ccna 7 chapter 2

ccna 7 chapter 2.Although switch ports all have a default port overhead associated with them, port overhead is configurable. The ability to configure individual port overheads gives administrators the flexibility to manually control the spanning tree path to the root bridge.To configure the port overhead for a port, enter spanning-tree cost value in interface configuration mode, with a value of 1. 200000000.In the example, switch port F0/1 has been configured with a port overhead of 25 using the spannlng-tree cost 25 interface configuration mode.To restore the port overhead to the default value of 19, you can enter the nospanning-tree costo in the interface configuration mode commandThe path overhead is equal to the sum of all port overheads on the path to the root bridge. The path with the minimum overhead value is called the preferred one, and all other redundant paths will be blocked. In the example, the path overhead from S2 to the root bridge S1 has an overhead of 19 on path 1 (based on the individual path overheads specified by IEEE) and an overhead of 38 on path 2. Since path 1 has the overall minimum overhead to the root bridge, it becomes the preferred path. STP then configures the other redundant paths to a blocking state to prevent loop generation.To verify the port and path overhead to the root bridge. enter the show spanning-tree command. The Cost field is the full path overhead to the root bridge. The value depends on how many switch ports must be traversed to get to the root bridge. In the output, each interface is also identified using a separate port overhead 19.6.2.4.5 802.1DBPDU Frame FormatThe Spanning Tree algorithm identifies the root bridge by switch BPDUs. The BPDU frame contains 12 different fields that are used to pass path and preferenceprecedence information that identifies the root bridge and the path to the root bridge. The first 4 fields are used to identify the protocol, version, message type, and status flag.