PT-7728 User’s Manual Featured Functions

3-31

y Bridge A has been selected as the Root Bridge, since it was determined to have the lowest

Bridge Identifier on the network.

y Since Bridge A is the Root Bridge, it is also the Designated Bridge for LAN segment 1. Port 1

on Bridge A is selected as the Designated Bridge Port for LAN Segment 1.

y Ports 1 of Bridges B, C, X, and Y are all Root Ports sine they are nearest to the Root Bridge,

and therefore have the most efficient path.

y Bridges B and X offer the same Root Path Cost for LAN segment 2. However, Bridge B was

selected as the Designated Bridge for that segment since it has a lower Bridge Identifier. Port

2 on Bridge B is selected as the Designated Bridge Port for LAN Segment 2.

y Bridge C is the Designated Bridge for LAN segment 3, because it has the lowest Root Path

Cost for LAN Segment 3:

y The route through Bridges C and B costs 200 (C to B=100, B to A=100)

y The route through Bridges Y and B costs 300 (Y to B=200, B to A=100)

y The Designated Bridge Port for LAN Segment 3 is Port 2 on Bridge C.

Using STP on a Network with Multiple VLANs

IEEE Std 802.1D, 1998 Edition, does not take into account VLANs when calculating STP

information—the calculations only depend on the physical connections. Consequently, some

network configurations will result in VLANs being subdivided into a number of isolated sections

by the STP system. You must ensure that every VLAN configuration on your network takes into

account the expected STP topology and alternative topologies that may result from link failures.

The following figure shows an example of a network that contains VLANs 1 and 2. The VLANs

are connected using the 802.1Q-tagged link between Switch B and Switch C. By default, this link

has a port cost of 100 and is automatically blocked because the other Switch-to-Switch

connections have a port cost of 36 (18+18). This means that both VLANs are now

subdivided—VLAN 1 on Switch units A and B cannot communicate with VLAN 1 on Switch C,

and VLAN 2 on Switch units A and C cannot communicate with VLAN 2 on Switch B.

Block

802.1Q tagged,

10BaseTx

half-duplex Link

camies VLAN1, 2

(path cost = 100)

100BaseTX

full-duplex Link;

only carries VLAN1

(path cost =18)

100BaseTX

full-duplex Link;

only carries VLAN2

(path cost =18)

LNK/ACTSPEEDFDX/HDX
FAULTMASTERCOUPLER
STATPWR1PWR2
RING PORTCOUPLER
PORT
MODE
37
48
26
15
37
48
26
15
4
3
2
1
37
4826
15
M1
M2
M3
M4
LNK/ACTSPEEDFDX/HDX
FAULTMASTERCOUPLER
STATPWR1PWR2
RING PORTCOUPLER
PORT
MODE
37
4826
15
37
4826
15
4
32
1
37
48
26
15
M1
M2
M3
M4
1
2
3
4
5
6
7
8
M3
PT-7728
M4
1
2
3
4
4
1
3
2
135
246
M1
TXRX TX RX TX RX
246
135
M2
TXRX TX RX TX RX
LNK/ACTSPEEDFDX/HDX
FAULTMASTERCOUPLER
STATPWR1PWR2
RING PORTCOUPLER
PORT
MODE
37
48
26
15
37
48
26
15
4
3
2
1
37
4826
15
M1
M2
M3
M4
LNK/ACTSPEEDFDX/HDX
FAULTMASTERCOUPLER
STATPWR1PWR2
RING PORTCOUPLER
PORT
MODE
37
4826
15
37
4826
15
4
32
1
37
48
26
15
M1
M2
M3
M4
1
2
3
4
5
6
7
8
M3
PT-7728
M4
1
2
3
4
4
1
3
2
135
246
M1
TXRX TX RX TX RX
246
135
M2
TXRX TX RX TX RX
LNK/ACTSPEEDFDX/HDX
FAULTMASTERCOUPLER
STATPWR1PWR2
RING PORTCOUPLER
PORT
MODE
37
48
26
15
37
48
26
15
4
3
2
1
37
4826
15
M1
M2
M3
M4
LNK/ACTSPEEDFDX/HDX
FAULTMASTERCOUPLER
STATPWR1PWR2
RING PORTCOUPLER
PORT
MODE
37
4826
15
37
4826
15
4
32
1
37
48
26
15
M1
M2
M3
M4
1
2
3
4
5
6
7
8
M3
PT-7728
M4
1
2
3
4
4
1
3
2
135
246
M1
TXRX TX RX TX RX
246
135
M2
TXRX TX RX TX RX

To avoid subdividing VLANs, all inter-switch connections should be made members of all

available 802.1Q VLANs. This will ensure connectivity at all times. For example, the connections

between Switches A and B, and between Switches A and C should be 802.1Q tagged and carrying

VLANs 1 and 2 to ensure connectivity.

See the Configuring Virtual LANs section for more information about VLAN Tagging.