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Understanding Spanning-Tree Instance Interfaces

 

An instance is analogous to one computer process. The 802.1Q standard defines one unique Spanning-Tree instance to be used by all VLANs in the network. STP runs on the Native VLAN so that it can communicate with both 802.1Q and non-802.1Q compatible switches. This single instance of STP is also referred to as 802.1Q Mono Spanning Tree or Common Spanning Tree (CST).

Benefits of Spanning-Tree Instance Interface Configuration

The interface mode allows RSTP, MSTP, and VSTP to converge faster than the original STP on point-to-point links. The protocol does not need to wait for timers on point-to-point links. Configure interfaces that have a point-to-point link to another Layer  2 bridge as p2p. This parameter is ignored if the STP is configured to run the original spanning-tree version.

If the interface (Spanning Tree) mode is configured at both the VSTP global and VLAN levels, the configuration at the VLAN level overrides the global configuration.

How Many Instances Do Spanning Tree Protocols Have?

STP and RSTP are limited to a single instance on any physical interface. Use the interface (Spanning Tree) statement to configure interfaces to participate in the STP or RSTP instance.

MSTP supports multiple instances on a single physical interface. Again, use the interface (Spanning Tree) statement to configure which logical interfaces participate in MSTP.

For VSTP, interfaces can be configured at the global level or at the VLAN level. Interfaces configured at the global VSTP level will be enabled for all the configured VLANs. If an interface is configured at both the global and VLAN levels, the configuration at the VLAN level overrides the global configuration.

Spanning-Tree Instance Interfaces Have Priorities

The reason that instances must have priorities is because a root port for a spanning tree is the interface on the nonroot bridge with the lowest path cost to the root bridge. When multiple interfaces have the same path cost to the root bridge, the interface with the lowest interface priority is selected as the root port.

If the interface priority is not configured and multiple interfaces have the same path cost to the root bridge, the interface with the lowest interface identifier is selected as the root port.

If the interface priority is configured under the MSTP protocol, this becomes the default value for all interfaces. If the interface priority is configured under the MSTI interface, the value overrides the default for that interface.

If the interface priority is configured at both the VSTP global and VLAN levels, the configuration at the VLAN level overrides the global configuration.

What is Spanning-Tree Instance Interface Cost?

The path cost used to calculate the root path cost from any given LAN segment is determined by the total cost of each link in the path. By default, the link cost is determined by the speed of the link. The interface cost can be configured to override the default cost and control which bridge is the designated bridge and which port is the designated port. In MSTP the CIST external path cost is determined by the link speed and the number of hops.

If the interface cost is not configured, the cost is determined by the speed of the interface. For example, a 100-Mbps link has a default path cost of  19, a 1000-Mbps link has a default path cost of  4, and a 10-Gbps link has a default path cost of  2.

If the interface cost is configured under MSTP, this becomes the default value for all interfaces. If the interface cost is configured under the MSTI interface, the value overrides the default for that interface.

If the interface cost is configured at both the VSTP global and VLAN levels, the configuration at the VLAN level overrides the global configuration.

Tip

The interface cost should be set the same for all interfaces connected to the same LAN segment.