with legacy RSTP by disabling or blocking BPDUs on certain planned segments. In this way you create domains of segments and rings where spanning-tree is enabled, and keep the segments manageable. It is also essential to choose a root bridge and backup root bridge carefully. Path-costs should be modified so that the network administrator knows exactly what will happen to the traffic in the event of a failed segment anywhere in the network.

Another myth is that L2 Ethernet MAN connections remove the need for using L3 routers or L3 switches. This is also not true. While equipment will operate just fine over your new metro-ethernet gear on L2 without a router. The whole point is to provide low latency transport. Why send unnecessary broadcast traffic over a metro-ethernet connection that you are probably paying for by Mbps? In most situations routing over your metro-ethernet connection will keep your broadcast traffic down to a bare minimum and help utilize your connection's bandwidth for real traffic, not superfluous packets. This is especially important with more and more nodes on each end of the connection. Routers are not very expensive. If you are paying out hundreds or thousands monthly for a metro-ethernet connection, spend the extra money and get a good router.

SONET/SDH-based Ethernet MANs

A SONET/SDH based Ethernet MAN is usually used as an intermediate step in the transition from a traditional, time-division based network, to a modern statistical network (such as Ethernet). In this model, the existing SDH infrastructure is used to transport high-speed Ethernet connections. The main advantage of this approach is the high level of reliability, achieved through the use of the native SDH protection mechanisms, which present a typical recovery time of 50 ms for severe failures. On the other hand, an SDH-based Ethernet MAN is usually more expensive, due to costs associated with the SDH equipment that is necessary for its implementation. Traffic engineering also tends to be very limited. Hybrid designs use conventional Ethernet switches at the edge of the core SDH ring to alleviate some of these issues, allowing for more control over the traffic pattern and also for a slight reduction in cost.

MPLS-based Ethernet MANs

An MPLS based Metro Ethernet network uses MPLS in the Service Provider's Network. The subscriber will get an Ethernet interface on Copper (ex:-100BASE-TX) or fiber (ex:-100BASE-FX). The customer's Ethernet packet is transported over MPLS and the service provider network uses Ethernet again as the underlying technology to transport MPLS. So, it is Ethernet over MPLS over Ethernet.

Here, Label Distribution Protocol (LDP) signaling is used as site to site signaling for the inner label (VC label) and Resource reSerVation Protocol-Traffic Engineering (RSVP-TE) or LDP may be used as Network signaling for the outer label.

One of the restoration mechanisms used in an MPLS based Metro Ethernet Networks is Fast ReRoute (FRR) to achieve sub-50ms convergence of MPLS local protection. For each deployment situation the benefit versus cost of MPLS must be weighed carefully, so if not implemented on a carrier's distribution network there might be more benefit for MPLS the core network. In some situations the cost may not warrant the benefits, particularly if sub 50ms convergence time is already being achieved with pure Ethernet.

A comparison of MPLS-based Metro Ethernet against a pure Ethernet MAN:

Maturity of Metro Ethernet

The Metro Ethernet Forum (MEF) has defined four types of services that can be delivered through Metro Ethernet. Each service type has 2 sub types namely; Port-Based (All-to-One Bundling) and VLAN-based (Service Multiplexed):

1) E-Line (Point-to-Point Ethernet Virtual Circuit (EVC)):

1.1 Port-Based: Ethernet Private Line (EPL)
1.2 VLAN-based: Ethernet Virtual Private Line (EVPL)

Outside the MEF context, E-Line is also known as Virtual Private Wire Service (VPWS), Virtual Leased Line (VLL).

2) E-LAN (multipoint-to-mulipoint EVC):

2.1 Port-Based: Ethernet Private LAN (EP-LAN)
2.2 VLAN-based: Ethernet Virtual Private LAN (EVP-LAN)

Outside the MEF context, E-LAN is also known as Virtual Private LAN Services (VPLS), Transparent LAN Services.

3) E-Tree (rooted multipoint EVC):

3.1 Port-Based: Ethernet Private Tree (EP-Tree)
3.2 VLAN-based: Ethernet Virtual Private Tree (EVP-Tree)

4) E-Access :

4.1 Port-Based: Access Ethernet Private Line (Access EPL)
4.2 VLAN-based: Access Ethernet Virtual Private Line (Access EVPL)

Various access services can be provided with E-Access including; High Speed Internet access and IP/VPN access.

There are lot of vendors supplying equipment for Metro Ethernet deployments. They include ADTRAN, ADVA Optical Networking, Alcatel-Lucent, BTI Systems, C-COR, Fujitsu Network Communications (FNC), Ciena, Cisco, Creanord,, DATACOM, Dahili Network, Ericsson, Extreme Networks, Foundry Networks, Hatteras Networks, Huawei, IPITEK, Juniper Networks, MAIPU, MRV, Nortel Networks, RAD Data Communications, Redback Networks an Ericsson Company, Tejas Networks, Tellabs, ZTE and many more.

In June 2002, HKBN built the largest Metro Ethernet IP network in the world, covering 1.62 million homes in Hong Kong. and it will continue to expand towards the 2.0 million target by 2010.

In late September 2007 Verizon Business announced that it is implementing a Metro Ethernet solution across Asia-Pacific including Australia, Singapore, Japan and Hong Kong using Nortel equipment.[4]

Africa's largest and most developed privately owned MPLS Based Metro Ethernet Network is in Kenya. Reaching more than 5000 corporate entities, Kenya Data Networks is providing High End Services using Alcatel Core and Siemens Access equipment. KDN is now moving into FTTH projects and intends to cover more than 100 000 buildings in East Africa within the next 3 years.

In May 2011, Comcast announced that it was launching its own Metro Ethernet line of services to business customers in the United States.[5]

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