206 CHAPTER 23: MSTP CONFIGURATION
transition in STP, a newly elected root port or designated port requires twice
the forward delay time before transitioning to the forwarding state, when the
new configuration BPDU has been propagated throughout the network.
■ Hello time is the time interval at which a device sends hello packets to the
surrounding devices to make sure that the paths are fault-free.
■ Max age is a parameter used to determine whether a configuration BPDU held
in the device has expired. A configuration BPDU beyond the max age will be
discarded.
Introduction to MSTP Why MSTP
1 Disadvantages of STP and RSTP
STP does not support rapid state transition of ports. A newly elected root port or
designated port must wait twice the forward delay time before transitioning to the
forwarding state, even if it is a port on a point-to-point link or it is an edge port,
which directly connects to a user terminal rather than to another device or a
shared LAN segment.
The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows
a newly elected root port or designated port to enter the forwarding state much
quicker under certain conditions than in STP. As a result, it takes a shorter time for
the network to reach the final topology stability.
n
In RSTP, a newly elected root port can enter the forwarding state rapidly if
this condition is met: The old root port on the device has stopped forwarding
data and the upstream designated port has started forwarding data.
In RSTP, a newly elected designated port can enter the forwarding state rap-
idly if this condition is met: The designated port is an edge port or a port con-
nected with a point-to-point link. If the designated port is an edge port, it can
enter the forwarding state directly; if the designated port is connected with a
point-to-point link, it can enter the forwarding state immediately after the de-
vice undergoes handshake with the downstream device and gets a response.
Although RSTP support rapid network convergence, it has the same drawback as
STP does: All bridges within a LAN share the same spanning tree, so redundant
links cannot be blocked based on VLANs, and the packets of all VLANs are
forwarded along the same spanning tree.
2 Features of MSTP
The multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP
and RSTP. In addition to support for rapid network convergence, it also allows data
flows of different VLANs to be forwarded along their own paths, thus providing a
better load sharing mechanism for redundant links. For description about VLANs,
refer to “Introduction to VLAN” on page 83.
MSTP features the following:
■ MSTP supports mapping VLANs to MST instances by means of a
VLAN-to-instance mapping table. MSTP can save communication overheads
and resource usage by mapping multiple VLANs to one MST instance.
■ MSTP divides a switched network into multiple regions, each containing
multiple spanning trees that are independent of one another.
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