FabricPath's biggest advantages over STP are in the areas of bandwidth and design versatility. STP provides redundancy and prevents loops, but it uses an active/standby model to do so. As a result, STP networks offer two potential paths for any flow, only one of which can forward traffic. Spanning tree designs further constrain bandwidth and increase complexity by requiring routers to move traffic between broadcast domains. The routers in turn add latency and require additional connections for redundancy.
In contrast, FabricPath creates a single switch fabric across all participating switches, increasing available bandwidth within a single layer-2 domain. FabricPath uses equal-cost multipath (ECMP) routing to distribute traffic across all available links, making it an active/active technology.
An expanded layer-2 domain also requires fewer layer-3 routing hops, reducing latency, and makes it simpler to migrate services such as VMware's Vmotion. And a larger layer-2 domain simplifies change management, since moving attached hosts no longer requires IP address and/or VLAN configuration changes.
FabricPath also reduces broadcast flooding and media access control (MAC) address table size, both well-known issues with large layer-2 networks. FabricPath switches use multicast rather than flooding to forward frames sent to unknown destinations, and compute routing tables with information learned from the fabric and source MAC addresses learned on each edge switch.
Moreover, using an approach called "conversational learning," switches populate MAC address tables only for ports actually involved in conversations. This differs from conventional switching, where switches see all flooded traffic within a broadcast domain and put every address into their MAC tables. In contrast, FabricPath switches don't need gargantuan MAC address tables, even when layer-2 domains encompass tens of thousands of hosts.
Cisco claims FabricPath scales up to 256 active paths, with each path comprising up to 16 links using link aggregation (or EtherChannel, in Cisco's terminology). We did not verify that claim; doing so would require nearly 10,000 test ports. However, we did verify two building blocks used to make this claim -- the ability of one switch to support 16 concurrently active paths, and the ability to support up to 16 links per path. (See our test methodology.)