Rstp; Pvst; Mstp; Stp, Rstp, And Pvst Limitations - HP 3100 Series Configuration Manual

RSTP

RSTP achieves rapid network convergence by allowing a newly elected root port or designated port
to enter the forwarding state much faster than STP.
A newly elected RSTP root port rapidly enters the forwarding state if the old root port on the device
has stopped forwarding data and the upstream designated port has started forwarding data.
A newly elected RSTP designated port rapidly enters the forwarding state if it is an edge port (a port
that directly connects to a user terminal rather than to another network device or a shared LAN
segment) or it connects to a point-to-point link (to another device). Edge ports directly enter the
forwarding state. Connecting to a point-to-point link, a designated port enters the forwarding state
immediately after the device receives a handshake response from the directly connected device.

PVST

PVST was introduced to improve link bandwidth usage in network environments where multiple
virtual LANs (VLANs) exist. Unlike STP and RSTP whose bridges in a LAN must forward their VLAN
packets in the same spanning tree, PVST allows each VLAN to build a separate spanning tree.
PVST uses the following BPDUs:
•
STP BPDUs—Sent by access ports according to the VLAN status, or by trunk ports and hybrid
ports according to the status of VLAN 1.
•
PVST BPDUs—Sent by trunk port and hybrid ports according to the status of permitted VLANs
except VLAN 1.

MSTP

STP, RSTP, and PVST limitations

STP does not support rapid state transition of ports. A newly elected port must wait twice the forward
delay time before it transits to the forwarding state, even if it connects to a point-to-point link or is an
edge port.
Although RSTP supports rapid network convergence, it has the same drawback as STP. All bridges
within a LAN share the same spanning tree, and the packets from all VLANs are forwarded along the
same spanning tree, so redundant links cannot be blocked based on VLAN and traffic cannot be
load-shared among VLANs.
The number of PVST BPDUs generated grows with that of permitted VLANs on trunk ports. When
the status of a trunk port transitions, network devices might be overloaded to re-calculate a large
number of spanning trees.

MSTP features

Developed based on IEEE 802.1s, MSTP overcomes the limitations of STP, RSTP, and PVST. In
addition to supporting rapid network convergence, it provides a better load sharing mechanism for
redundant links by allowing data flows of different VLANs to be forwarded along separate paths.
MSTP provides the following features:
•
MSTP divides a switched network into multiple regions, each of which contains multiple
spanning trees that are independent of one another.
•
MSTP supports mapping VLANs to spanning tree instances by means of a VLAN-to-instance
mapping table. MSTP can reduce communication overheads and resource usage by mapping
multiple VLANs to one instance.
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