Spanning Tree Protocol (Stp); Rapid Stp; Stp Terminology; How Stp Works - ZyXEL Communications ZYWALL 2 PLUS User Manual

Chapter 7 Bridge Screens

7.2 Spanning Tree Protocol (STP)

STP detects and breaks network loops and provides backup links between switches, bridges or
routers. It allows a bridge to interact with other STP-compliant bridges in your network to
ensure that only one route exists between any two stations on the network.

7.2.1 Rapid STP

The ZyWALL uses IEEE 802.1w RSTP (Rapid Spanning Tree Protocol) that allow faster
convergence of the spanning tree (while also being backwards compatible with STP-only
aware bridges). Using RSTP, topology change information does not have to propagate to the
root bridge and unwanted learned addresses are flushed from the filtering database. In RSTP,
the port states are Discarding, Learning, and Forwarding.

7.2.2 STP Terminology

The root bridge is the base of the spanning tree.
Path cost is the cost of transmitting a frame from the root bridge to that port. It is assigned
according to the speed of the link to which a port is attached. The slower the media, the higher
the cost - see the next table.

Table 26 STP Path Costs

Path Cost
Path Cost
Path Cost
Path Cost
Path Cost
Path Cost
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 bridge 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.

7.2.3 How STP Works

After a bridge determines the lowest cost-spanning tree with STP, it enables the root port and
the ports that are the designated ports for connected LANs, and disables all other ports that
participate in STP. Network packets are therefore only forwarded between enabled ports,
eliminating any possible network loops.
STP-aware bridges exchange Bridge Protocol Data Units (BPDUs) periodically. When the
bridged LAN topology changes, a new spanning tree is constructed.
136
LINK RECOMMENDED
SPEED VALUE
4Mbps 250
10Mbps 100
16Mbps 62
100Mbps 19
1Gbps 4
10Gbps 2
RECOMMENDED
RANGE
100 to 1000
50 to 600
40 to 400
10 to 60
3 to 10
1 to 5
ZyWALL 2 Plus User's Guide
ALLOWED
RANGE
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535