Metro Ethernet

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A Metro Ethernet is a computer network based on the Ethernet standard and which covers a metropolitan area. It is commonly used as a metropolitan access network to connect subscribers and businesses to a Wide Area Network, such as the Internet. Large businesses can also use Metro Ethernet to connect branch offices to their Intranets.

Ethernet has been a well known technology for decades. An Ethernet interface is much less expensive than a SDH or PDH interface of the same bandwidth. Ethernet also supports high bandwidths with fine granularity, 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 into corporate and, more recently, residential networks. Therefore, bringing Ethernet in to the Metropolitan Area Network (MAN) introduces a lot of advantages to both the service provider and the customer (corporate and residential).

A typical service provider Metro Ethernet network is a collection of Layer 2 or 3 switches or routers connected through optical fiber. The topology could be a ring, hub-and-spoke (star), full mesh or partial mesh. The network will also have a hierarchy; core, distribution and access. The core in most cases is an existing IP/MPLS backbone.

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.

1 Pure Ethernet MANs
2 SDH-based Ethernet MANs
3 MPLS-based Ethernet MANs
4 Further reading
5 External links

[edit] Pure Ethernet MANs

A pure Ethernet MAN uses only layer 2 switches for all its internal structure. This allows for a very simple and cheap design, and also for a relatively simple initial configuration. The original Ethernet technology wasn't 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 90's 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 customer's traffic from each other and from the core network internal signaling traffic.

There are three main shortcomings with a pure Ethernet MAN approach:

By design, layer 2 switches use fixed tables to direct traffic based on the MAC address of the endpoints. As the network gets larger, the number of MAC address transiting through the network may grow beyond the capacity of the core switches. If the core table gets full, the result is a catastrophic loss of performance due to the flooding of packets over the entire network structure.
Network stability is relatively fragile, especially if compared to the more advanced SDH and MPLS approaches. The recovery time for the standard spanning tree protocol is in the range of tens of seconds, much higher than what can be obtained in the alternative networks (usually a fraction of second). There are a number of optimizations, some standardized through the IEEE, and others vendor-specific, that seek to alleviate this problem. The clever use of such features allow the network to achieve good stability and resilience, at the cost of a more complex configuration and possible use of non-standard, vendor-specific, mechanisms.
Traffic engineering is very limited. There are few tools to manage the topology of the network; also, the fact that forwarding is done hop-by-hop, added to the possibility of broadcasts even for unicast packets (for instance, while learning new addresses), makes predicting the real traffic pattern very difficult. There are techniques that allow for some control of the preferential traffic paths; these techniques rely on the use of multiple spanning trees, or "per VLAN spanning trees", and are closely connected to the solutions used to achieve better stability and resiliency in the network.

Despite these shortcomings, pure Ethernet-based MANs are used for two primary purposes:

For small scale deployments (under a few hundred customers), a pure Ethernet MAN can be highly cost-effective. It also has the advantage of not requiring advanced knowledge of IP and related protocols, such as BGP and MPLS, that are necessary for a MPLS-based deployment.
In large scale Metro Ethernets, it's common for the access part of the network to use a pure layer 2 design. At this level, the pure layer 2 design is deemed to be cheaper while still operating under its design limitations. From the distribution layer and above, traffic is aggregated and routed using a MPLS-based Metro Ethernet design.

[edit] SDH-based Ethernet MANs

An 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/DWDM 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.

[edit] MPLS-based Ethernet MANs

A MPLS based Metro Ethernet network uses MPLS in the Service Provider Network. The subscriber will get an Ethernet interface on Copper (100BASE-TX) or fiber (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. The following figure explains this;

Fig.1 Ethernet over MPLS and MPLS signaling

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

One of the restoration mechanisms used in a MPLS based Metro Ethernet Networks is Fast ReRoute-FRR (MPLS local protection)

The main advantages of a MPLS-based Metro Ethernet against a pure Ethernet are:

Scalability: pure Ethernet MAN are limited to a maximum of 4,096 VLANs for the whole network, when using MPLS, Ethernet VLANs have local meaning only (like Frame Relay PVC). Same scalability considerations applies to the MAC addresses where in a pure Ethernet MAN all MAC addresses are being shared among the network while only having local meaning in the MPLS context.
Resiliency: pure Ethernet network resiliency relays on STP or RSTP (30 to 1 sec convergence) while MPLS-based MANs use MPLS-based mechanism (i.e. MPLS Fast Reroute) to achieve SDH-like (50 msecs) convergence times.
Multiprotocol convergence: with the maturity on pseudowires standards (ATM VLL, FR VLL, etc.) an MPLS-based Metro Ethernet can backhaul not only IP/Ethernet traffic but virtually any type of traffic coming from customers networks or other access networks (i.e. ATM aggregation for UMTS).
End to End OAM: MPLS-based MAN offers a wider set of troubleshooting and OAM MPLS-based tools which enrich Service Providers ability to effectively troubleshoot and diagnose network problems.

The Metro Ethernet Forum (MEF) has defined two types of services that can be delivered through Metro Ethernet:

E-Line also known as Virtual Leased Line and Point-to-Point.
E-LAN also known as Virtual Private LAN Services, Transparent LAN Services and MultiPoint-to-MultiPoint.

Additionally, various access services can be provided with Metro Ethernet including; High Speed Internet access and IP/VPN access.

There are lot of vendors supplying equipment for Metro Ethernet deployments. They include Alcatel-Lucent, Ericsson, C-COR, Cisco Systems, Extreme Networks, Foundry Networks, Huawei, Nortel Networks, Tellabs, ZTE and many more.

In June 2002, HKBN built the largest Metro Ethernet IP network in the world, covering 1.2 million homes.