| Exam 200-120 | Question id=124 | Troubleshooting |
switch# sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 30482
Address 0017.9425.fc00
Cost 38
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0013.19a3.e5c0
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 19 128.1 P2p
Fa0/2 Desg FWD 38 128.1 P2p
Fa0/3 Desg FWD 38 128.1 P2p
Fa0/4 Desg FWD 38 128.1 P2p
Why has this switch not been elected the root bridge for VLAN1?
| A. |
It has more than one interface that is connected to the root network segment. | |
| B. |
It is running RSTP while the elected root bridge is running 802.1d spanning tree. | |
| C. |
It has a higher MAC address than the elected root bridge. | |
| D. |
It has a higher bridge ID than the elected root bridge. |
When a switch receives a BPDU, it first compares priority, the lower number wins. If a tie, compare MAC, the smaller one wins. Here Switch has 32769 priority which is greater than 20481 so switch will not elect for root bridge. It says the bridge priority for Switch is 32769, and the root priority is 30482.