Spanning-Tree Operation
Overview
802.1s Spanning Tree
Protocol
Default Menu CLI Web
Viewing the MSTP Status
and Configuration
Enable/Disable MSTP and
Configure Global
Parameters
Configuring Basic Port
Connectivity Parameters
Configuring MSTP Instance
Parameters
Configuring MSTP Instance
Per-Port Parameters
Enabling/Disabling MSTP
Spanning Tree Operation
Enabling an Entire MST
Region at Once
n/a — page 6-71 —
Disabled — page 6-57 —
edge-port: No — page 6-61 —
mcheck: Yes and
hello-time: 2 following
path-cost: auto
point-to-point MAC: Force-True
priority: 128 (multiplier: 8)
instance (MSTPI): none — page 6-63 —
priority: 32768 (multiplier: 8)
Auto — page 6-66 —
Disabled — page 6-69 —
n/a — page 6-69 —
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
6-3
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