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25 February 2019

Data Center Bridging (DCB)



I wrote about Bridging, Provider Bridging, Provider Backbone Bridging (PBB) and Shortest Path Bridging (SPB) two years ago when I was teaching these technologies to a group of network administrators. These technologies are useful for Cloud Service Providers or Internet Service Providers. However, I want to write today about Data Center Bridging (DCB) which is useful for most Data Centers where there is a high demanding Ethernet traffic such as virtual SAN (vSAN) or Fibre Channel over Ethernet (FCoE) traffic.

Data Center Bridging is a set of open standards Ethernet extensions developed through the IEEE 802.1 working group to improve clustering and storage networks. These extensions are Priority-based Flow Control (PFC), which is included in the 802.1Qbb standard; Enhanced Transmission Selection (ETS), which is included in the 802.1Qaz standard; Data Center Bridging Capabilities Exchange Protocol (DCBX), which is included in the 802.1az standard; and Congestion Notification, which is included in the 802.1Qau standard.

The first one, Priority-based Flow Control (PFC), is going to create eight virtual links on the physical link where PFC provides the capability to use pause on a single virtual link without affecting traffic on the other virtual links. Pauses will be enabled based on user priority or classes of service. This extension allows administrators to create lossless links for traffic requiring no-drop service, such as vSAN or FCoE, while retaining packet-drop congestion management for IP traffic.

Priority-based Flow Control

The second extension is Enhanced Transmission Selection (ETS) which provides bandwidth management between traffic types for multiprotocol links. For instance, a virtual link can share a percentage of the overall link with other traffic classes. Therefore, ETS is able to create priority groups and it allows differentiation between traffic of the same virtual link.

Enhanced Transmission Selection
 
These two extensions, PFC and ETS, have to be configured in switches and endpoints. This configuration can be deployed easily with Data Center Bridging Capabilities Exchange Protocol (DCBX). The third extension is going to exchange the configuration between devices. It is going to exchange parameters such as priority groups in ETS, congestion notification, PFC, Applications, etc. In addition, DCBX is able to discover peers and detect mismatched configuration.

DCBX Deployment Scenario
 
The last extension, which is optional and not required into the Data Center Bridging architecture, is Congestion Notification. This extension is useful for actively managing traffic flows and, thus, avoid traffic jams. It’s interesting because an aggregation-level switch with this feature can send control frames to access-level switches asking them to throttle back their traffic and, therefore, rate limit policies for congestion can be enforced close to the source.

Congestion Notification
 
These four specifications (PFC, ETS, DCBX and Congestion Notification) improve clustering and storage networks as well as responding to future data center network needs such as the new Hyper-Convergence Infrastructure of VMware vSAN or Nutanix.

Keep reading and keep learning!!

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