Using 802.1s Multiple Spanning Tree
Comparing MSTP with STP and RSTP
Using MSTP (802.1s) has the following items in common with STP (802.1D) and RSTP (802.1w) proto-
cols:
Each protocol ensures one data path between any two switches within the network topology. This
•
prevents network loops from occurring while at the same time allowing for redundant path configura-
tion.
Each protocol provides automatic reconfiguration of the network Spanning Tree topology in the event
•
of a connection failure and/or when a switch is added to or removed from the network.
All three protocols are supported in the flat Spanning Tree operating mode.
•
The flat mode CST instance automatically determines port states and roles across VLAN port and
•
MSTI associations. This is because the CST instance is active on all ports and only one BPDU is used
to forward information for all MSTIs.
MSTP is based on RSTP.
•
Using MSTP differs from STP and RSTP as follows:
MSTP is only supported when the switch is running in the flat Spanning Tree mode. STP and RSTP
•
are supported in both the 1x1 and flat modes.
MSTP allows for the configuration of up to 16 Multiple Spanning Tree Instances (MSTI) in addition to
•
the CST instance. Flat mode STP and RSTP protocols only use the single CST instance for the entire
switch. See
"What is a Multiple Spanning Tree Instance (MSTI)" on page 6-7
MSTP applies a single Spanning Tree instance to an MSTI ID number that represents a set of VLANs;
•
a one to many association. STP and RSTP in the flat mode apply one Spanning Tree instance to all
VLANs; a one to all association. STP and RSTP in the 1x1 mode apply a single Spanning Tree
instance to each existing VLAN; a one to one association.
The port priority and path cost parameters are configurable for an individual MSTI that represents the
•
VLAN associated with the port.
The flat mode 802.1D or 802.1w CST is identified as instance 1. When using MSTP, the CST is identi-
•
fied as CIST (Common and Internal Spanning Tree) instance 0. See
nal Spanning Tree Instance" on page 6-9
MSTP allows the segmentation of switches within the network into MST regions. Each region is seen
•
as a single virtual bridge to the rest of the network, even though multiple switches may belong to the
one region. See
"What is a Multiple Spanning Tree Region" on page 6-8
MSTP has lower overhead than a 1x1 configuration. In 1x1 mode, because each VLAN is assigned a
•
separate Spanning Tree instance, BPDUs are forwarded on the network for each VLAN. MSTP only
forwards one BPDU for the CST that contains information for all configured MSTI on the switch.
What is a Multiple Spanning Tree Instance (MSTI)
An MSTI is a single Spanning Tree instance that represents a group of VLANs. Alcatel switches support
up to 16 MSTIs on one switch. This number is in addition to the Common and Internal Spanning Tree
(CIST) instance 0, which is also known as MSTI 0. The CIST instance exists on every switch. By default,
all VLANs not mapped to an MSTI are associated with the CIST instance. See
Internal Spanning Tree Instance" on page 6-9
OmniSwitch 6600 Family Network Configuration Guide
for more information.
for more information.
April 2006
MST General Overview
for more information.
"What is the Common and Inter-
for more information.
"What is the Common and
page 6-7