Metro Ethernet Overview
A Metro Ethernet is a flexible transport architecture that uses some combination of optical, Ethernet, and IP technologies in the metropolitan area. Exactly what the mix of technologies is depends on how the service provider has designed its infrastructure.
This topic provides an overview of Metro Ethernet service models and architectures.
Metro Ethernet Service Model
Metro Ethernet leverages a service provider multiservice core.
The technology inside a Metro Ethernet network is not visible to customers; they see only the Ethernet services connection at their premises. The service provider is responsible for provisioning these services across its core network.
Metro Ethernet is a large market for the service provider, because there is an opportunity to provide services to customers with millions of existing Ethernet interfaces. Although the service provider might not want to disclose the backbone infrastructure, the more the customers know about the provider core, the more informed they can be about the quality of the services they will be receiving and the potential problems that may arise.
| Note | Appropriate SLAs for the advanced WAN services are discussed in the “Implementing Advanced WAN Services” section of this chapter. |
Metro Ethernet Architecture
The service provider provides Ethernet as a network infrastructure for metropolitan connectivity, possibly using various Layer 1 transport technologies. The Metro Ethernet architecture is illustrated in Figure 4-6.
The service provider may use SONET/SDH rings or point-to-point links, WDM, or RPR technology for its Metro Ethernet architecture. Edge aggregation devices or user provider edge (UPE) devices may multiplex multiple customers onto one optical circuit to the network provider edge (NPE) device. NPE devices connect to core provider (P) devices. The Ethernet service provided might include multiple services, such as LAN interconnection, IP telephony, and Internet access. It might also include varying levels of SLA and QoS for different customer needs. Edge aggregation allows the service provider to support oversubscription.
The actual implementation for the Metro Ethernet MAN service may be based on one or several of the following approaches:
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A pure Ethernet MAN uses only Layer 2 switches for all its internal structure. The switches may be in a loop-free topology and may not be running Spanning Tree Protocol (STP).
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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 SONET/SDH infrastructure is used to transport high-speed Ethernet connections.
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An MPLS based Metro Ethernet network uses Layer 2 MPLS VPNs in the provider network (P-network). The subscriber will get an Ethernet interface on copper or fiber, at 10-Mb/s to 1-Gb/s rates. The customer Ethernet packets are transported over Multiprotocol Layer Switching (MPLS), and the P-network may use Ethernet again as the underlying technology to transport MPLS.
Each of these approaches offers different oversubscription characteristics. Switched Ethernet and Ethernet over MPLS (EoMPLS) use statistical multiplexing (stat muxing), with no differentiation between customers or types of traffic unless QoS is provided. Ethernet over SONET implementations are not oversubscribed unless the SONET infrastructure does not go end-to-end in the provider network, in which case there may be portions of the network subject to oversubscription.
One advantage of edge aggregation is that service providers can now customize the service to customers without changing the infrastructure. For instance, with oversubscription, a provider web page might allow customers to increase their bandwidth limits.
Metro Ethernet LAN Services
Cisco offers a scalable Metro Ethernet solution over an existing SONET/SDH network, switched Ethernet network, or IP MPLS network, which provides multiple classes of service and bandwidth profiles to support critical data, voice, video, and storage applications. The Cisco Optical Metro Ethernet solution supports several Metro Ethernet Forum (MEF) service types:
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Ethernet Private Line (EPL) service: A port-based point-to-point Ethernet-line (E-line) service that maps Layer 2 traffic directly onto a TDM circuit
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Ethernet Relay Service (ERS): A point-to-point VLAN-based E-line service that is used primarily for establishing a point-to-point connection between customer routers
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Ethernet Wire Service (EWS): A point-to-point port-based E-line service that is used primarily to connect geographically remote LANs over a P-network
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Ethernet Multipoint Service (EMS): A multipoint-to-multipoint port-based emulated LAN (ELAN) service that is used for transparent LAN applications
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Ethernet Relay Multipoint Service (ERMS): A multipoint-to-multipoint VLAN-based ELAN service that is used primarily for establishing a multipoint-to-multipoint connection between customer routers
Metro Ethernet services are characterized by the UNI and Ethernet Virtual Circuit (EVC) attributes. EVCs can be point-to-point or point-to-multipoint services. Some UNIs can support multiple EVCs. The EPL, ERS, and EWS service types map to the E-line services defined by the MEF. The EMS and ERMS service types map to the ELAN services defined by the MEF.
Cisco Ethernet Services also include Layer 3 MPLS VPN services, which may be based on Ethernet or other underlying transport technologies. Figure 4-7 provides an illustrated overview of Cisco Ethernet-based services.
Ethernet services can be used in conjunction with Ethernet switches or with routers. For organizations with the skills and interest for managing their own routing, Layer 2 Ethernet connectivity provides routing-neutral connectivity similar to that of leased lines, Frame Relay, and ATM circuits. One potential difference of service provider Ethernet services is that using multipoint Ethernet could vastly increase the number of routing peers in the organization.
When implementing service provider Ethernet services, customers must decide whether they want to outsource routing to the service provider, or do their own routing. Outsourced routing, or routing in cooperation with the service provider, is typically done using Layer 3 MPLS VPNs.
| Note | Metro Ethernet switching and large multipoint router-based architectures have design and scalability implications. |
Ethernet Private Line Service
An EPL service is a dedicated point-to-point connection from one customer-specified location to another, with guaranteed bandwidth and payload transparency end to end.
EPL typically uses SONET/SDH transport. Because the bandwidth is dedicated with no oversubscription, a simple SLA concerning uptime may support all the customer requirements. SONET protection can provide high availability for EPL service.
The Cisco EPL service is ideal for transparent LAN interconnection and data center integration, for which wire-speed performance and VLAN transparency are important. Whereas TDM and Optical Carrier (OC)-based facilities have been the traditional means of providing EPL service, the Cisco EPL service also supports DWDM/CWDM, Ethernet over SONET/SDH, and dedicated Ethernet platforms interconnected via fiber.
The EPL service is typically used for the following:
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Mission-critical links
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Mainframe-to-mainframe links
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Data center or storage-area network (SAN) extension links
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Business continuity links
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Network consolidation—joining sites in MAN
| Note | Organizations need to be careful of provider-managed CE devices handling the speed transitions from the customer to the P-network, because the provider-managed CE devices may impede the organization from being able to easily implement their own QoS policies. |
Ethernet Relay Service
Cisco ERS is a point-to-point VLAN-based E-line service that supports service multiplexing, where multiple instances of service or EVCs can be multiplexed onto a single customer UNI.
Service multiplexing means that many connections can be provided over one link. The multiplexed UNI supports point-to-point or point-to-multipoint connections between two or more customer-specified sites, similar to a Frame Relay service. Instead of the data-link connection identifier (DLCI), the connection identifier in ERS is a VLAN tag. Each customer VLAN tag is mapped to a specific Ethernet virtual connection.
| Note | ERS uses the VLAN to indicate destination. Therefore, the Ethernet service is not transparent to Layer 2 Ethernet frames—the VLAN tag dictates destination. The ERS EVC does not act like a trunk where all VLANs go from one site to one or multiple sites. |
ERS uses different point-to-point VLANs to connect one site to other remote sites.
| Note | If multipoint connections are available, the service is referred to as an ERMS. |
Service multiplexing provides scalability for large sites, minimizing the number of Ethernet connections to the MAN or WAN Ethernet service. A router is typically the customer premise device.
ERS also provides Ethernet access through service interworking to other Layer 2 services, such as Frame Relay and ATM, so that the customers can begin using Ethernet services without replacing their existing legacy systems. With service interworking, traffic on a DLCI or virtual path identifier / virtual channel identifier (VPI/VCI) at a remote site is converted to an Ethernet frame by the provider, and arrives within a VLAN at headquarters.
The provider may offer tiers of service, based on bandwidth, CoS, and distance. A typical SLA might be based on committed information rate (CIR) or peak information rate (PIR), burst capacity, and packet-loss rate.
ERS is ideal for interconnecting routers in an enterprise network, and for connecting to Internet service providers (ISP) and other service providers for direct Internet access, VPN services, and other value-added services. Service providers can multiplex connections from many end customers onto a single Ethernet port at the service provider’s point of presence (POP) for efficiency and ease of management.
Ethernet Wire Service
The Cisco EWS is a point-to-point connection between a pair of sites. Cisco EWS differs from Cisco EPLS in that it is typically provided over a shared, switched infrastructure within the service provider network that can be shared among customers. Oversubscription of the service provider network is handled using stat muxing. The benefit of EWS to the customer is that it is typically offered with a wider choice of committed bandwidth levels up to wire speed. To help ensure privacy, the service provider segregates each subscriber’s traffic by applying VLAN tags on each EVC, typically using queue-in-queue (QinQ) tunneling. Customer SLA capabilities are typically based on CoS.
EWS is considered a port-based service. With EWS, the carrier network is transparent to all customer Ethernet traffic. EWS provides all-to-one bundling, where all customer packets are transmitted to the destination port transparently and the VLAN tags from the customer are preserved through the P-network. The CE device might be a router or a switch.
EWS is commonly used for point-to-point LAN extension, access to storage resources, and data center connectivity.
Ethernet Multipoint Service
EMS is a multipoint-to-multipoint service that is typically provided over a shared, switched infrastructure within the P-network.
EMS is a multipoint version of EWS, and shares the same technical access requirements and characteristics.
In EMS, the P-network acts as a virtual switch for the customer, providing the ability to connect multiple customer sites and allow for any-to-any communication. The enabling technology is Virtual Private Line Service (VPLS), implemented at the NPE. The service provider can use rate limiting to minimize the impact of a customer broadcast storm on other customers.
Oversubscription of the P-network is also handled with EMS using stat muxing. EMS is typically offered to the customer with a choice of committed bandwidth levels up to wire speed. To help ensure privacy, the service provider segregates each subscriber’s traffic by applying VLAN tags on each EVC, typically using QinQ tunneling. Customer SLA capabilities are typically based on CoS.
EMS provides all-to-one bundling, where all customer packets are transmitted to the destination ports transparently and the VLAN tags from the customer are preserved through the P-network. The CE device might be a router or a switch.
For example, Verizon Transparent LAN Services (TLS) is a commercial EMS. It is based on a loop-free topology using Cisco 6500 switches and fiber-based Gigabit Ethernet links between them. TLS uses 802.1Q QinQ encapsulation to maintain customer traffic separation.
EMS is commonly used for multipoint LAN extension, LAN extension over the WAN, and disaster recovery.
Ethernet Relay Multipoint Service
ERMS is a hybrid of EMS and ERS.
ERMS offers the any-to-any connectivity characteristics of EMS, and the service multiplexing of ERS. This combination enables a single UNI to support a customer’s intranet connection and one or more additional EVCs for connection to outside networks, ISPs, or content providers. Some EVCs might be point to point, and others might be multipoint. The service provider can use rate limiting to minimize the impact of a customer broadcast storm on other customers.
ERMS can be used for many applications, including branch Layer 2 VPNs, Layer 3 VPNs for intranet and extranet access, Internet access through the ISP, and disaster recovery.
End-to-End QoS
Metro Ethernet offerings should provide end-to-end QoS across the network.
A service provider can use IEEE 802.1Q tunneling to support end-to-end QoS for their customers. In Figure 4-8, the CE device is connected to the service provide UPE device using 802.1Q. The CE device adds an 802.1Q tag to all frames and supports the CoS across the network. The UPE devices add a second 802.1Q frame to support QinQ encapsulation of the customer traffic. Depending on the agreement with the service provider, the type of service (ToS) can be extended across the network. The two 802.1Q tags can be seen in the frame in the middle of the chart. The outer 802.1Q tag added by the UPE acts as a customer ID.
Switches and other devices in the service provider backbone transport the encapsulated Ethernet frame based on the outer 802.1Q tag and ToS. The outer 802.1Q tag is stripped off when the frame reaches the destination or destinations indicated in the outer tag. At the remote UPE, the Ethernet frame is transparently forwarded based on the original CE 802.1Q tag with the original CoS.
The destination MAC is preserved end to end, so multicast traffic will be seen by the provider network as having a multicast destination MAC address. If the service is point to multipoint, one multicast frame sent into the provider network will be received at multiple customer sites, in accordance with multicast flooding within a VLAN.
| Note | If any remote site receives a multicast stream, the stream will flood to all sites in that VLAN. |
Because the service providers do not need to coordinate the customer VLANs with QinQ encapsulation, the customer VLANs can be preserved across the network, and the network supports VLAN transparency. With the QinQ encapsulation, customer VLANs can overlap.
An example of the 802.1Q encapsulation technique is a large service provider using Ethernet over MPLS to break up VLAN domains with a routed domain in the middle.
Choosing the Right Service
Figure 4-9 shows a decision tree a customer could use to help choose the appropriate Metro Ethernet service. For example, customers needing only point-to-point service can use EPL, EWS, or ERS; whereas customers needing multipoint services should use EMS or ERMS.
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