User`s manual

TN-5516/5518 Series Featured Functions
3-36
• Bridge A has been selected as the Root Bridge, since it was determined to have the lowest Bridge Identifier
on the network.
• 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.
• 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.
• 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.
• Bridge C is the Designated Bridge for LAN segment 3, because it has the lowest Root Path Cost for LAN
Segment 3:
 The route through Bridges C and B costs 200 (C to B=100, B to A=100)
 The route through Bridges Y and B costs 300 (Y to B=200, B to A=100)
• 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)
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.