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C
HAPTER
4: U
SING
R
ESILIENCE
F
EATURES
Default Behavior
This section contains important information to note when using the RSTP
and Fast Start features, particularly if you already have existing Switch
4200 units in your network with an older version of software.
RSTP Default
Behavior
When using the RSTP feature, note that the Switch will have RSTP
enabled by default.
Fast Start Default
Behavior
When using the Fast Start feature note that the Switch will have Fast Start
enabled by default.
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 10 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.