Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
You could configure DSW2 as the root bridge for virtual LAN (VLAN) 20 to fix the problem and maintain the maximum amount of redundancy in the design. In this scenario, the switches are configured to use Per- VLAN Spanning Tree Plus (PVST+). PVST+ is a revision of the Cisco-proprietary Per-VLAN Spanning Tree (PVST), which enables a separate spanning tree to be established for each VLAN. Therefore, a per- VLAN implementation of STP, such as PVST+, enables the location of a root switch to be optimized on a per-VLAN basis.
Because all of the VLANs use DSW1 as the root bridge in this scenario, all traffic from the access layer switches, regardless of VLAN, flows first to DSW1. Traffic from VLAN 10 is therefore already optimized because VLAN 10 uses DSW1 as its default gateway. However, VLAN 20 uses DSW2 as its default gateway. Therefore, traffic from VLAN 20 will most likely flow first to DSW1 and then across the PortChannel 1 EtherChannel interface to DSW2 for forwarding.
Although configuring DSW2 as the root bridge for all VLANs would cause traffic from VLAN 20 devices to flow directly to DSW2, traffic from VLAN 10 devices would no longer flow in an optimized fashion.
Configuring DSW2 as the root bridge for all VLANs would cause traffic from VLAN 10 devices to flow first to DSW2, then across the PortChannel 1 EtherChannel interface to DSW1 for forwarding. Because PVST+ is configured in this scenario, it is not necessary and not optimal to deploy a single root bridge for all VLANs.
You should not configure DSW2 as the root bridge for VLAN 10, because traffic from VLAN 10 is already flowing in an optimized fashion. Configuring DSW2 as the root bridge for VLAN 10 would cause traffic from VLAN 10 devices to flow first to DSW2 and then across the PortChannel 1 EtherChannel interface to DSW1 for forwarding.
You should not disable any links in the topology. Disabling links would not preserve the maximum amount of redundancy in the design. Like Spanning Tree Protocol (STP), PVST+ blocks redundant ports until a configuration change occurs that would allow the redundant port to enter the forwarding state without creating a switching loop.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 3, STP Design Considerations, pp. 101-103 Cisco: InterSwitch 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