© Copyright Lenovo 2017
Chapter 10: Spanning Tree Protocols
187
Multiple Spanning Tree Protocol
Multiple
Spanning
Tree
Protocol
(MSTP)
extends
Rapid
Spanning
Tree
Protocol
(RSTP),
allowing
multiple
Spanning
Tree
Groups
(STGs)
which
may
each
include
multiple
VLANs.
MSTP
was
originally
defined
in
IEEE
802.1s
(2002)
and
was
later
included
in
IEEE 802.1Q (2003).
In
MSTP
mode,
the
CN4093
supports
up
to
32
instances
of
Spanning
Tree,
corresponding
to
STGs 1
‐
32,
with
each
STG
acting
as
an
independent,
simultaneous
instance
of
STP.
MSTP
allows
frames
assigned
to
different
VLANs
to
follow
separate
paths,
with
each
path
based
on
an
independent
Spanning
Tree
instance.
This
approach
provides
multiple
forwarding
paths
for
data
traffic,
thereby
enabling
load
‐
balancing,
and
reducing
the
number
of
Spanning
Tree
instances
required
to
support
a
large
number
of
VLANs.
Due
to
Spanning
Tree’s
sequence
of
discarding,
learning,
and
forwarding,
lengthy
delays
may
occur
while
paths
are
being
resolved.
Ports
defined
as
edge
ports
bypass
the
Discarding
and
Learning
states,
and
enter
directly
into
the
Forwarding
state.
Note:
In
MSTP
mode,
Spanning
Tree
for
the
management
ports
is
turned
off
by
default.
MSTP Region
A
group
of
interconnected
bridges
that
share
the
same
attributes
is
called
an
MST
region.
Each
bridge
within
the
region
must
share
the
following
attributes:
Alphanumeric
name
Revision
number
VLAN
‐
to
STG
mapping
scheme
MSTP
provides
rapid
re
‐
configuration,
scalability
and
control
due
to
the
support
of
regions,
and
multiple
Spanning
‐
Tree
instances
support
within
each
region.
Common Internal Spanning Tree
The
Common
Internal
Spanning
Tree
(CIST)
provides
a
common
form
of
Spanning
Tree
Protocol,
with
one
Spanning
‐
Tree
instance
that
can
be
used
throughout
the
MSTP
region.
CIST
allows
the
switch
to
interoperate
with
legacy
equipment,
including
devices
that
run
IEEE
802.1D
(1998)
STP.
CIST
allows
the
MSTP
region
to
act
as
a
virtual
bridge
to
other
bridges
outside
of
the
region,
and
provides
a
single
Spanning
‐
Tree
instance
to
interact
with
them.
CIST
port
configuration
includes
Hello
time,
Edge
port
enable/disable,
and
Link
Type.
These
parameters
do
not
affect
Spanning
Tree
Groups
1–32.
They
apply
only
when
the
CIST
is
used.
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