STP Root Path Cost

In this article we will discuss STP Root Path Cost, will make brief discussion on STP Root Path Cost, In last article we discuss about STP Root Bridge.

When the root bridge has been elected for the spanning tree instance, the STA starts the process of determining the best paths to the root bridge from all destinations in the broadcast domain.

The path information, known as the internal root path cost, is determined by summing up the individual port costs along the path from the switch to the root bridge.

Note: Switches send BPDUs, which include the root path cost. This is the cost of the path from the sending switch to the root bridge. When a switch receives the BPDU, it adds the ingress port cost of the segment to determine its internal root path cost.

The default port costs are defined by the speed at which the port operates. 10 Gb/s Ethernet ports have a port cost of 2, 1 Gb/s Ethernet ports have a port cost of 4, 100 Mb/s Fast Ethernet ports have a port cost of 19, and 10 Mb/s Ethernet ports have a port cost of 100.

Although switch ports have a default port cost associated with them, the port cost is configurable. The ability to configure individual port costs gives the administrator the flexibility to manually control the spanning tree paths to the root bridge.

In the example, switch port F0/1 has been configured with a port cost of 25 using the spanning-tree cost 25 interface configuration mode command on the F0/1 interface.

To restore the port cost to the default value of 19, enter the no spanning-tree cost interface configuration mode command.

The internal root path cost is equal to the sum of all the port costs along the path to the root bridge. Paths with the lowest cost become preferred, and all other redundant paths are blocked. In the example, the internal root path cost from S2 to the root bridge S1 over path 1 is 19 (based on the IEEE-specified individual port cost) while the internal root path cost over path 2 is 38. Because path 1 has a lower overall path cost to the root bridge, it is the preferred path. STP configures the redundant path to be blocked, which prevents a loop from occurring.

To verify the port and internal root path cost to the root bridge, enter the show spanning-treecommand. The Cost field near the top of the output is the internal root path cost, the total path cost to the root bridge. This value changes depending on how many switch ports must be traversed to get to the root bridge. In the output, each interface is also identified with an individual port cost of 19.

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