metro ethernet service

8/8/2019 Metro Ethernet Service

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METRO ETHERNET SERVICEMETRO ETHERNET SERVICE



A Metro Ethernet is a computer network that covers a metropolitanarea and that is based on the Ethernet standard.It is commonly used as a metropolitan access network to connect

subscribers and businesses toa larger service network or the Internet.Businesses can also use Metro Ethernet to connect branch offices totheir Intranet.

Ethernet has been a well known technology for decades.

An Ethernet interface is much less expensive than a SONET/SDHor 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 isthat it can be easily connected to the customer network,

due to the prevalent use of Ethernet in corporate and, morerecently, residential networks.Therefore, bringing Ethernet in to the Metropolitan Area Network(MAN) introducesa lot of advantages to both the service provider and the customer(corporate and residential).



Metro Ethernet system

A typical service provider Metro Ethernet network is acollection of Layer 2 or/and Layer 3 switchesor/and routers connected through optical fiber.The topologycould 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, butmay migrate to newer forms of Ethernet Transport in the formof 10G or 100G speeds.



Ethernet on the MAN can be used as pure Ethernet, Ethernet overSDH, 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 experimentaldeployments.SDH-based deployments are useful when there is an existing SDHinfrastructure already in place,its main shortcoming being the loss of flexibility in bandwidthmanagement due to the rigid hierarchy imposed by the SDH

network.MPLS based deployments are costly but highly reliable andscalable, and are typically used by large service providers.

Contents :

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



Pure Ethernet MANsA pure Ethernet MAN uses only layer 2 switches for all of itsinternal structure. This allows for a very simple and cheap design, and also for a relatively simple initial configuration. The original

Ethernet technolog y was not well suited for service provider applications;as a shared-media network, it was impossible to keep trafficisolated, 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 asVLAN T unneling), and VLAN T ranslation, it became possible to isolate the customers' traffic from eachother and from the core network internal signaling traffic.

However, Ethernet is constantly evolving and has now carrier classfeatures with the recent addition of IEEE 802.1ad (Provider Bridges)(also known as QinQ or stacked VLANs) and IEEE802.1ah (Provider Backbone Bridges)(also known as MAC in MAC or PBB) and IEEE 802.1Qay (Provider Backbone T ransport) (also known as PBT or PBB-TE).

Spanning-tree, broadcast packets and dynamic MAC learning are disabled and sub 50ms failover features are introduced.



* Network stability is relatively fragile, especially ifcompared to the more advanced SDH and MPLSapproaches.The recovery time for the standard spanning tree

protocol is in the range of tens of seconds, muchhigher than whatcan be obtained in the alternative networks (usually afraction of second). There are a number ofoptimizations,some standardized through the IEEE, and others

vendor-specific, that seek to alleviate this problem.The clever use of such features allow the network toachieve good stability and resilience, at the cost ofa more complex configuration and possible use ofnon-standard, vendor-specific, mechanisms.Some vendor's implementations of RSTP achieve sub

50ms convergence in typical sized rings.RSTP also provides for easy deployment of complexdesigns such as multi-ring, figure eight, etc.If designed appropriately, in many networks thefragility in this network design can beovercome without the additional expense of MPLS.



* Traffic engineering is very limited. There are few tools tomanage the topology of the network; also,the fact that forwarding is done hop-by-hop, added to thepossibility of broadcasts even for unicast packets (forinstance, while learning new addresses),makes predicting the real traffic pattern very difficult for anetworking novice. Custom tools, such as topology maps thatoutline where blocking ports occur inthe network during normal and backup conditions may need tobe built to fully understand and troubleshoot the network

quickly.* 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 ofIP and related protocols, such as BGP and MPLS,which are necessary for an MPLS-based deployment. Even for

larger scale deployments for thousands and thousands ofcustomers can be achieved if careful network design rules arefollowed. In order to do this effectively skilled networkingprofessionals need to be utilized.



* In large scale Metro Ethernets, it is common for the accesspart of the network to use a pure layer 2 design.At this level, the pure layer 2 design is deemed to be cheaperwhile still operating under its design limitations.

From the distribution layer and above, traffic is aggregated androuted using an MPLS-based Metro Ethernet design.In very large networks MPLS may be unavoidable, but withcareful network design, the use of both PBT and MPLS and theirassociated cost and complexity can be postponed if not eliminatedentirely by careful network planning and design.

The biggest myth being propagated regarding pure metro-ethernet orcarrier ethernet is that there are 4094 VLANsavailable 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 topoint z, but can be reused anywhere else in the networkas long as the paths are separated. Larger pure ethernet aggregationdevices allow for traffic classification up totwo tags deep. This allows for up to 16.7 million paths on a device ofthis nature, which should be used to aggregatedevices that can only classify traffic based on 4094 VLAN ids. So withproper network design, in most networks



Spanning tree should be segmented and designed in small domains to besuccessful. A spanning tree domain is an area in whichBPDUs will propagate. While advanced features of MSTP can be utilized,so can building manual spanning tree domains withlegacy RSTP by disabling or blocking BPDUs on certain planned segments.In this way you create domains of segments and ringswhere spanning-tree is enabled, and keep the segments manageable. It isalso essential to chose a root bridge and backuproot bridge carefully. Path-costs should be modified so that the networkadministrator knows exactly what will happen

to the traffic in the event of a failed segment anywhere in the network.

Another myth is that L2 metro-ethernet connections remove the need forusing L3 routers or L3 switches.This is also not true. While equipment will operate just fine over your newmetro-ethernet gear on L2 without a router.The whole point is to provide low latency transport. Why send unnecessarybroadcast traffic over a metro-ethernet connectionthat you are probably paying for by Mbps? In most situations routing overyour metro-ethernet connection will keep your broadcasttraffic down to a bare minimum and help utilize your connection's bandwidthfor real traffic, not superfluous packets.This is especially important with more and more nodes on each end of theconnection. Routers are not very expensive.



If you are paying out hundredsor thousands monthlyfor 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 intermediatestep in the transition from a traditional,time-division based network, to a modern statistical network (such asEthernet). In this model, the existing SDH infrastructureis 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 severefailures. On the other hand, an SDH-based Ethernet MAN is usuallymore expensive, due to costs associated with the SDHequipment that is necessary for its implementation. Traffic engineering

also tends to be very limited. Hybrid designs useconventional Ethernet switches at the edge of the core SDH ring toalleviate some of these issues, allowing for more control over the trafficpattern and also for a slight reduction in cost.



MPLS-based Ethernet MANs

An MPLS based Metro Ethernet network uses MPLS in the Service ProviderNetwork. The subscriber will get an Ethernet interfaceon Copper (ex:-100BASE-TX) or fiber (ex:-100BASE-FX). The customer'sEthernet packet is transported over MPLS and the serviceprovider network uses Ethernet again as the underlying technology totransport MPLS. So, it is Ethernet over MPLS over Ethernet.Here, Label Distribution Protocol (LDP) signaling is used as site to sitesignaling for the inner label (VC label) and Resource

reSerVation Protocol-Traffic Engineering (RSVP-TE) or LDP may be used asNetwork signaling for the outer label.

One of the restoration mechanisms used in an MPLS based Metro EthernetNetworks is Fast ReRoute-FRR (MPLS local protection)The main advantages of an MPLS-based Metro Ethernet against a pure

Ethernet are:* Scalability: There is a horrible myth in MPLS proponents that pureEthernet MAN are limited to a maximum of 4,094 VLANsfor the whole network, and that when using MPLS, Ethernet VLANs have localmeaning only (like Frame Relay PVC).This is untrue. Each local segment that data traverses is limited to 4094VLANs even without MPLS, not the whole network.



In a properly designed simple VLAN network, each switched path can have 4094single tag VLANs. Some larger aggregation equipmentcan classify traffic by two VLANs, so with properly placed two tag aggregationdevices in the center of the network, up to 16.7 million

different virtual switches can be assigned network wide³which eliminates manyof the reasons to use MPLS. When crossing the agg devices,there are 16.7 million vlan combinations³so then end segments and rings ofsingle tag devices can receive only the traffic that theyneed³leaving 4094 instances per segment or ring, but 16.7 million differenttagged paths for the whole network(4094x4094).

* Resiliency: pure Ethernet network resiliency relies on STP, RSTP or MSTP(30 to sub 50ms sec convergence) while MPLS-based MANsuse MPLS-based mechanism (i.e. MPLS Fast Reroute) to achieve SDH-like (50msecs) convergence times. Some vendors RSTP convergence is alsosub-50ms, but this convergence time can't be relied on from vendor to vendor.For each deployment situation the benefit versus cost of

MPLS must be weighed carefully. In some situations the cost may not warrantthe benefits, particularly if sub 50ms convergence time is alreadybeing achieved.

* Multiprotocol convergence: with the maturity on pseudowires standards(ATM Virtual Leased Line VLL, FR VLL, etc.) an MPLS-basedMetro Ethernet can backhaul not only IP/Ethernet traffic but virtually any type

of traffic coming from customer networks or other access networks(i.e. ATM aggregation for UMTS or TDM aggregation for GSM).



* End to End OAM: MPLS-based MAN offers a wider set oftroubleshooting and OAM MPLS-based tools which enrich Service Providersabilityto effectively troubleshoot and diagnose network problems. These includefor example, MAC ping, MAC traceroute, LSP ping etc.

The Metro Ethernet Forum (MEF) has defined three types ofservices that can be delivered through Metro Ethernet:

* E-Line also known as Virtual Private Wire Service (VPWS),Virtual Leased Line (VLL), Point-to-Point, or Ethernet Private WireService (EPVS).

* E-LAN also known as Virtual Private LAN Services (VPLS),Transparent LAN Services and MultiPoint-to-MultiPoint.

* E-TREE also known as Rooted-MultiPoint.



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

There are lot of vendors supplying equipment for Metro Ethernetdeployments. They include ADTRAN, ADVA Optical Networking, Alcatel-Lucent,C-COR, Fujitsu Network Communications (FNC), Ciena, Cisco, Creanord,DATACOM, Dahili Network, Ericsson, Extreme Networks, Foundry Networks,Hatteras Networks, Huawei, Juniper Networks, MAIPU, MRV, Nortel

Networks, RAD Data Communications, Redback Networks an EricssonCompany,Tejas Networks, Tellabs, ZTE and many more.

In June 2002, HKBN built the largest Metro Ethernet IP network in theworld, 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 implementinga Metro Ethernet solution across Asia-Pacific including Australia,Singapore, Japan and Hong Kong using Nortel equipment.[1]



Africa's largest and most developed privately owned MPLSBased Metro Ethernet Network is in Kenya.Reaching more than 5000 corporate entities, Kenya Data

Networks is providing High End Services using Alcatel Coreand SiemensAccess equipment. KDN is now moving into FTTH projects andintends to cover more than 100 000 buildings in East Africawithinthe next 3 years.

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