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Table 247 MSTP/RSTP Path Costs
LINK
SPEED
10 Mbps
100 Mbps
1 Gbps
10 Gbps
100 Gbps
1000 Gbps
10 000 Gbps 2
On each bridge, the root port is the port through which this bridge communicates with the root.
It is the port on this switch with the lowest path cost to the root (the root path cost). If there is
no root port, then this switch has been accepted as the root bridge of the spanning tree
network.
For each LAN segment, a designated bridge is selected. This bridge has the lowest cost to the
root among the bridges connected to the LAN.
After a bridge determines the lowest cost-spanning tree with RSTP, it enables the root port and
the ports that are the designated ports for the connected LANs, and disables all other ports that
participate in RSTP. Network packets are therefore only forwarded between enabled ports,
eliminating any possible network loops.
Figure 331 STP Root Ports and Designated Ports
RSTP-aware devices exchange Bridge Protocol Data Units (BPDUs) periodically. When the
bridged LAN topology changes, a new spanning tree is constructed.
In RSTP, the devices send BPDUs every Hello Time. If an RSTP-aware device does not get a
Hello BPDU after three Hello Times pass (or the Max Age), the device assumes that the link to
the neighboring bridge is down. This device then initiates negotiations with other devices to
reconfigure the network to re-establish a valid network topology.
MSC1000G/1024G/1224G Series User's Guide
RECOMMENDED VALUE
2 000 000
200 000
20 000
2000
200
20
RECOMMENDED RANGE
200 000 to 20 000 000
20 000 to 2 000 000
2000 to 200 000
200 to 20 000
20 to 2000
2 to 200
1 to 20
Chapter 15 Switch Screens
ALLOWED
RANGE
1 to 200 000 000
1 to 200 000 000
1 to 200 000 000
1 to 200 000 000
1 to 200 000 000
1 to 200 000 000
1 to 200 000 000
481
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Troubleshooting
