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

 

LINK

RECOMMENDED

RECOMMENDED

ALLOWED

 

SPEED

VALUE

RANGE

RANGE

Path Cost

4Mbps

250

100 to 1000

1 to 65535

 

 

 

 

 

Path Cost

10Mbps

100

50 to 600

1 to 65535

 

 

 

 

 

Path Cost

16Mbps

62

40 to 400

1 to 65535

 

 

 

 

 

Path Cost

100Mbps

19

10 to 60

1 to 65535

 

 

 

 

 

Path Cost

1Gbps

4

3 to 10

1 to 65535

 

 

 

 

 

Path Cost

10Gbps

2

1 to 5

1 to 65535

 

 

 

 

 

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.

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