Qtech QSW-2800v2 Руководство пользователя онлайн [185/230] 481607

Qtech QSW-2800v2 Руководство пользователя онлайн [185/230] 481607
Chapter8 Reliability Configuration
8-3
In order to solve the convergence rate defect of STP, the IEEE defined RSTP based
on IEEE 802.1w standard in 2001. There are three important improvements in RSTP
based on STP. It quickens the convergence rate (in 1s at fastest).
Configure the two roles of Alternate Port and Backup Port for the root port and
designated port fast switching. When the root port is non-effective, the alternate
port will fast switch to be the new root port and go into the forwarding state
without delay. When the designated port is non-effective, the backup port will fast
switch to be the new designated port and go into the forwarding state without
delay.
In the point to point link only connecting two Switch interfaces, the designated
interface only needs to shake hand with the downstream bridge once time and
then can enter the forwarding state without any delay. If it is the sharing link
connecting three bridges or above, the downstream bridge will not response to
the handshake request sent out by the designated interface of upstream bridge
and only can wait two times of Forward Delay time to enter the forwarding state.
The interface directly connects with the terminal and does not connect with other
bridges is defined to be the Edge Port. The edge port can go into the forwarding
state directly without any delay. Because the bridge cannot know whether the
port connecting with the terminal or not, it needs to be configured manually.
RSTP Disadvantage
The RSTP has many improvements compared with the STP and can be compatible
with the STP downward and can be mixed network. But, RSTP belongs to the SST
(Single Spanning Tree) as STP. It has so many disadvantages mainly displaying the
following three aspects.
Because the whole network has only one tree, the convergence time will be too
long when the network size is large.
Because the RSTP is single spanning tree protocol and all VLAN share one tree,
every VLAN in the network must be continuously distributed along the tree path
direction in order to ensure normal communication in VLAN. Otherwise, there will
be some VLAN separated due to the internal link is blocked. This will cause the
problem that bridges in VLAN cannot communicate with each other.

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