Tuesday, October 20, 2009

802.1aq and TRILL

There are a number of drivers increasing demand for bandwidth in the data center:
  • Multi-core processors and blade servers greatly increase computational density, creating a corresponding demand for bandwidth.
  • Networked storage increases demand for bandwidth.
  • Virtualization and server consolidation ensures that servers are fully utilized, further increasing demand for bandwidth.
Virtual machine mobility (e.g. VMWare vMotion, Citrix XenMotion or Xen Live Migration) require a large, flat layer 2 network so that machines can be moved without reconfiguring their network settings. The increasing size of the layer 2 network, combined with the increasing demand for bandwidth challenges the scalability of current Ethernet switching technologies.

The diagram illustrates the problem. Currently, Ethernet uses the spanning tree protocol to determine forwarding paths. The tree structure forces traffic to the network core, creating a bottleneck. In addition, the tree structure doesn't allow traffic to flow on backup links, further limiting usable bandwidth. An alternative forwarding technique (used by routers), is to select shortest paths through the network. Shortest path forwarding allows traffic flows to bypass the core, reducing the bottleneck. The added benefit of shortest path forwarding is its ability to make use of all the links, including backup links, further increasing capacity.

There are two emerging standards for shortest path forwarding in switches:
  • TRILL (Transparent Interconnect of Lots of Links)
  • IEEE 802.1aq (Shortest Path Bridging)
Both TRILL and 802.1aq use IS-IS (Intermediate System to Intermediate System) routing to select shortest paths through the network. The protocols are very similar, but are being proposed by two different standards bodies: TRILL is being developed by IETF and 802.1aq by the IEEE.

It is surprising to see the IETF working on a LAN bridging protocol. The IETF is responsible for Internet protocols (TCP/IP, routing) and the IEEE is responsible for LAN protocols (802.11, Ethernet, bridging/switching). Adopting the IEEE 802.1aq standard makes the most sense, since it will ensure interoperability with the IEEE data center bridging standards being developed to support FCoE and facilitate data center convergence.

Finally, while more efficient network topologies will help increase network capacity, the days of relying on network over-provisioning are over. Much tighter control of bandwidth is going to be required in order to cope with converged data center workloads. Selecting switches that support the sFlow standard provides the visibility and control needed to manage the increasing demand for bandwidth.

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