3Com 3C17300A Implementation Guide - Page 48

Using STP on a Network with Multiple VLANs, on Switch B.

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48 CHAPTER 4: USING RESILIENCE FEATURES Using STP on a Network with Multiple VLANs The IEEE 802.1D standard 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 12 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 12 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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48
C
HAPTER
4: U
SING
R
ESILIENCE
F
EATURES
Using STP on a
Network with
Multiple VLANs
The IEEE 802.1D standard 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 12
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 12
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”
.