Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
Most likely, the packet will travel from ASW3 to DSW2, to DSW1, and then to Router1. Because all of the virtual LANs (VLANs) use DSW2 as the root bridge in this scenario, all traffic from the access layer switches, regardless of VLAN, flows first to DSW2. Traffic from VLAN 20 is therefore already optimized because VLAN 20 uses DSW2 as its default gateway. However, VLAN 10 uses DSW1 as its default gateway. Therefore, traffic from VLAN 10 will most likely flow first to DSW2 and then across the PortChannel 1 EtherChannel interface to DSW1 for forwarding.
Because every access layer switch has been physically connected to every distribution layer switch in this scenario, the topology contains a high level of redundancy. In addition, the use of Rapid Per-VLAN Spanning Tree Plus (RPVST+) ensures that the network will converge quickly. However, if you were to configure the root bridge on a per-VLAN basis, the network could be further optimized because the location of the root switch could be optimized on a per-VLAN basis. For example, configuring DSW1 as the preferred root bridge for devices that operate on VLAN 10 would cause VLAN 10 traffic from ASW1, ASW2, or ASW3 to flow directly to DSW1 for forwarding to Router1. VLAN 20 traffic would remain optimized to flow directly to DSW2 from ASW1 or ASW3.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 3, STP Design Considerations, pp. 101-103 Cisco: Inter-Switch Link and IEEE 802.1Q Frame Format: Background Theory Cisco: Catalyst 3750X and 3560X Switch Software Configuration Guide, Release 12.2(55)SE: Configuring the Switch Priority of a VLAN