User`s manual

EDS-405A/408A Series User’s Manual Featured Functions
3-36
Differences between RSTP and STP
RSTP is similar to STP, but includes additional information in the BPDUs that allow each bridge
to confirm that it has taken action to prevent loops from forming when it decides to enable a link
to a neighboring bridge. Adjacent bridges connected via point-to-point links will be able to enable
a link without waiting to ensure that all other bridges in the network have had time to react to the
change. The main benefit of RSTP is that the configuration decision is made locally rather than
network-wide, allowing RSTP to carry out automatic configuration and restore a link faster than
STP.
STP Example
The LAN shown below has three segments, with adjacent segments connected using two possible
links. The various STP factors, such as Cost, Root Port, Designated Bridge Port, and Blocked Port
are shown in the figure.
Bridge A
LAN Segment 3
LAN Segment 2
LAN Segment 1
Port 2
(Root Bridge)
Port 1
(Designated
Bridge Port)
Bridge B
Port 2
(Designated
Bridge Port)
Port 1
(Root Port)
Cost =100
Bridge X
Port 2
(Blocked Port)
Port 1
(Root Port)
Cost =100
Bridge C
Port 2
(Designated
Bridge Port)
Port 1
(Root Port)
Cost =100
Bridge Y
Port 2
(Blocked Port)
Port 1
(Root Port)
Cost =200
y Bridge A has been selected as the Root Bridge, since it was determined to have the lowest
Bridge Identifier on the network.
y Since Bridge A is the Root Bridge, it is also the Designated Bridge for LAN segment 1. Port 1
on Bridge A is selected as the Designated Bridge Port for LAN Segment 1.
y Ports 1 of Bridges B, C, X, and Y are all Root Ports since they are nearest to the Root Bridge,
and therefore have the most efficient path.
y Bridges B and X offer the same Root Path Cost for LAN segment 2. However, Bridge B was
selected as the Designated Bridge for that segment since it has a lower Bridge Identifier. Port
2 on Bridge B is selected as the Designated Bridge Port for LAN Segment 2.
y Bridge C is the Designated Bridge for LAN segment 3, because it has the lowest Root Path
Cost for LAN Segment 3:
¾ The route through Bridges C and B costs 200 (C to B=100, B to A=100)