Mstp Overview; Background Of Mstp - H3C S3100-52P Operation Manual

Operation Manual – MSTP
H3C S3100-52P Ethernet Switch
root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur.
3) 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.

1.2 MSTP Overview

1.2.1 Background of MSTP

I. 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.
1-11
Chapter 1 MSTP Configuration