3Com 3CR17660-91, Configuration Manual

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20
MSTP C
ONFIGURATION
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 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.
MSTP Protocol Data Unit 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 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. In this
case, the BPDUs carry MSTP configuration information of the switches.
Basic MSTP
Terminologies
Figure 36 illustrates basic MSTP terms (assuming that MSTP is enabled on each switch
in Figure 36).