Using STP on a Network with Multiple VLANs 43
Using STP on a
Network with
Multiple VLANs
The IEEE Std 802.1D, 1998 Edition does not take into account VLANs
when it calculates STP information — the calculations are only performed
on the basis of physical connections. For this reason, some network
configurations can result in VLANs being subdivided into a number of
isolated sections by the STP system. Therefore, you must ensure that any
VLAN configuration on your network takes into account the expected STP
topology and alternative topologies that may result from link failures.
For example, Figure 13
shows a network containing VLANs 1 and 2. They
are connected using the 802.1Q-tagged link between Switch B and
Switch C. By default, this link has a path cost of 100 and is automatically
blocked because the other Switch-to-Switch connections have a path 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.
Figure 13 Configuration that separates VLANs
To avoid any VLAN subdivision, it is recommended that all inter-Switch
connections are made members of all available 802.1Q VLANs to 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.
For more information about VLAN Tagging, see Chapter 8
“Setting Up
Virtual LANs”.
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