Spanning Tree And Redundant Connectivity - Cisco NCS 4200 Series Configuration Manual

Lan switching

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Configuring PVST+ and RPVST+

Spanning Tree and Redundant Connectivity

becomes the root switch, you force a spanning-tree recalculation to form a new topology with the ideal switch

as the root.

Figure 6: Spanning-Tree Topology

When the spanning-tree topology is calculated based on default parameters, the path between source and

destination end stations in a switched network might not be ideal. For instance, connecting higher-speed links

to an interface that has a higher number than the root port can cause a root-port change. The goal is to make

the fastest link the root port.

For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on Switch B

(a 10/100 link) is the root port. Network traffic might be more efficient over the Gigabit Ethernet link. By

changing the spanning-tree port priority on the Gigabit Ethernet port to a higher priority (lower numerical

value) than the root port, the Gigabit Ethernet port becomes the new root port.

You can create a redundant backbone with spanning tree by connecting two switch interfaces that are

participating in spanning tree to another device or to two different devices, as shown in the figure below.

Spanning tree automatically disables one interface but enables it if the other one fails. If one link is high-speed

and the other is low-speed, the low-speed link is always disabled. If the speeds are the same, the port priority

and port ID are added together, and spanning tree disables the link with the lowest value.

Figure 7: Spanning Tree and Redundant Connectivity

LAN Switching Configuration Guide, Cisco IOS XE Everest 16.6.1 (Cisco NCS 4200 Series)

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