Chapter 13 Spanning Tree Protocol

13.1.2 What You Need to Know

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

The Switch uses IEEE 802.1w RSTP (Rapid Spanning Tree Protocol) that allows faster convergence of the spanning tree than STP (while also being backwards compatible with STP-only aware bridges). In RSTP, topology change information is directly propagated throughout the network from the device that generates the topology change. In STP, a longer delay is required as the device that causes a topology change first notifies the root bridge that then notifies the network. Both RSTP and STP flush unwanted learned addresses from the filtering database. In RSTP, the port states are Discarding, Learning, and Forwarding.

Note: In this user’s guide, “STP” refers to both STP and RSTP.

STP Terminology

The root bridge is the base of the spanning tree.

Path cost is the cost of transmitting a frame onto a LAN through that port. The recommended cost is assigned according to the speed of the link to which a port is attached. The slower the media, the higher the cost.

Table 27 STP Path Costs

 

LINK

RECOMMENDED

RECOMMENDED

ALLOWED

 

SPEED

VALUE

RANGE

RANGE

Path

4Mbps

250

100 to 1000

1 to 65535

Cost

 

 

 

 

 

 

 

 

 

Path

10Mbps

100

50 to 600

1 to 65535

Cost

 

 

 

 

 

 

 

 

 

Path

16Mbps

62

40 to 400

1 to 65535

Cost

 

 

 

 

 

 

 

 

 

Path

100Mbps

19

10 to 60

1 to 65535

Cost

 

 

 

 

 

 

 

 

 

Path

1Gbps

4

3 to 10

1 to 65535

Cost

 

 

 

 

 

 

 

 

 

Path

10Gbps

2

1 to 5

1 to 65535

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 switch has been accepted as the root bridge of the spanning tree network.

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GS2200-24 User’s Guide