Addition of a Link Between SW-D1 and SW-D2
The link between SW-D1 and SW-D2 continues the VLAN numbering convention by using VLAN 906, and a dynamic routing neighbor relationship is formed between SW-D1 and SW-D2 using this link. As a result, when the link between the SW-A2 and SW-D1 is lost, traffic bypasses the core switches and is routed via the distribution layer. Seeing this new traffic flow, administrators may wonder why any design would exclude the link between SW-D1 and SW-D2. Although there is certainly nothing wrong with adding the link, it does add cost to the design because each pair of distribution switches will require an extra connection, and is really only useful in a scenario where an access layer link has failed or when a device is single attached to a distribution layer switch. Access layer failures should be rare, and single-attached devices to the distribution layer should be avoided whenever possible.