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Chapter 4: Web-Based Management
Creating a Stable STP Topology
It is to make the root port a fastest link. If all switches have STP enabled with
default settings, the switch with the lowest MAC address in the network will
become the root switch. By increasing the priority (lowering the priority number)
of the best switch, STP can be forced to select the best switch as the root switch.
When STP is enabled using the default parameters, the path between source
and destination stations in a switched network might not be ideal. For instance,
connecting higher-speed links to a port that has a higher number than the current
root port can cause a root-port change.
STP Port States
The BPDUs take some time to pass through a network. This propagation delay
can result in topology changes where a port that transitioned directly from a
Blocking state to a Forwarding state could create temporary data loops. Ports
must wait for new network topology information to propagate throughout the
network before starting to forward packets. They must also wait for the packet
lifetime to expire for BPDU packets that were forwarded based on the old
topology. The forward delay timer is used to allow the network topology to
stabilize after a topology change. In addition, STP specifies a series of states a
port must transition through to further ensure that a stable network topology is
created after a topology change.
Each port on a switch using STP exists is in one of the following five states:
Blocking - the port is blocked from forwarding or receiving packets.
Listening - the port is waiting to receive BPDU packets that may tell the port
to go back to the blocking state.
Learning - the port is adding addresses to its forwarding database, but not yet
forwarding packets.
Forwarding - the port is forwarding packets.
Disabled - the port only responds to network management messages and
must return to the blocking state first.
A port transitions from one state to another as follows:
From initialization (switch boot) to blocking.
From blocking to listening or to disabled.
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GE-DS-242-PoE Managed Ethernet Switch User Manual
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