The increasing popularity of Metro Ethernet is the result of the cost effectiveness, availability and seamless protocol transition it offers from the campus network to the WAN. Ethernet is already optimized for use with the TCP/IP protocol. There is no WAN protocol transition (handoff) to another WAN protocol such as SONET or Serial T3 links for example. With Metro Ethernet, there is no need to buy and manage a WAN router. Most enterprise companies have already deployed high speed Cisco routers and network switches with GE and 10 GE interfaces. Those same interface can be used for Metro Ethernet services. In addition there is the Cisco 3750M and 3400ME switches optimized and designed for Metro Ethernet services. The comparative cost per Mbps for a Metro Ethernet circuit is astoundingly cost effective compared with T1, T3 circuits and SONET. The disadvantage of Metro Ethernet is with long haul WAN connectivity. Most service providers offer services for state and multi-state connectivity. The service providers have deployed SONET for years and use that with Metro Ethernet for network connectivity on a national scale. Metro Ethernet and DWDM technology make it a viable alternative to SONET for long haul circuits as service providers continuing to increase fiber network infrastructure across the country.
Metro Ethernet Components
The Metro Ethernet network include customer edge (CE) devices, a user network interface (UNI), service provider edge equipment (PE) and the provider core (PC) network devices.
Customer Edge (CE)
The Customer Edge (CE) is a router or switch with an Ethernet interface preferably optimized for Metro Ethernet services. Some companies now deploy Metro Ethernet from a Cisco 6500 switch Gigabit interface. The Gigabit port bandwidth is then configured with QoS traffic shaping to limit bandwidth according to the service provider CIR selected by the company as part of the service level agreement.
User Network Interface (UNI)
This is the demarcation point between what the customer manages and what the service provider manages. The management of various network services is defined by the UNI interface to the Metro Ethernet service provider. Those services include QoS, security, encryption, etc.
Provider Edge (PE)
The provider edge equipment is typically a carrier class device that aggregates hundreds of Metro Ethernet connections from customers onto a DWDM or SONET network for long haul connectivity
Provider Core (PC)
The provider core network is a high speed DWDM or SONET switched network that aggregates and forwards packets from thousands of customers using a carrier class DWDM switched core. There is typically no quality of service added to customer packets. The customer network stream is tunneled across the core using a private VLAN where packets are forwarded to the provider edge router. The provider edge router then forwards packets to the customer edge router or switch.
Metro Ethernet Service Types
Native Ethernet Service
The Native Ethernet service is comprised of Layer 2 end to end native Ethernet signaling over the Metro Ethernet network where available. There is no transition of packets to any other WAN protocol and no VLAN encapsulation required. The use of native Ethernet services is mostly available for metropolitan connectivity. As the distance between offices increases, the use of a WAN transport such as SONET must be used. Traffic shaping or rate limiting of packets to the CIR should be done at the customer edge Ethernet interface to make sure packets are not dropped by the service provider.
Ethernet Private Line (EPL)
The Ethernet Private Line service is used to deploy private line WAN connectivity across the Metro network. Typically the Ethernet service will forward packets to a long haul SONET network where Ethernet packets are encapsulated in SONET frames.
The Ethernet packets are stripped off (de-encapsulated) at the SONET Provider Edge (PE) equipment and forwarded to the local Metro service provider network. The Ethernet private line is similar to any WAN link where VLAN information isn’t sent between routers. The service provider does the rate limiting of traffic based on the CIR selected by the company. The CIR is the guaranteed data rate service level agreement with the ISP. Traffic shaping or rate limiting of packets should be done at the Customer Edge (CE) Ethernet interface to make sure packets are not dropped by the service provider. Some other options for transport include Packet over SONET (PoS) and Resilient Packet Ring (RPR).
Ethernet Virtual Private Line (EVPL)
The Ethernet virtual private line service is deployed for trunking of multiple VLANs across a Metro network. The 802.1q encapsulation protocol is the new standard that works with Cisco and other vendor equipment. The customer edge device uses 802.1q protocol to tag each Ethernet packet with VLAN membership before forwarding it across the virtual point to point Metro link. QoS is applied at the Customer Edge (CE) Ethernet interface using per VLAN or per Class per VLAN traffic shaping.
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