Chapter 5 View

5.2 STP/RSTP

(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.

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

5.2.1STP Terminology

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 - see the next table.

Table 15 STP Path Costs

LINK SPEED

RECOMMENDED VALUE

RECOMMENDED RANGE

ALLOWED RANGE

4Mbps

250

100 to 1000

1 to 65535

 

 

 

 

10Mbps

100

50 to 600

1 to 65535

 

 

 

 

16Mbps

62

40 to 400

1 to 65535

 

 

 

 

100Mbps

19

10 to 60

1 to 65535

 

 

 

 

1Gbps

4

3 to 10

1 to 65535

 

 

 

 

10Gbps

2

1 to 5

1to 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 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.

5.2.2 STP Port States

STP assigns five port states (see next table) to eliminate packet looping. A bridge port is not allowed to go directly from blocking state to forwarding state so as to eliminate transient loops.

Table 16 STP Port States

PORT STATE

DESCRIPTION

Disabled

STP is disabled (default).

 

 

Blocking

Only configuration and management BPDUs are received and processed.

 

 

Listening

All BPDUs are received and processed.

 

 

 

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NetAtlas Workgroup User’s Guide