*** see also the MEF (Metro Ethernet Forum) http://www.metroethernetforum.org/
Metro Ethernet is very difficult to explain, because it encompasses so much !! It is an extremely complex set of recommendations (not standardized yet)
Traditional Ethernet is used to connect devices and allow Layer 2 data communication - forming a LAN. This allowed distributed processing and peer-to-peer networking. It worked well and for the most part, replaced much of the previous centric network model (i.e. Mainframes and Dumb terminals). But companies needed to communicate with their "sister sites" in different cities. After numerous methods have been used for many years to connect Ethernet LAN's - what was missing was standards to address the problems of speed, QoS, and scalability. Metro Ethernet is an answer to those missing standards. First, a few important points:
Ethernet Forum) - currently the MEF is running the show and
making the recommendations. They do ratify their suggested
standards, but they are NOT a standards body !!! The IEEE may end up creating Metro Ethernet
standards, but they will almost certainly be based on the work the MEF has
done. Here are two important MEF documents:
Metro Ethernet Brochure
Metro Ethernet Technical White Paper (dated 8/2004 - for most recent goto MEF website)
MEF defines Ethernet 802.3 technologies, but is not concerned only with the Ethernet Layer (Layer 2). The MEF specifies Layer 2-7 management, protocols and architecture
10GEA (10GbE Alliance) is concerned with 10GbE transport and interoperability with existing legacy networks and demonstrations of conforming products at the MAC layer and below. (Layer 1-2)
the three Metro Ethernet speeds - Fast Ethernet (100 Mbps), GbE (1 Gbps = 1000 Mbps), or 10GbE (10 Gbps). Standard 10 Mbps Ethernet is not used with Metro Ethernet !!
MEN (Metro Ethernet Network) - an acronym to know
Metro Ethernet is transport agnostic
- or is it ?? the MEF says that "Metro Ethernet is
Transport Agnostic". This statement is drilled into you several
times as you read the MEF website. BUT then they contradict
themselves. In their primary brochure,
they state their mission:
"The mission of the Forum is to accelerate the adoption of optical Ethernet as the technology of choice in metro networks worldwide."
Hmmm . . . sounds like Metro Ethernet is transport over fiber (optical) - not very agnostic, is it? The better phrase would be:
"Metro Ethernet uses an optical transport medium for Layer 1 and high-speed Ethernet for layer 2, but is still in the process of being standardized for all OSI Layers, 1-7".
But it does run over fiber, using various protocols. Many people think that Metro Ethernet = Ethernet over SONET. But Metro Ethernet can run over SONET, WDM, Dark Fiber, etc.
Example - two MEN's Connected by a WAN Link
(Metro Ethernet Networks) Definition
are primarily point-to-point, and can be either Fast Ethernet (100 Mbps),
GbE (1 Gbps) or 10GbE (10 Gbps).
Nodes can be either switches or routers, depending on
(a) Their location in the MEN
(b) The nature of services being provisioned
(c) The level of service resilience / protection required. Nodes are meshed to whatever degree necessary to
provide the desired connectivity, services and protection.
are primarily point-to-point, and can be either Fast Ethernet (100 Mbps),
GbE (1 Gbps) or 10GbE (10 Gbps).
WAN links connect MEN’s together across large distances.
Ethernet Services can be classified topologically into either point-to-point (as shown) or point-to-multi-point. Services are then further classified according to the bandwidth provisioned and used. This usage can be exclusive and/or shared across multiple users. Bandwidth is provisioned on demand from 1Mb/s to 1Gb/s, in increments as fine as 1Mbps.
Network resilience - Hierarchically implemented using a combination of various techniques. Protection could be end-to-end (as shown) and/or node-to-node.
Quality of Service - realized using a combination of various techniques, providing both ‘hard’ and ‘soft’ bandwidth guarantees. Implemented on both an end- to-end (as shown) and node-to-node basis.
Metro Ethernet areas that the MEF is Working On
MEF Metro Ethernet "Service Areas"
Ethernet services Definition - Phase 1
Uses 2 models . . . E-Line, a point-to-point link, and E-LAN, an any-to-any LAN interconnection similar to Transparent LAN Service (TLS), but in a multipoint configuration. At the business-user level, the MEF is positioning E-Line and E-LAN as the basis for applications such as LAN extension, dedicated Internet access, and Ethernet over Virtual Private LAN Service
*** for the other 3 service areas - Traffic Management, CES, and PDH - see the MEF Technical White Paper.(8/2004 - for most recent goto MEF website)
History of Metro Ethernet
Traditional TDM ways of getting to the customer, with traditional SDH/SONET-based transmission, carriers pay a high price in operational complexity, cost and provisioning delay. Ethernet's advantages include fast provisioning, fine-grained bandwidth granularity (inherent in packet technologies) and a scalability from kbps to Gbps. The customer also expects a lower cost service, although carrier pricing remains volatile, partially from fear of cannibalising their existing connectivity revenues.
Generic Framing Procedure (GFP)
Carriers with already deployed SONET/SDH networks naturally consider how to use them efficiently to carry Ethernet. The problem of mapping continuously-scalable packet flows into the lumpy SONET/SDH bandwidth hierarchy is well-described. Generic Framing Procedure (GFP) is becoming an increasingly popular adaptation layer between Ethernet (and other packet protocols such as PPP, Fiber Channel, FICON/ESCON) and SDH/SONET, implemented via the evolution of SONET/SDH devices into Multi-Service Provisioning Platforms (MSPPs). The bandwidth mismatches are addressed via Virtual Concatenation (VCAT) and Link Capacity Adjustment Scheme (LCAS).
Halabi's "Metro Ethernet" Book Summary
Perhaps the best book on Metro Ethernet is summarized here:
Ethernet-over-SONET/SDH as just described is a pure transport mechanism. To create an Ethernet analogue of add-drop multiplexing and to support traffic aggregation, L2 switching functionality needs to be added to the basic SDH/SONET box. (This is a well-worn path for transmission vendors - the same model was proposed for ATM). Different customers' Ethernet streams need to be identifiable, and carrier VLAN tagging is a possibility, although MPLS provides a more scalable solution.
In L2 switching in ring topologies, bandwidth fairness and efficient protection switching is difficult to achieve. The new "Resilient Packet Ring" (RPR) MAC protocol was developed to address these issues, and RPR can be run over GFP, and therefore supported in SONET/SDH devices which understand the RPR protocol. Of course, one can dispense with SONET/SDH equipment altogether (especially if you are a new operator and never installed it). Halabi briefly touches on the deployment and management of Gigabit Ethernet switches with direct interconnect.
Ethernet services must be understood, such as L2 switching, MAC learning, flooding, broadcast/multicast, VLANs and spanning tree protocol, and then we can get down to the services.
The Metro Ethernet Forum has defined two Ethernet service types: Ethernet Line Service (ELS) and Ethernet LAN service (E-LAN). Point-to-point vs. multipoint-to-multipoint, or transport vs. transport-and-switching if you prefer. ELS issues include traffic and performance management, class-of-service, VLAN support. Additional issues for E-LAN services focus on mechanisms for customer-separation, address-management and scalability.
Halabi identifies a number of issues along the way: with the VLAN tag length restricting operators to 4,096 customer-id values, operational services cannot scale; Ethernet does not have the kind of embedded OA&M facilities which allow carrier services such as SONET/SDH to be monitored and provisioned; the spanning tree protocol for loop-prevention does not scale and is inefficient; VPN configuration is hard to scale.
Chapter 4: "hybrid L2 and L3 IP/MPLS networks" unveils the solution. In a nutshell it is to adapt Ethernet to MPLS at the network edge, and use the power of BGP/MPLS VPN technology to scale the service. Halabi starts by reviewing standard L3 VPNs, both IP tunnels (GRE, not IPsec) and BGP/MPLS rfc 2547.
He then notes that Ethernet can be carried over MPLS via the IETF Pseudowire standard - this is fine for Ethernet Line Services. An E-LAN service such as Virtual Private LAN Service requires more work from the CE/PE devices, however. Specifically, the CEs think they are talking to an Ethernet switch on their link to the PE. The PE needs the additional functionality of a VLAN switch, and maps a specific MPLS label to each VLAN on a per-customer basis. This "broadcast domain" identification inner label is then augmented by an outer traffic-engineering label to forward traffic to the correct destination PE across the Service Provider network. Halabi describes in detail the mechanisms, which are similar to rfc 2547 VPNs at L3.
In practice there are still scaling issues, and the concept of "Decoupled Transparent LAN Service" is introduced. This creates an additional customer-premises PE which specialises in L2 MAC address management and customer segmentation, while the network POP PE can specialise in L3 MPLS tunnel and connectivity management. Given the detailed technical treatment, this is one of the harder chapters in the book. However, this is the last chapter actually devoted to Ethernet.
With Chapter 5, we enter part II of the book, which is more focused on traffic engineering and GMPLS. This chapter is a fast review of MPLS for traffic engineering of IP networks. Chapter 6 extends this discussion to cover the details of RSVP-TE for LSP establishment, specifically for fast-reroute. In Chapter 7 and the final Chapter 8, we see how MPLS (specifically GMPLS) can be used as a generalised control plane for virtual circuit management in the SONET/SDH and the optical layers.
The Internet is awash with white papers on all aspects of metro Ethernet. This book was published, by Cisco Press, in September 2003 so it's hot off the press. But sections still appear to be slightly dated in what is an incredibly fast-moving area.
Why buy it? Because Halabi knows what he's talking about, and gets down into the detail of how everything works with great intellectual clarity. Although it can be hard to see the wood from the trees in this book, it is the ideal "in one place" reference for both services and technologies for carrier Ethernet. I consider chapters 5-8 as an MPLS bonus, as they actually have nothing specifically to do with carrier Ethernet.