Configuration Guide User guide

FastIron Configuration Guide 1111

53-1002494-02

STP feature configuration

The following example (Figure 126) explains role assignments in a simple RSTP topology.

NOTE

All examples in this document assume that all ports in the illustrated topologies are point-to-point

links and are homogeneous (they have the same path cost value) unless otherwise specified.

The topology in Figure 126 contains four bridges. Switch 1 is the root bridge since it has the lowest

bridge priority. Switch 2 through Switch 4 are non-root bridges.

FIGURE 126 Simple 802.1W topology

Assignment of ports on Switch 1

All ports on Switch 1, the root bridge, are assigned Designated port roles.

Assignment of ports on Switch 2

Port2 on Switch 2 directly connects to the root bridge; therefore, Port2 is the Root port.

The bridge priority value on Switch 2 is superior to that of Switch 3 and Switch 4; therefore, the

ports on Switch 2 that connect to Switch 3 and Switch 4 are given the Designated port role.

Furthermore, Port7 and Port8 on Switch 2 are physically connected. The RST BPDUs transmitted

by Port7 are superior to those Port8 transmits. Therefore, Port8 is the Backup port and Port7 is the

Designated port.

Assignment of ports on Switch 3

Port2 on Switch 3 directly connects to the Designated port on the root bridge; therefore, it assumes

the Root port role.

The root path cost of the RST BPDUs received on Port4/Switch 3 is inferior to the RST BPDUs

transmitted by the port; therefore, Port4/Switch 3 becomes the Designated port.

Switch 1

Bridge priority = 100

Switch 2

Bridge priority = 200

Switch 3

Bridge priority = 300

Switch 4

Bridge priority = 400

Port2

Port3

Port4

Port7

Port8