Mstp Overview - 3Com 7757 Configuration Manual

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MSTP Overview

MSTP Protocol Data Unit
Basic MSTP
Terminologies
MSTP C
ONFIGURATION
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 transit to the
forwarding state even if the port is on a point-to-point link or the port is an edge
port. This slows down the spanning tree convergence of STP.
Rapid spanning tree protocol (RSTP) enables the spanning tree to converge rapidly,
but it suffers from the same drawback as that of 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 by VLANs.
As well as the above two protocols, 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.
Besides this, MSTP can also provide multiple redundant paths for packet
forwarding and balances the forwarding loads of different VLANs.
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 with redundant links.
Bridge protocol data unit (BPDU) 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 the information that is needed for switches
to figure out the spanning tree.
BPDUs used in STP fall into the following two categories:
Configuration BPDUs: BPDUs of this type are used to maintain the spanning
■
tree topology.
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 to figure out spanning trees too.
Besides, the BPDUs of MSTP carry MSTP configuration information of the switches.
Figure 60 illustrates basic MSTP terms (assuming that MSTP is enabled on each
switch in this figure).