R&M Data Center Handbook

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Shortest Path Bridging
In Shortest Path Bridging (IEEE 802.1aq) the Shortest Path Tree (SPT) is calculated for each individual node,
using the Link State Protocol (LSP). The LSP protocol is used to detect and show the shortest connection
structures for all bridges/switches in the SPB area. Depending on the SPB concept, MAC addresses for
participating stations and information on interface service affiliations are distributed to those stations that are not
participating. This SPB method is called SPBM, which stands for SPB-MAC. The topology data helps the
calculation computer to determine the most economical connections from each individual node to all other
participating nodes. That way each switch or router always has an overall view of all paths available between the
nodes.
In case of a change in topology, for example a switch is added or a link deactivated or activated, all switches must
re-learn the entire topology. This process may sometimes last several minutes and depends upon the number of
switches and links participating in the switchover process.
Nevertheless, Shortest-Path Bridging offers a number of advantages:
• It is possible to construct networks in the Ethernet that are truly meshed.
• Network loads are distributed equally over the entire network topology.
• Redundant connections are no longer deactivated automatically, but used to actively transport the data
that is to be transmitted.
• The network design becomes flatter and performance between network nodes increases.
Dynamic negotiation must be mentioned as one disadvantage of Shortest Path Bridging. Transmission paths in
this topology change very quickly, which makes traffic management and troubleshooting more difficult.
The first products to support the new “trill” and “SPB” switch mechanisms are expected by the middle of 2011. It
still remains to be seen whether present switches can be upgraded to these standards. Network design will
change drastically as a result of Shortest Path Bridging. Therefore one should prepare intensively for future
requirements and start to lay the groundwork for a modern LAN early on.
3.8.7. Data Center Bridging
The trend in the data center is moving toward convergent networks. Here, different data streams like TCP/IP,
FCoE and iSCSI are transported over Ethernet. However, Ethernet is not especially suited for loss-free transmission.
This is why Data Center Bridging (DCB) is being developed by the IEEE. Cisco uses the term “Data
Center Ethernet” for this technology. IBM uses the name “Converged Enhanced Ethernet”.
Since DCB can only be equipped with specialized hardware, upgrading switches that have been used up to this
point is not an easy matter. One must therefore make sure that DCB is supported when purchasing switches, for
purposes of investment security.
Four different standards are behind Data Center Bridging, for the purpose of drastically increasing network controllability,
reliability and responsiveness:
• Priority-based Flow Control (PFC, 802.1Qbb)
• Enhanced Transmission Selection (ETS, 802.1Qaz)D
• CB Exchange Protocol (DCBX, 802.1Qaz)
• Congestion Notification (CN, 802.1Qau)
All four elements are independent of one another. However, they only achieve their full effect when working
together:
• PCF enhances the current Ethernet stop mechanism. The current mechanism stops the entire
transmission, PCF only one channel. There are then eight independent channels for this one channel.
They can be stopped and restarted.
• DCBX requires switches, to exchange configurations with one another.
• ETS is used to define different priority classes within a PCF channel and to process it in different ways.
• CN stands for a type of traffic management that restricts the flow of traffic (rate limiting). A notification
service causes the source to lower its transmission speed. (optional)
DCB allows multiple protocols with different requirements to be handled over layer 2 in a 10 gigabit Ethernet infrastructure.
This method can therefore also be used in FCoE environments.
Up to this point, there are only three manufacturers who are known to support DCB: Cisco, Brocade and Blade
Network Technologies.
Page 108 of 156 © 08/2011 Reichle & De-Massari AG R&M Data Center Handbook V2.0