© Copyright Lenovo 2017
Chapter 10: Spanning Tree Protocols
173
PVRST Mode
Note:
Per
‐
VLAN
Rapid
Spanning
Tree
(PVRST)
is
enabled
by
default
on
the
CN4093.
Using
STP,
network
devices
detect
and
eliminate
logical
loops
in
a
bridged
or
switched
network.
When
multiple
paths
exist,
Spanning
Tree
configures
the
network
so
that
a
switch
uses
only
the
most
efficient
path.
If
that
path
fails,
Spanning
Tree
automatically
sets
up
another
active
path
on
the
network
to
sustain
network
operations.
Enterprise
NOS
PVRST
mode
is
based
on
IEEE
802.1w
RSTP.
Like
RSTP,
PVRST
mode
provides
rapid
Spanning
Tree
convergence.
However,
PVRST
mode
is
enhanced
for
multiple
instances
of
Spanning
Tree.
In
PVRST
mode,
each
VLAN
may
be
automatically
or
manually
assigned
to
one
of
127
available
STGs,
with
each
STG
acting
as
an
independent,
simultaneous
instance
of
STP.
PVRST
uses
IEEE
802.1Q
tagging
to
differentiate
STP
BPDUs
and
is
compatible
with
Cisco
R
‐
PVST/R
‐
PVST+
modes.
The
relationship
between
ports,
LAGs,
VLANs
and
Spanning
Trees
is
shown
in
.
Port States
The
port
state
controls
the
forwarding
and
learning
processes
of
Spanning
Tree.
In
PVRST,
the
port
state
has
been
consolidated
to
the
following:
discarding
,
learning
or
forwarding
.
Due
to
the
sequence
involved
in
these
STP
states,
considerable
delays
may
occur
while
paths
are
being
resolved.
To
mitigate
delays,
ports
defined
as
edge
ports
)
may
bypass
the
discarding
and
learning
states,
and
enter
directly
into
the
forwarding
state.
Table 17.
Ports,
LAGs
and
VLANs
Switch Element
Belongs To
Port
LAG
or
one
or
more
VLANs
LAG
One
or
more
VLANs
VLAN
(non
‐
default)
PVRST:
One
VLAN
per
STG
RSTP:
All
VLANs
are
in
STG
1
MSTP:
Multiple
VLANs
per
STG
Summary of Contents for Flex System Fabric CN4093
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