22

Spanning Tree Protocol

This chapter introduces the Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP).

22.1 RSTP and STP

RSTP adds rapid reconfiguration capability to STP. The IES-612-51A supports RSTP and the earlier STP. RSTP and STP detect and break network loops and provide backup links between switches, bridges or routers. They allow a device to interact with other RSTP or STP-aware devices in your network to ensure that only one path exists between any two stations on the network. The Integrated Ethernet Switch uses RSTP by default but can still operate with STP switches (although without RSTP’s benefits).

The root bridge is the base of the spanning tree; it is the bridge with the lowest identifier value (MAC address). Path cost is the cost of transmitting a frame onto a LAN through 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, as illustrated in the following table.

Table 48

Path Cost

 

 

 

 

 

LINK SPEED

RECOMMENDED

RECOMMENDED

ALLOWED RANGE

 

 

VALUE

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 Integrated Ethernet Switch with the lowest path cost to the root (the root path cost). If there is no root port, then this Integrated Ethernet 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.

 

175

IES-612-51A User’s Guide