Introduction

Recently, a number of techniques have emerged in the industry to transport Carrier Ethernet services. One such technology is called Provider Backbone Bridging — Traffic Engineering (PBB-TE), which addresses several existing challenges of natively extending Ethernet services across a provider’s network. Until now, other technologies such as SONET/SDH or MPLS have been required to build large-scale networks. However, many carriers are actively seeking native Ethernet networks to reduce operational and capital costs while enabling them to efficiently deliver a wide range of existing and emerging applications and services. Due to its ubiquity and ease-of-use, Ethernet, in the form of PBB-TE, is positioned to capitalize on this sizeable opportunity of delivering and transporting Carrier Ethernet services.

Carrier Ethernet Services

The telecommunications industry has experienced continued erosion of legacy technologies (e.g., FR, TDM) that are being supplanted by Carrier Ethernet. Defined by the Metro Ethernet Forum (a 100-member organization that works with standards bodies such as the IEEE and ITU-T) Carrier Ethernet represents an exciting new flavor of Ethernet. Five specific service provider-influenced attributes distinguish it from traditional enterprise-class Ethernet.

• Standardized Services
• Quality of Service
• Service Management
• Reliability
• Scalability

One thrust of MEF members is to reduce complexity and simplify the operation of provider networks. Transitioning from complex routed architectures and protocols, to an easier-to-use switch-based approach has advantages and, admittedly, some drawbacks, particularly in the area of scalability and reliability. PBB-TE aims to address these shortcomings.

Legacy Ethernet’s Scalability Limitations

Traditional Ethernet switch-based networks have two defining characteristics that bound their topological size, namely learning and loop avoidance. When a switch receives a packet destined for an end station it does not yet know about, it replicates the packet down every link on which it is connected (known as flooding). As the switch inspects every packet it receives, the switch remembers or learns the association of sending stations and ingress links. Each switch in the layer two domain learns the address and associated link for every device in the network.

While many metro and core devices may be able to support hundreds of thousands of addresses, requiring each device in a provider’s network to handle this number of addresses is cost prohibitive and impacts protection switching schemes. When a link or device experiences a failure, the network must react to the changing topology. Often, as the number of addresses increases, the time to failover and restore network connectivity increases.

One alternative is to introduce routers, which segment the layer two network into multiple sub-networks (subnets). While providing a level of hierarchy, routing also introduces several complex and difficult to operate protocols (e.g. border gateway protocol), requires more sophisticated configuration and operation, expensive hardware and ever faster processors. In addition, routing does not provide the same level of service transparency as switch-based Ethernet service transport. For these reasons, the industry has devoted resources to enhancing several aspects of switch-based Ethernet networks.

Briefly mentioned earlier, layer two networks routinely flood traffic to unknown destinations. Many mesh, hub-and-spoke, and ring topologies contain physical loops in the form of redundant and sometimes inadvertent connections between devices. These loops must be logically prevented in order for flooded traffic to propagate through the network properly. If loops were allowed to remain, the flooded traffic would replicate and multiply, wreaking havoc on the network. For these reasons, Spanning Tree Protocol (and the later enhancements known as Multiple Spanning Tree Protocol and Rapid Spanning Tree Protocol) was invented to detect and eliminate these loops.

The figure below shows a provider network with a mesh interconnecting several Ethernet switch-based devices known as Provider Bridges. Provider Bridges are under the control of the provider. These devices support one or more Ethernet service transport technologies, such as the standard IEEE 802.1ad Provider Bridges (PB). As shown below, the physical topology, while providing some redundancy, contains several loops.

The figure below shows the same provider network with IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) detecting the loops and logically blocking a few of links. Now, no loops exist in the network. Switch-based flooding and learning occur normally. The blocked links remain on standby in the event an active link or device experiences a fault.

The figure below depicts the path a Carrier Ethernet service may take across the provider network. RSTP blocked links are avoided and all of the customer locations are interconnected.

While moderate scale provider networks are possible using RSTP and Multiple Spanning Tree Protocol (MSTP), some carriers remain skeptical about the performance during failover situations. Larger networks have significantly more links and MAC addresses to manage straining the capability of RSTP and MSTP.

World Wide Packets LightningEdge® solution is proven to support standards-based RSTP failover and restoration times of sub-50 ms for many PB configurations.

Carriers want to minimize, and avoid if possible, sources of service disruption. PBB-TE represents one approach for addressing these concerns.

• Part 1:
  Introduction
• Part 2:
  Customer Service Delivery and Transparency
• Part 3:
  Carrier Ethernet Transport Scalability and Resiliency
• Part 4:
  Summary