Spanning-Tree #
802.1D or STP is used to help provide a loop-free network. Possible states:
•Disabled: port is admin down
•Blocking: port is enabled but not forwarding traffic
•Listening: the switch has moved from a blocking state and can send and receive BPDU’s
•Learning: the port can now modify the MAC address table with traffic it receives
•Forwarding: the port can forward all network traffic
•Broken: a configuration or operational problem with the port
spanning-tree path cost comes into modes short and long short-mode allows up to unique path costs up to 20Gbps and long-mode unique path costs up to 10Tbps
Spanning tree pathcost method long
This command must be configured on every device in the layer 2 topology
Root election by default will use the lowest MAC in the environment.
•This can be overridden by setting the priority of the root bridge.
•If a switch receives an inferior BPDU it will disregard it
•If a switch receives a BPDU that is preferred it will update its BPDU to include the new root bridge
The following command allow you to see information to verify the root bridge
Switch#sh spanning-tree root
Root Hello Max Fwd
Vlan Root ID Cost Time Age Dly Root Port
---------------- -------------------- --------- ----- --- --- ------------
VLAN0001 1 001d.7034.8100 4 2 20 15 Gi0/1
VLAN0002 2 001d.7034.8100 4 2 20 15 Gi0/1
VLAN0003 32771 aca0.1649.5d80 0 2 20 15
The following process is used to determine which ports should be blocked between 2 non-root bridges
- The interface is a designated port
- The switch with the lowest path cost to the root will win
- The system priority of the local switch compared to the priority of the remote switch traffic is blocked if the remote switch is lower
- The system MAC of the local switch is compared to the remote switch. Traffic is blocked if the remote switch is lower
command to help track topology changes
Switch#show spanning-tree detail | i ieee|from|occur|exec
VLAN0001 is executing the rstp compatible Spanning Tree protocol
Number of topology changes 6 last change occurred 2w4d ago
from GigabitEthernet1/0/4
VLAN0002 is executing the rstp compatible Spanning Tree protocol
Number of topology changes 3 last change occurred 3w5d ago
from GigabitEthernet1/0/24
802.1W RSTP Rapid Spanning Tree Possible states:
•Discarding: port is enabled but not forwarding any traffic.
•Learning: the port can now modify the MAC address table with traffic it receives
•Forwarding: the port can forward all network traffic
Port types: Edge port: a port where a host connects and cannot form a loop. Directly correlates to spanning-tree port fast being enabled. Root port: port with the best path cost to the root bridge Point to point: any port that connects to another RSTP switch with full duplex