Hirschmann GREYHOUND GRS1020 - Page 500

Redundancy
12.3
Spanning Tree
160
UM Config GRS
Release
8.0 09/2019
Root Path Cost
Each path that connects 2 bridges is assigned a cost for the transmission (path cost). The device
determines this value based on the transmission speed (see table 28). It assigns a higher path cost
to paths with lower transmission speeds.
Alternatively, the Administrator can set the path cost. Like the device, the Administrator assigns a
higher path cost to paths with lower transmission speeds. However, since the Administrator can
choose this value freely, he has a tool with which he can give a certain path an advantage among
redundant paths.
The root path cost is the sum of the individual costs of those paths that a data packet has to traverse
from a connected bridge‘s port to the root bridge.
Figure 32: Path costs
Port Identifier
The port identifier consists of 2 bytes. One part, the lower-value byte, contains the physical port
number. This provides a unique identifier for the port of this bridge. The second, higher-value part
is the port priority, which is specified by the Administrator (default value: 128). It also applies here
that the port with the smallest number for the port identifier has the highest priority.
Figure 33: Port Identifier
Table 28: Recommended path costs for RSTP based on the data rate.
Data rate Recommended value Recommended range Possible range
≤100 Kbit/s
200 000000
1
1. Bridges that conform with IEEE 802.1D 1998 and only support 16-bit values for the path costs should use the value
65,535 (FFFFH) for path costs in cases where they are used in conjunction with bridges that support 32-bit values for
the path costs.
20000000-200000000 1-200000000
1 Mbit/s
20000000
a
2000000-200000000 1-200000000
10 Mbit/s
2000000
a
200000-20000000 1-200000000
100 Mbit/s
200000
a
20000-2000000 1-200000000
1 Gbit/s 20000 2000-200000 1-200000000
10 Gbit/s 2000 200-20000 1-200 000000
100 Gbit/s 200 20-2000 1-200000000
1 TBit/s 20 2-200 1-200000000
10 TBit/s 2 1-20 1-200000000
PC = 200 000 000
PC = 200
000
PC = 200
000
1
2 3
Ethernet (100 Mbit/s)
Ethernet (10 Mbit/s)
PC Path costs
Priority Port number
MSB LSB