j2sw Blog

As networking trends yo-yo between layer-3 and layer-2 centric different protocols have emerged. Protocols such as Transparent Interconnection of Lots of Links (TRILL), Shortest Path Bridging (SPB), and Virtual Extensible LAN (VXLAN) have emerged to address the need of scalability at Layer2.   Cloud scalability, spanning tree bridging issues, and big broadcast networks start to become a problem in large data center or cloud environments.

To figure out if things like TRILL is a solution for you, you must understand the problem that is being addressed by TRILL. The same goes for the rest of the mentioned protocols. When it boils down to it the reason for looking at such protocols is you want high switching capacity, low latency, and redundancy.  The current de facto standard of Spanning Tree Protocol (STP) simply is unable to meet the needs of modern layer2 networks.  TRILL addresses the problem of STP’s ability to only allow one network path between switches or ports.  STP prevents loops by managing active layer -2 paths.   TRILL applies Intermediate System-to-Intermediate System protocol (IS-IS), which is a layer3 routing protocol translated to Layer 2 devices.

For those who say TRILL is not the answer things like SPB also known as 802.1aq, and VXLAN are the alternative. A presentation at NANOG 50 in 2010 addressed some of the SPB vs TRILL debate. This presentation goes into great detail on the differences between the two.

The problem, which is one most folks overlook, is that you can only make a layer 2 network so flat.  The trend for a while, especially in data centers, is to flatten out the network. Is TRILL better? Is SPB better? The problem isn’t what is the better solution to use.  What needs to be addressed is the design philosophy behind why you need to use such things.   Having large Layer2 networks is generally a bad idea. Scaling issues can almost always be solved by Layer-3.

So, and this is where the philosophy starts, is TRILL, SPB, or even VXLAN for you? Yes, but with a very big asterisk. TRILL is one of those stop gap measures or one of those targeted things to use in specific instances. TRILL reduces complexity and makes layer-2 more robust when compared to MLAG. Where would you use such things? One common decision of whether to use TRILL or not comes in a virtualized environment such as VSPHERE.

Many vendors such as Juniper, have developed their own solutions to such things.  Juniper and their Virtual Chassis solution does away with spanning tree issues, which is what TRILL addresses.   Cisco has FabricPath, which is Cisco’s proprietary TRILL based solution. Keep in mind, this is still TRILL.   If you want to learn some more about Fabric Path this article by Joel Knight gets to the heart of Fabric path.

Many networks see VXLAN as their upgrade path.  VXLAN allows layer 2 to be stretched across layer 3 boundaries. If you are a “Microsoft person” you probably hear an awful lot about Network Virtualization using Generic Routing Encapsulation (NVGRE) which can encapsulate a layer two frame into IP.

The last thing to consider in this entire debate is how does Software Defined Networking (SDN) play into this. Many folks think controllers will make ECMP and MLAG easy to create and maintain. If centralized controllers have a complete view of the network there is no longer a need to run protocols such as TRILL.   The individual switch no longer makes the decision, the controller does.

Should you use Trill, VXLAN, or any of the others mentioned? If you have a large Layer-2 virtualized environment it might be something to consider.  Are you an ISP, there is very little case for running TRILL in anything other than your data center. Things such as Carrier Ethernet and MPLS are the way to go.