I’m not a car person and I don’t worry too much about what’s under the hood. That means that I’m just a car user, I only want to turn the ignition key and drive. In the Data Center world, the server team is typically a user of the network. Server guys don’t want to know how the network is implemented. They just want their VLANs to extend to the whole network so that they can connect their devices with no constraint, without having to worry about high availability, risk containment, link provisioning… network stuff. That’s precisely what FabricPath is designed to offer them: a network that looks like a single switch, the simplest networking entity. This “Fabric” offers efficient any-to-any connectivity with high bandwidth and low latency, all without having to understand how it works.
Of course, this user perspective is an abstraction. The following Figure 2 represents an example of the physical topology of the network, a Clos fabric, typical in Data Center environments. Note that this could just as well be a ring, a star, or even a network distributed across two sites. FabricPath turns an arbitrary topology into a Fabric and does not lock you into a particular model.
The network view is critical to the network admin. How could you manage this Fabric without exposing those links? How would you troubleshoot errors on a port? How would you enforce a security policy that would constraint traffic to a subset of the links? How would you provision bandwidth and prioritize traffic? However nice the dashboard might be, the mechanic cannot maintain a car without looking under the hood. Based on an open standard (TRILL), FabricPath provides full access to all the networking elements.
Finally, let’s get our hands dirty examining how traffic is switched in a network. In Figure 3, a host on switch 2 sends a frame to a peer on switch 4. When this frame reaches switch 1, a switching decision must be taken. This decision *has* to be made based on some information in the frame, there is no way around this process. Switch 1 extracts some bits from the frame, does a lookup on an internal table, and the result determines the exit port.
Now, what are those bits, where are they coming from? In a typical Layer 2 network, switches do a lookup on the destination mac address of the frame. Of course, some switches can perform more intelligent things and are capable of looking even further into the frame. However, if this deeper lookup does not really impact latency, it requires additional hardware resources and complex provisioning, making the solution more expensive.
Figure 4 illustrates how TRILL (and thus FabricPath) operates. When the frame enters the Fabric on switch 2, a full mac address lookup is performed. The result is “4”, a value that identifies a destination switch across the Fabric. This result is added to the frame when it is forwarded by switch 2 to switch 1. Instead of doing a full mac address lookup, switch 1 now only needs to parse this smaller switch ID (here 4) and do a lookup in an smaller internal routing table in order to identify the exit link (here L3). So the full processing is only done once at the edge, the devices inside the Fabric like switch 1 don’t even implement a mac address table.
So let’s summarize some of important properties of FabricPath described here:
- FabricPath does not perform any mac address lookup inside the Fabric. I could then pretend that FabricPath is performing a single switching decision at the edge.
- With the above and because FabricPath looks like a single switch from the perspective of its users, I could say that FabricPath is a single switch, and that the result is a 1-tier network.
- So if this Fabric is in fact a switch, performing a single switching decision, I could finally suggest that with FabricPath, every port is “directly” connected to all the others.
The messaging about 1-tier networks circulating in the industry is based on those above claims that could be applied to FabricPath. Do you believe in 1-tier networks now you see where they’re coming from? As a network engineer, I consider it’s just marketing buzz. Hopefully, having an opinion on this is not important. The technical aspects described here are real, and FabricPath’s performance and availability is no hype. That’s all that matters;-)
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