Metro Ethernet - MPLS-based Ethernet MANs

MPLS-based Ethernet MANs

An MPLS based Metro Ethernet network uses MPLS in the Service Provider's Network. The subscriber will get an Ethernet interface on Copper (ex:-100BASE-TX) or fiber (ex:-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.

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

One of the restoration mechanisms used in an MPLS based Metro Ethernet Networks is Fast ReRoute (FRR) to achieve sub-50ms convergence of MPLS local protection. For each deployment situation the benefit versus cost of MPLS must be weighed carefully, so if not implemented on a carrier's distribution network there might be more benefit for MPLS the core network. In some situations the cost may not warrant the benefits, particularly if sub 50ms convergence time is already being achieved with pure Ethernet.

A comparison of MPLS-based Metro Ethernet against a pure Ethernet MAN:

• Scalability: In a properly designed Ethernet VLAN network, each switched path can have 4094 single tag VLANs. Some aggregation and core switches can classify traffic by two VLANs using IEEE 802.1ad VLAN stacking, so with such aggregation devices properly placed in the center of a network, end segments and rings of single tag devices can receive only the traffic that they need. When using MPLS, Ethernet VLANs have local meaning only (like Frame Relay PVC). Same scalability considerations apply to the MAC addresses where in a pure Layer 2 Ethernet MAN all MAC addresses are being shared across the network, although this issue can be managed by smart network design and choosing switches with MAC tables sufficient for the size of network segments.
• Resiliency: pure Ethernet network resiliency relies on Spanning Tree Protocols STP, IEEE 802.1w RSTP or IEEE 802.1s MSTP (30 to sub 50ms sec convergence depending on network design) while MPLS-based MANs use mechanisms such as MPLS Fast Reroute to achieve SDH-like (50 msecs) convergence times. Metro Ethernet can also utilise Link aggregation or Resilient Packet Ring where appropriate to add link redundancy and recovery in distribution networks. Some Ethernet vendors' RSTP convergence is also sub-50ms, but this convergence time may vary from vendor to vendor. Ethernet protection switching is also standardised in (ITU G.8031).
• Multiprotocol convergence: with the maturity on pseudowires standards (ATM Virtual Leased Line VLL, FR VLL, etc.) an MPLS-based Metro Ethernet can backhaul IP/Ethernet traffic together with virtually any type of traffic coming from customer or other access networks (i.e. ATM aggregation for UMTS or TDM aggregation for GSM), while this could be more challenging in a pure Ethernet scenario.
• End to End OAM: MPLS-based MAN offers a wide set of troubleshooting and OAM MPLS-based tools which enrich Service Providers ability to effectively troubleshoot and diagnose network problems. These include for example, MAC ping, MAC traceroute, LSP ping etc. However there are now Ethernet OAM tools defined in IEEE 802.1ab, IEEE 802.1ag and Ethernet in the First Mile (IEEE 802.3ah) for monitoring and troubleshooting Ethernet networks. EOAM (Ethernet Operations, Administration, and Maintenance) is a protocol for installing, monitoring, and troubleshooting MANs and WANs.