RSTP (IEEE 802.1W Standard) – Rapid Spanning Tree Protocol
The main flaw of the STP protocol is the huge convergence time in the event of a network outage
The RSTP protocol is identical to the STP in the following respects:
– Election of the Root-Bridge.
– Election of root port on non-Root Switch.
– Election of the Designated port.
– Same logic for ports in The Forwarding and Blocking states, except that RSTP uses the term Discarding instead of Blocking.
The convergence time of this protocol is 6 seconds.
The big differences are:
– MaxAge time lapse from 10 times between two Hello BPDu (20seconds) to three (6 seconds)
– Appearances of new terminology:
– Alternate Port: Replaces the Root Port in the event of a breakdown
– Backup Port: Replaces the Designated Port in the event of a breakdown
Let's take our STP examples:
Our Root-Port are elected on our "Non-root switches"
So far, nothing changes!!
Our Non-Root bridge with interfaces in "Discarding" mode will look for an alternative path to the Root-Bridge following the same logic to elect the Root-Port.
Case number 1
All the metric swings are equal, so it will look at who has the weakest Bridge-ID.
Case number 2
He'll look at the cost of our connections:
For the Switch C, there is only one port in mode, DISCARDING. So he has no choice but to switch it to ALTERNATE mode
For the Switch D, there are 3 paths:
– C,B and A , i.e. a cost of 4-4-4 – 12
– B and A , i.e. a cost equal to 19-4 – 23
– Directly, i.e. a cost equal to 19
It will switch its port to the Switch C in ALTERNATE mode.
For the Switch E, there are two ways to go:
– B and A , i.e. a cost of 4-4 – 8
– A , i.e. a cost equal to 19
It will switch to the Switch B in ALTERNATE mode.
In order to understand this mode, we will take the following architecture:
Between switch B and C, there is a HUB. The latter makes the two ports of the Switch B going to the Switch C are in the same collision area! The RSTP concludes:
If my Designated port comes to fall, I have yet another port in this collision area!! So I place this second port in BackUP mode (BackUp – Back online)
Different port states
In the RSTP, our ports can take several states:
This connection is a full-DUPLEX connection, RSPT believes that this link is a point-to-point link.
This connection is a HALF-DUPLEX connection, it is shared (Shared) between several devices via a Hub.
This connection is a connection between a Switch and a Customer
Hoping this article has been helpful to you! Don't hesitate to let me know!!
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