H3C Technologies H3C S3100 Series Switches - Manual (Page 176)

Operation Manual – MSTP
H3C S3100-52P Ethernet Switch
Chapter 1 MSTP Configuration
1-1
Chapter 1 MSTP Configuration
1.1 MSTP Overview
Spanning tree protocol (STP) cannot enable Ethernet ports to transit their states rapidly.
It costs two times of the forward delay for a port to turn to the forwarding state even if
the port is on a point-to-point link or the port is an edge port.
Rapid spanning tree protocol (RSTP) enables the spanning tree to converge rapidly,
but it suffers from the same drawback as STP: all bridges in a LAN share one spanning
tree; packets of all VLANs are forwarded along the same spanning tree, and therefore
redundant links cannot be blocked based on VLANs.
As well as the two protocols above, multiple spanning tree protocol (MSTP) can
disbranch a ring network to form a tree-topological ring-free network to prevent packets
from being duplicated and forwarded endlessly in the ring network. Additionally, MSTP
can also provide multiple redundant paths for packet forwarding to implement
VLAN-based load balancing.
MSTP is compatible with both STP and RSTP. It overcomes the drawback of STP and
RSTP. It not only enables spanning trees to converge rapidly, but also enables packets
of different VLANs to be forwarded along their respective paths to provide a better load
balancing mechanism for redundant links.
1.1.1 MSTP Protocol Data Unit
Bridge protocol data unit (BPDU), which is also called configuration message, is the
protocol data unit (PDU) that STP and RSTP use.
The switches in a network transfer BPDUs between each other to determine the
topology of the network. BPDUs carry enough information needed for spanning tree
calculation.
BPDUs used in STP fall into the following two categories:
z
Configuration BPDUs: BPDUs of this type are used to maintain the spanning tree
topology.
z
Topology change notification BPDU (TCN BPDN): BPDUs of this type are used to
notify the switches of network changes.
Similar to STP and RSTP, MSTP uses BPDUs for spanning tree calculation too.
Besides, the BPDUs of MSTP carry MSTP configuration information of the switches.