In a routed access hierarchical campus design, the access-to-distribution Layer 2 uplink trunks are replaced with Layer 3 point-to-point routed links. Why is it recommended that VLANs are confined on a single access switch rather than span across multiple access switches?
A. to allow for better convergence time
B. to prevent the occurrence of Layer 2 loops
C. to allow for fault isolation
D. to prevent routing black holes
Correct answer is ‘A’,
http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/HA_campus_DG/hacampusdg.html
Only span VLANs across multiple access layer switches if you must.
Throughout this document we have discussed the challenges with an environment in which VLANs span access layer switches. This design is less than optimal from a convergence perspective. If you follow the rules, you can achieve deterministic convergence. However, there are many opportunities to increase your availability and optimize convergence with alternative designs.