ZyXEL Communications MSC1000G User Manual page 523

Table 277 MSTP/RSTP Path Costs
LINK SPEED
1000 Gbps
10 000 Gbps
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 356 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.
In STP, once a stable network topology has been established, all devices listen for Hello BPDUs
transmitted from the root bridge. If an STP-aware device does not get a Hello BPDU after a
predefined interval (Max Age), the device assumes that the link to the root bridge is down. This
device then initiates negotiations with other devices to reconfigure the network to re-establish a
valid network topology.
Management Switch Card User's Guide
RECOMMENDED VALUE
20
2
RECOMMENDED RANGE
2 to 200
1 to 20
Chapter 13 Switch Screens
ALLOWED RANGE
1 to 200 000 000
1 to 200 000 000
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