EDS-510A Series User’s Manual Featured Functions
3-46
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.
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