A 2 Rstp - Rapid Spanning Tree Protocol - Phoenix Contact SCX 4POE 2LX User Manual

Loops
IEEE 802.1w
Root bridge
BPDU
Path costs
109008_en_01
A 2 RSTP – Rapid Spanning Tree Protocol
The RSTP protocol enables the use of Ethernet networks with redundant data paths. These
networks form a meshed topology, initially with impermissible loops. These loops can lead
to data packets circulating endlessly within the network or even being duplicated. As a con-
sequence, the network becomes overloaded due to circulating data packets. Communica-
tion is interrupted.
Therefore, the meshed structure is replaced with a logical, deterministic path using the
Rapid Spanning Tree algorithm. The path has a tree structure which no longer contains any
loops. In the event of data path failures, some of the previously disabled connections are
reconnected. This ensures that the network operates without interruption.
The RSTP protocol is standardized in the IEEE 802.1w standard. RSTP is event driven. As
a result, the switch-over times are significantly shorter than with time-based STP.
Example
To ensure continued access to all participants in the network in the event of a data path fail-
ure, there are six redundant paths in the following network topology. These redundant paths
are impermissible loops. The RSTP protocol automatically converts this topology into a tree
by disabling selected ports. In this case the root (root bridge) of the tree is one switch. Every
other switch can be accessed from the root bridge via just one data path.
US1
US2
FAIL
XF
1
1
XF
2
XF
3
XF
4
Root
XD1
3
Figure 8-3 Possible tree structure with RSTP
The switch with the lowest bridge priority is the root bridge. If this root bridge fails, the next
root bridge is selected based on the bridge priority. If two switches have the same bridge
priority value, the root bridge with the lower MAC address is selected.
The root bridge continuously sends BPDUs (Bridge Protocol Data Units) at the set hello time
interval. If a topology change is detected, alternative paths are calculated.
The path costs are used to decide which ports are to be blocked and which are to be pre-
ferred. The path costs are determined automatically based on the bandwidth. However, you
can also specify a value manually.
MODE
XF
5
XF
6
XF
7
XF
8
4
1 3 5 7 9 11 13
XF
2 4 6 8 10 12 14
US1 US2 FAIL LNK/ACT
SWITCH 2000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
XD1 XG1
US1 GND US2 GND R1 R2 SPD
RSTP – Rapid Spanning Tree Protocol
CF0
1 3 5 7 9 11 13 15
XF
2 4 6 8 10 12 14 16
US1 US2 FAIL LNK/ACT
SWITCH 2000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
XD1 XG1
US1 GND US2 GND R1 R2 SPD
CF0
15
16
SWITCH 2000
XD1 XG1
US1 GND US2 GND R1 R2
PHOENIX CONTACT
2
5
CF0
1 3 5 7 9 11 13 15
XF
2 4 6 8 10 12 14 16
US1 US2 FAIL LNK/ACT
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
SPD
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