Thursday, May 24, 2012

Future of STP, and Layer2 Multipathing?

Many changes are happening in data center architectures because of various Layer2 problems.  Layer2 Multipathing seems to be the new buzzword that people are talking about to solve these problems.  So I have been doing a bit of research to familiarize myself with terms and the technology in theory.

Layer2 Multipathing is essentially a replacement to the STP 802.1D protocol that has ruled the Ethernet switch market for a very long time.  Essentially, what L2 Multipathing does is turn switching in to link state routing.  This is needed because of the scalability problems, suboptimal path selections, and inefficient use of network links with STP as we know it today.  That's right, no more BPDU, or mac flooding for switches to learn there place in the network.

There are two flavors of Layer2 Multipathing that are gaining traction, TRILL, and SPB.  TRILL with stands for Transparent Interconnection of Lots of Links and was developed by the IETF. SPB or Shortest Path Bridging was developed by the IEEE and is designated 802.1aq.

TRILL uses the routing protocol IS-IS to calculate a layer 2 path through the LAN.  Any Layer2 frame a switch running TRILL (known as Rbridges) receives at its edge is encapsulated within a TRILL frame.  A lookup of a destination Rbridge is preformed and the frame is sent accross the TRILL domain to that Rbridge.  Once the frame is received it de-encapsulates and passed to the end host.  Rbidges learn where each other are in the network via hellos via IS-IS, and IS-IS calculates the shortest path between each Rbridge.

SPB again utilizes IS-IS to determine a shortest path through the LAN, and a switch that runs SPB builds a Shortest Path Tree or SPT to determine the optimal forwarding path.  Essentially this is a MAC-in-MAC (Like PBB) or a Q-in-Q like solution.  Designated MAC or VLAN (I think of it as a Provider VLAN or the outer tag) is assigned for each switch and frames received are encapsulated within it, the SPT is looked up for the destination, and the frame switched(or forwarded) to the edge switch which again de-encapsulated and the frame is sent to the end device.

As can be seen, both protocols are very similar in the way they work but the encapsulation protocols are very different in operation.  But as with routed protocols, we can now load balance frames across muliple equal cost links, we can preform PBR type functions, or influence forwarding decisions just as if it was routing.  The TRILL and SPB frames also keep track of hop counts to implement an IP like loop prevention/broadcast storm control.  I am very excited to see how these protocols develop among the vendors, and how it will be implemented within both the data center and with service provider environments.

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