MSTP configuration
The MSTP feature is available only on a SAP interface card in bridging mode.
As a Layer 2 management protocol, the STP eliminates Layer 2 loops by selectively blocking redundant
links in a network, and in the meantime, allows for link redundancy.
Like many other protocols, STP evolves as the network grows. The later versions of STP are the RSTP and
the MSTP. This chapter describes the features of STP, RSTP, and MSTP and the relationship among them.
Why STP
STP was developed based on the 802.1d standard of IEEE to eliminate loops at the data link layer in a
LAN. Devices running this protocol detect loops in the network by exchanging information with one
another and eliminate loops by selectively blocking certain ports to prune the loop structure into a
loop-free tree structure. This avoids proliferation and infinite cycling of packets that would occur in a loop
network and prevents decreased performance of network devices caused by duplicate packets received.
In the narrow sense, STP refers to IEEE 802.1d STP. In the broad sense, STP refers to the IEEE 802.1d STP
and various enhanced spanning tree protocols derived from that protocol.
Protocol packets of STP
STP uses BPDUs, also known as configuration messages, as its protocol packets.
STP-enabled network devices exchange BPDUs to establish a spanning tree. BPDUs contain sufficient
information for the network devices to complete spanning tree calculation.
In STP, BPDUs come in two types:
Configuration BPDUs, used for calculating a spanning tree and maintaining the spanning tree
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topology.
TCN BPDUs, used for notifying the concerned devices of network topology changes, if any.
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Basic concepts in STP
Root bridge
A tree network must have a root. Hence, the concept of root bridge was introduced in STP.
There is only one root bridge in the entire network, and the root bridge can change along with changes
of the network topology. Therefore, the root bridge is not fixed.
Upon initialization of a network, each device generates and sends out configuration BPDUs periodically
with itself as the root bridge. After network convergence, only the root bridge generates and sends out
configuration BPDUs at a certain interval, and the other devices forward the BPDUs.
Root port
On a non-root bridge, the port nearest to the root bridge is called the root port. The root port is
responsible for communication with the root bridge. Each non-root bridge has only one root port. The root
bridge has no root port.
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