Spanning-Tree Operation
Overview
Without spanning tree, having more than one active path between a pair of
nodes causes loops in the network, which can result in duplication of mes-
sages, leading to a “broadcast storm” that can bring down the network.
Single-Instance spanning tree operation (802.1D STP and 802.1w RSTP)
ensures that only one active path at a time exists between any two nodes in a
physical network. In networks where there is more than one physical, active
path between any two nodes, enabling single-instance spanning tree ensures
one active path between such nodes by blocking all redundant paths.
Multiple-Instance spanning tree operation (802.1s) ensures that only one
active path exists between any two nodes in a spanning-tree instance. A
spanning-tree instance comprises a unique set of VLANs, and belongs to a
specific spanning-tree region. A region can comprise multiple spanning-tree
instances (each with a different set of VLANs), and allows one active path
among regions in a network. Applying VLAN tagging to the ports in a multiple-
instance spanning-tree network enables blocking of redundant links in one
instance while allowing forwarding over the same links for non-redundant use
by another instance. For example, suppose you have three switches in a region
configured with VLANs grouped into two instances, as follows:
VLANs Instance 1 Instance 2
10, 11, 12 Yes No
20, 21, 22 No Yes
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