Spanning Tree - Supermicro SSE-F3548S User Manual

6 Spanning Tree

Switches are interconnected to provide network access to a large number of end stations. In complex
networks, it is possible to have multiple network paths between any two end devices. The multiple paths
form network loops that lead to a flooding of packets by forwarding broadcast and multicast packets
repeatedly over the looped connections. Flooding makes the network unusable until the looped
connections are disconnected and the flooding stopped.
Spanning tree protocol helps to avoid flooding on network loops. Spanning tree protocols form a loop-
free, tree structured logical network topology over physical network connections.
Spanning tree enabled switches exchange spanning tree protocol messages (BPDU) to form a loop-free
topology. Based on the exchanged BPDU information,the spanning tree algorithm selects one of the
switches on the network as the root switch for the tree topology. All other switches on the networks
choose the best loop-free path to reach the root switch. The redundant paths to root switch are blocked
to form a loop-free topology.
The spanning tree algorithm assigns one of the following roles to every port on the switch.
Root Port
Designated Port
Alternate Port
Blocked Port
When the network connections status changes, spanning tree recalculates the paths to form a loop-free
topology. Spanning tree calculations are based on the following three key factors:
Bridge Identifier: Combination of switch MAC address and switch spanning tree priority
Path Cost: Spanning tree path cost to the root switch
Port Identifier: Combination of port number and port priority
When a switch boots up, it assumes its role as the root switch. It sends out spanning tree BPDUs with its
bridge ID as the root bridge ID. When a switch receives spanning tree BPDUs, it compares the received
BPDU information. If the received BPDU information is superior, the switch uses the received BPDU
information to decide the root bridge and recalculates the spanning tree. If the received BPDU
information is inferior, the switch ignores the received BPDU.
Spanning tree operates the switch ports in different states while calculating the loop-free topology. The
BPDU exchange between switches takes a few seconds in large LANs. To avoid any temporary loops while
forming spanning tree topology, the switch ports are moved through different states to reach the
forwarding state. Switch ports stay in one of the following spanning tree states:
•Port to reach the root switch with lowest path cost
•Root ports forwards the traffic
•Loop-free connection to other switches on the LAN
• Designated ports forward the traffic
• Redundant path to the root switch
•Alternate ports do not forward the traffic
•Redundant path to other switches on the LAN
•Blocked ports do not forward the traffic
Supermicro SSE-F3548S/SSE-F3548SR Configuration User's Guide 159