Netis ST3326(ST-3302) [47/118] Mstp overview

topology change continue forwarding data through the old path. If the new root port and

designated port begin to forward data as soon as they are elected, a temporary loop may occur.

7) STP timers

The following three time parameters are important for STP calculation:

 Forward delay, the period a device waits before state transition.

A link failure triggers a new round of spanning tree calculation and results in changes of the

spanning tree. However, as new configuration BPDUs cannot be propagated throughout the

network immediately,if the new root port and 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 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 configuration BPDU that

has “expired” is discarded by the switch.

MSTP Overview

Background of MSTP

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.

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.