QTECH
Software Configuration Manual
15-192
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Note :
To facilitate description, the spanning tree computing process in this example is simplified, while the
actual process is more complicated.
2) The BPDU forwarding mechanism in STP
·
Upon network initiation, every switch regards itself as the root bridge, generates configuration BPDUs with
itself as the root, and sends the configuration BPDUs at a regular interval of hello time.
·
If it is the root port that received the configuration BPDU and the received configuration BPDU is superior
to the configuration BPDU of the port, the device will increase message age carried in the configuration
BPDU by a certain rule and start a timer to time the configuration BPDU while it sends out this configuration
BPDU through the designated port.
·
If the configuration BPDU received on the designated port has a lower priority than the configuration BPDU
of the local port, the port will immediately sends out its better configuration BPDU in response.
·
If a path becomes faulty, the root port on this path will no longer receive new configuration BPDUs and the
old configuration BPDUs will be discarded due to timeout. In this case, the device will generate a
configuration BPDU with itself as the root and sends out the BPDU. This triggers a new spanning tree
computing process so that a new path is established to restore the network connectivity.
However, the newly computed configuration BPDU will not be propagated throughout the network immediately,
so the old root ports and designated ports that have not detected the topology change continue forwarding data
through the old path. If the new root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur. For this reason, STP uses a state transition mechanism. Namely, 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.
15.1.2
Introduction to MSTP
15.1.2.1
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.
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Note :
l
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.
l
In RSTP, a newly elected designated port can enter the forwarding state rapidly if this
condition is met : The designated port is an edge port or a port connected 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 device 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
VLAN Configuration
in the
Access Volume
.
