ccna exam fee hk

ccna exam fee hk.Figure 14-1 STP Port State Transition and Dwell TimeThe convergence time for STP typically takes 30.50 seconds. If the port is connected to a computer or other device that is not running STP, this means that the port has to wait 2 forwarding delays after the port is turned on before the port can work properly. If the port is connected to an IP phone, the default is 30 seconds before it can be used, which is obviously unbearable. To reduce the convergence time, PortFast technology can be used. This technique allows a switch port to enter the forwarding state as soon as a device is connected to the port, instead of waiting for spanning tree convergence. After a port is set up with PortFast (the port is configured with the command spanning-treeportfast), the port is turned on or off and the switch will no longer send TCN messages.14.1.3 STP ConvergenceConvergence is the process by which the network determines the switch that will act as the root bridge over time, goes through all the different port states and sets all switch ports to their final spanning tree port roles, and all potential switching loops are eliminated. The STP convergence process is divided into the following 3 steps.1. Elect the root bridgeIn order to create a loop-free topology in the network, the switches in the network first elect the root when STP is running.220.Cisco Networking Lab CCNA Lab Guide (Release 2)bridge. Each switch has a unique bridge ID o When the switch powers up. assuming that it is the root bridge. and then begins sending BPDU frames in which the root bridge ID is equal to its own bridge ID. each switch compares the root ID within the received BPDU frame with its own root ID when it receives a BPDU frame from a neighboring switch. If the root ID of the received BPDU frame is smaller than its own current root 1D, the root ID field is updated to indicate the new best candidate running for the root bridge role. How is the bridge ID size compared? The priority is first compared, and if the priority is the same, the MAC address is compared. After the Root ID field on the switch is updated, the switch will then include the new Root ID in all subsequent BPDU frames, which ensures that the smallest Root ID is eventually passed to all other neighboring switches in the network. The root bridge election process will eventually converge, which means that the switches in the network will eventually agree that a switch is the root bridge, and the root bridge election is complete. An example of the STP convergence process is shown in Figure 14-2. The STP priority of VLAN1 of all three switches is the same (default value 32768+1 =32769), however, the MAC address of switch S1 is AA-AA-AA-AA-AA-AA, which is smaller than the MAC addresses of other switches, so it is elected as the root bridge, and all ports on root bridge S1 are designated ports.2. Electing the Root PortAfter the root bridge is elected, the switch starts to configure the port roles for each switch port. The first role to be determined is the root port. The root port is the switch port with the lowest path overhead to the root bridge. The principle for determining the root port campaign winner is done in the following order, and once the size is compared, no further comparisons are made.