31 Days Before Your CCNA Exam

84 31 Days Before Your CCNA ExamInterface Role Sts Cost Prio.Nbr Type---------------- ---- --- --------- -------- --------------------------------Fa0/1 Desg FWD 19 128.1 P2pFa0/2 Desg FWD 19 128.2 P2pBecause an extended system ID is used in the BID, the value of the priority includes the additionof the VLAN ID. So, a priority of 24576 plus a VLAN of 1 results in an priority output of 24577.PortFastTo speed up convergence for access ports when they become active, you can use Cisco’s proprietaryPortFast technology. After PortFast is configured and a port is activated, the port immediatelytransitions from the blocking state to the forwarding state. Example 24-2 shows the interface commandto configure PortFast.Example 24-2Configuring PortFastSwitch# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Switch(config)# interface f0/11Switch(config)# switchport mode accessSwitch(config-if)# spanning-tree portfastAlternatively, you can configure the global command spanning-tree portfast default, whichenables PortFast by default on all access ports.Configuring RSTPRemember, STP is the default operation of Cisco switches. To change to RSTP and PVRST, use asingle global command on all switches: spanning-tree mode rapid-pvst.Troubleshooting STPSTP runs by default on switches and rarely causes problems in small- to medium-sized networks.However, you might encounter STP troubleshooting problems on the exam. Use the followingsteps to analyze an STP problem:Step 1Step 2Step 3Determine the root switch.For each nonroot switch, determine its one root port (RP) and cost to reach the rootswitch through that RP.For each segment, determine the designated port (DP) and the cost advertised by theDP onto that segment.The information requested in each of the steps can be obtained from variations of the showspanning-tree command.