Pure Ethernet MANs (Metropolitan Area Networks)

Written by: From Wikipedia - Aug 20, 2008

A pure Ethernet MAN uses only layer 2 switches for all of its internal structure. This allows for a very simple and cheap design, and also for a relatively simple initial configuration. The original Ethernet technology wasn't well suited for service provider applications; as a shared-media network, it was impossible to keep traffic isolated, which made implementation of private circuits impossible. Ethernet MANs became feasible in the late 90's 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 as VLAN Tunneling), and VLAN Translation, it became possible to isolate the customer's traffic from each other and from the core network internal signaling traffic. However, ethernet is constantly evolving and has now carrier class features with the recent addition of IEEE 802.1ad (Provider Bridges) and IEEE 802.1Qay (Provider Backbone Transport). Spanning-tree, broadcast packets and dynamic mac learning are disabled and sub 50ms failover features are introduced.

There are three main shortcomings with a pure non PBT/PB enabled Ethernet MAN approach:

By design, layer 2 switches use fixed tables to direct traffic based on the MAC address of the endpoints. As the network gets larger, the number of MAC address transiting through the network may grow beyond the capacity of the core switches. If the core table gets full, the result is a catastrophic loss of performance due to the flooding of packets over the entire network structure. Network stability is relatively fragile, especially if compared to the more advanced SDH and MPLS approaches. The recovery time for the standard spanning tree protocol is in the range of tens of seconds, much higher than what can be obtained in the alternative networks (usually a fraction of second). There are a number of optimizations, some standardized through the IEEE, and others vendor-specific, that seek to alleviate this problem. The clever use of such features allow the network to achieve good stability and resilience, at the cost of a more complex configuration and possible use of non-standard, vendor-specific, mechanisms. Traffic engineering is very limited. There are few tools to manage the topology of the network; also, the fact that forwarding is done hop-by-hop, added to the possibility of broadcasts even for unicast packets (for instance, while learning new addresses), makes predicting the real traffic pattern very difficult. There are techniques that allow for some control of the preferential traffic paths; these techniques rely on the use of multiple spanning trees, or "per VLAN spanning trees", and are closely connected to the solutions used to achieve better stability and resiliency in the network. Despite these shortcomings, non PBT/PB enabled Ethernet-based MANs are used for two primary purposes:

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 of IP and related protocols, such as BGP and MPLS , which are necessary for an MPLS-based deployment. In large scale Metro Ethernets, it's common for the access part of the network to use a pure layer 2 design. At this level, the pure layer 2 design is deemed to be cheaper while still operating under its design limitations. From the distribution layer and above, traffic is aggregated and routed using an MPLS-based Metro Ethernet design.