Mstp Overview - 3Com 4210 PWR Configuration Manual

MSTP Overview

Background of MSTP
designated port begin to forward data as soon as they are elected, loops may
temporarily occur.
For this reason, the protocol uses a state transition mechanism. Namely, a newly
elected root port and the designated ports must go through a period, which is
twice the forward delay time, before they transit to the forwarding state. The
period allows the new configuration BPDUs to be propagated throughout the
entire network.
Hello time, the interval for sending hello packets. Hello packets are used to
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check link state.
A switch sends hello packets to its neighboring devices at a regular interval (the
hello time) to check whether the links are faulty.
Max time, lifetime of the configuration BPDUs stored in a switch. A
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configuration BPDU that has "expired" is discarded by the switch.
Disadvantages of STP and RSTP
STP does not support rapid state transition of ports. A newly elected root port or
designated port must wait twice the forward delay time before transiting to the
forwarding state, even if it is a port on a point-to-point link or it is an edge port
(an edge port refers to a port that directly connects to a user terminal rather than
to another device or a shared LAN segment.)
The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows
a newly elected root port or designated port to enter the forwarding state much
quicker under certain conditions than in STP. As a result, it takes a shorter time for
the network to reach the final topology stability.
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In RSTP, the state of a root port can transit fast under the following conditions:
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the old root port on the device has stopped forwarding data and the upstream
designated port has started forwarding data.
In RSTP, the state of a designated port can transit fast under the following
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conditions: the designated port is an edge port or a port connected with a
point-to-point link. If the designated port is an edge port, it can enter the
forwarding state directly; if the designated port is connected with a
point-to-point link, it can enter the forwarding state immediately after the
device undergoes handshake with the downstream device and gets a response.
RSTP supports rapid convergence. Like STP, it is of the following disadvantages: all
bridges in a LAN are on the same spanning tree; redundant links cannot be
blocked by VLAN; the packets of all VLANs are forwarded along the same
spanning tree.
Features of MSTP
The multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP
and RSTP. In addition to support for rapid network convergence, it also allows data
flows of different VLANs to be forwarded along their own paths, thus providing a
better load sharing mechanism for redundant links.
MSTP features the following:
MSTP Overview 147