How the Spanning Tree protocol organizes an Ethernet network

Scalability of Layer 2

The topology shown in Figure 7 scales well, but some new challenges arise in large networks because of the possibilities of virtualization. Figure  8 shows a sample topology. Rack  1 and Rack 2 use the same VLAN, which enables the migration of a virtual machine from Rack 1 to Rack 2 on the fly using a tool such as vMotion by VMware. However, often there is only one VLAN available per rack to keep the broadcast domain small, despite a large number of virtual machines – as is the case with Racks 3 and 4.

Figure 8: Virtual environments also create new challenges for the network.

In addition to the restriction on the mobility of virtual machines in multiple-chassis EtherChannel, the restriction to two pairs of switches is a limiting factor. Added to this are the long convergence times for Spanning Tree – if several hundred VLANs are in use – recomputing the spanning tree if the root bridge fails still takes some time.

To address these challenges, some proprietary protocols allow topologies as shown in Figure 9. The foundation for this approach was laid down by the official IETF Trill (Transparent Interconnection of Lots of Links) standard. The Trill standard allows Equal Cost Multipathing in Layer 2 networks.

Figure 9: The Trill standard and Cisco's FabricPath protocol allow the aggregation of network connections.

Cisco FabricPath is also based on Trill but offers advanced options. For example, in FabricPath, each switch only learns the MAC addresses that it really needs – on classic Layer 2 networks and Trill, however, each switch stores all the MAC addresses. The scaled address tables in FabricPath result in better performance and avoid switches hitting the memory limits of their hardware.

All connections between the switches in Figure 9 can be operated with Cisco devices in FabricPath mode, which is activated through the switchport mode fabricpath command. The Fabric Shortest Path First protocol handles Layer 2 routing, with IS-IS (Intermediate System to Intermediate System) in the background. The influence of the spanning tree ends at this point – the connection is no longer about Ethernet connections but about FabricPath. However, connections to classical Ethernet switches are still possible.

Conclusions

The Spanning Tree protocol is a fundamental part of an Ethernet network. If you manage an Ethernet network with multiple switches, a good understanding of Spanning Tree will help you ensure stability and optimize performance.