Operation Manual

107
Tips
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In an STP with stable topology, only the root port and designated port can forward data, and the
other ports are blocked. The blocked ports only can receive BPDUs.
RSTP (Rapid Spanning Tree Protocol), evolved from the 802.1D STP standard, enable Ethernet
ports to transit their states rapidly. The premises for the port in the RSTP to transit its state
rapidly are as follows.

The condition for the root port to transit its port state rapidly: The old root port of the
switch stops forwarding data and the designated port of the upstream switch begins to
forward data.

The condition for the designated port to transit its port state rapidly: The designated port
is an edge port or connecting to a point-to-point link. If the designated port is an edge port,
it can directly transit to forwarding state; if the designated port is connecting to a
point-to-point link, it can transit to forwarding state after getting response from the
downstream switch through handshake.

RSTP Elements
Edge Port: Indicates the port connected directly to terminals.
P2P Link: Indicates the link between two switches directly connected.
MSTP (Multiple Spanning Tree Protocol), compatible with both STP and RSTP and subject to
IEEE 802.1s standard, not only enables spanning trees to converge rapidly, but also enables
packets of different VLANs to be forwarded along their respective paths so as to provide
redundant links with a better load-balancing mechanism.
Features of MSTP:

MSTP combines VLANs and spanning tree together via VLAN-to-instance mapping table. It
binds several VLANs to an instance to save communication cost and network resources.

MSTP divides a spanning tree network into several regions. Each region has several
internal spanning trees, which are independent of each other.

MSTP provides a load-balancing mechanism for the packets transmission in the VLAN.

MSTP is compatible with both STP and RSTP.

MSTP Elements
MST Region (Multiple Spanning Tree Region): An MST Region comprises switches with the
same region configuration and VLAN-to-Instances mapping relationship.
IST (Internal Spanning Tree): An IST is a spanning tree in an MST.
CST (Common Spanning Tree): A CST is the spanning tree in a switched network that connects
all MST regions in the network.
CIST (Common and Internal Spanning Tree): A CIST, comprising IST and CST, is the spanning
tree in a switched network that connects all switches in the network.
The following figure shows the network diagram in MSTP.