You issue the show spanning-tree command on SwitchC and receive the following output:
SwitchC#sh spanning-tree vlan 300
VLAN0300
Spanning tree enabled protocol rstp
Root ID Priority 31574
Address 0017.1234.5600
Cost 19
Port 2 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32810 (priority 32768 sys-id-ext 300)
Address 0017.4321.6500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.2 P2p
Fa0/2 Desg FWD 4 128.3 P2p Peer (STP)
Which of the following statements are true regarding the switches on the network?
| A. |
SwitchA is the root bridge | |
| B. |
SwitchC is using the default STP timer values | |
| C. |
802.1D STP is running on SwitchB | |
| D. |
802.1D STP is running on SwitchD | |
| E. |
The BID of SwitchB is 32810.0017.4321.6500 | |
| F. |
The BID of SwitchC is 0017.4321.6500.32810 |
SwitchC is using the default Spanning Tree Protocol (STP) timer values, and 802.1D STP is running on SwitchD. STP uses three timer values: the hello timer value, the forward_delay timer value, and the max_age timer value. The hello timer value is the time between the sending of bridge protocol data units (BPDUs)? this value is set to two seconds by default. The forward_delay timer value is the time spent in the listening state and the learning state? this value is set to 15 seconds by default. The max_age timer value is the maximum length of time before BPDU information is aged out? this value is set to 20 seconds by default. The output of the show spanningtree command indicates that SwitchC is using these default STP timer values.
Traditional STP is defined in the Institute of Electrical and Electronics Engineers (IEEE) 802.1D standard. Rapid STP (RSTP), which is defined in the IEEE 802.1w standard, is used to improve the slow transition of an STP port to the forwarding state. The line of text Spanning tree enabled protocol rstp in the output of the show spanning-tree command indicates that SwitchC is running RSTP. If SwitchC were running 802.1D STP, the line of text Spanning tree enabled protocol ieee would be displayed in the output of the show spanningtree command.
To determine the type of STP running on neighbor switches, you should analyze the Type field in the show spanningtree command output. If the Type field displays P2P, the neighbor switch is running RSTP. If the Type field displays P2P Peer(STP), the neighbor switch is running traditional 802.1D STP. Therefore, SwitchB is running RSTP and SwitchD is running 802.1D STP.
SwitchA is not the root bridge. The output of the show spanningtree command indicates that the root bridge is reachable through the Fa0/1 interface. Therefore, either SwitchA or SwitchB is the root bridge. You can determine which switch is the root by analyzing the link cost, which is used to determine the best path to the root bridge. The link cost is based on the bandwidth of a link.
FastEthernet links have a bandwidth of 100 Mbps, so the link cost of traversing aFastEthernet link is 19. If packets must traverse two FastEthernet links, the link cost is 38. In this scenario, the output of the show spanningtree command indicates that the link cost is 19. Because packets must traverse only one FastEthernet link to reach the root bridge, SwitchB must be the root bridge.
The bridge ID (BID) of SwitchB is not 32810.0017.4321.6500. The BID is composed of a 2-byte bridge priority prefix and a 6byte Media Access Control (MAC) address suffix. The output of the show spanning-tree command indicates that the root bridge has a priority value of 31574 and a MAC address of 0017.1234.5600. Therefore, the BID of the root bridge, SwitchB, is 31574.0017.1234.5600.
The BID of SwitchC is not 0017.4321.6500.32810. The Bridge ID field in the output of the show spanning-tree command indicates the priority value and MAC
address of the local switch, SwitchC. In the BID, the bridge priority comes before the MAC address. Therefore, the BID of SwitchC is 32810.0017.4321.6500.