Customer Service Delivery and Transparency

One motivation for layer two virtual private networks (L2VPNs) has been the customer demand for interconnecting multiple sites. Customers want inexpensive, high-performance, transparent LAN services. In addition, most do not want to hassle with their switch or router configurations. They also resist exchanging route tables with carriers due to security concerns and operational complexity. Increasingly, customers desire to use Ethernet to natively interconnect their locations.

Connecting two sites creates an Ethernet-Line (E-Line) service utilizing a point-to-point Ethernet Virtual Connection (EVC) as shown below.

Customers with more than two locations want multi-site interconnectivity and would choose an E-LAN service, which supports multipoint-to-multipoint EVCs as depicted below.

Completed in December 2005, IEEE 802.1ad Provider Bridging is the first Ethernet bridging project expressly created for provider networks. PB standardizes the use of multiple Virtual Local Area Network (VLAN) tags in the same frame. The format of the 802.1ad Provider Bridge frame is shown below.

The existing fields of the customer frame are preserved. This allows a customer’s full 4K VLAN range to be transported seamlessly across a PB network to each of its other locations. Depicted below, Customer A interconnects its three locations using an E-LAN service. Customer B connects its two sites with an E-Line service. Both techniques provide secure, transparent L2VPNs across the PB network.

Each L2VPN enables complete customer separation. Both customers have complete freedom to use internal, customer VLANs (C-VLANs) as they choose. The provider configures up to 4K Service VLANs (S-VLANs) supporting up to 4K separate customers.

Often, the 4K maximum is not the limiting factor for the provider. Rather, the aggregate number of MAC addresses and/or the physical topology demands placed upon the RSTP and MSTP protocols force the provider to segment or use alternative transport facilities, such as PBB-TE.

Provider Bridge Network Quality of Service

While topology and address scaling are critical issues, an important development in IEEE 802.1ad PB is the inclusion of drop eligibility and packet marking capabilities. Rather than a fixed interpretation of the 3-bit priority field used by the legacy IEEE 802.1Q VLAN standard, PB allows a variety of priority code point (PCP) encodings. As shown in the following tables, four distinct priority/drop eligible interpretations are possible. For instance, 6P2D provides six classes of service with two of the classes supporting discard eligible (yellow) marking.

This table shows the usage of the PCP and drop eligibility fields.

IEEE 802.1ad Priority Code Point, Drop Eligibility Usage
		7	7DE	6	6DE	5	5DE	4	4DE	3	3DE	2	2DE	1	1DE	0	0DE
PCP, DE	8P8D	7	7	6	6	5	5	4	4	3	3	2	2	1	1	0	0
PCP	8P0D	7	7	6	6	5	5	4	4	3	3	2	2	1	1	0	0
	7P1D	7	7	6	6	5	4	5	4	3	3	2	2	1	1	0	0
	6P2D	7	7	6	6	5	4	5	4	3	2	3	2	1	1	0	0
	5P3D	7	7	6	6	5	4	5	4	3	2	3	2	1	0	1	0

This layer two coloring allows for efficient mechanisms to handle congestion situations without requiring inspection of layer 3 header information.

World Wide Packets LightningEdge solution supports dual-rate three color policing with random early discard (RED) enabling committed information rate (CIR) and excess information rate (EIR) service level assurance (SLA). Classification fields include ingress/egress port, S-VLAN/C-VLAN ID, PCP, IP precedence and DiffServ Code Point (DSCP). Selected devices also support flexible and powerful marking of either L2 or L3 fields based upon the classification and policing results.

An analysis of the benefits and limitations of PB will help in drawing comparisons to technologies that enable larger-scale Ethernet-based networks later.

Benefits	Limitations
Data plane	Data plane
Transparency of full 4K C-VID range Ability to determine layer-two drop eligibility Service PCP assigned by provider or determined by C-Tag Native support of E-LAN Services	4K services Topology constrained by number of aggregate connected devices PB devices learn all provider and customer MAC addresses Customer MAC addresses exposed on provider network Service ID derived from ingress port and C-VID
Control plane	Control plane
Separation of customer and provider control domains All customer layer two control protocols are transported through provider network	Sub-optimal provider network capacity due to RSTP/MSTP loop prevention

With the popularity, ease-of-use, and enhanced quality-of-service (QoS) features ensuring service predictability, more and more operators are moving to Carrier Ethernet. Despite some limitations, adoption is growing at a swift pace. Concerns about the inherent scalability issues are being alleviated by the promising MAC header encapsulation techniques.

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