3com 5500-SI Manual De Usuario
STP Overview 543
To facilitate the descriptions, the description of the example is simplified. For
example, the root ID and the designated bridge ID in actual calculation should
comprise both Switch priority and Switch MAC address. Designated port ID should
comprise port priority and port MAC address. In the updating process of a
configuration BPDU, other configuration BPDUs besides the first four items will
make modifications according to certain rules. The basic calculation process is
described below:
example, the root ID and the designated bridge ID in actual calculation should
comprise both Switch priority and Switch MAC address. Designated port ID should
comprise port priority and port MAC address. In the updating process of a
configuration BPDU, other configuration BPDUs besides the first four items will
make modifications according to certain rules. The basic calculation process is
described below:
Configuration BPDU
Forwarding Mechanism
in STP
Upon the initiation of the network, all the Switches regard themselves as the roots.
The designated ports send the configuration BPDUs of local ports at a regular interval
of HelloTime. If it is the root port that receives the configuration BPDU, the Switch will
enable a timer to time the configuration BPDU as well as increase MessageAge carried
in the configuration BPDU by certain rules. If a path goes wrong, the root port on this
path will not receive configuration BPDUs any more and the old configuration BPDUs
will be discarded due to timeout. Hence, recalculation of the spanning tree will be
initiated to generate a new path to replace the failed one and thus restore the
network connectivity.
The designated ports send the configuration BPDUs of local ports at a regular interval
of HelloTime. If it is the root port that receives the configuration BPDU, the Switch will
enable a timer to time the configuration BPDU as well as increase MessageAge carried
in the configuration BPDU by certain rules. If a path goes wrong, the root port on this
path will not receive configuration BPDUs any more and the old configuration BPDUs
will be discarded due to timeout. Hence, recalculation of the spanning tree will be
initiated to generate a new path to replace the failed one and thus restore the
network connectivity.
However, the new configuration BPDU as now recalculated will not be propagated
throughout the network right away, so the old root ports and designated ports that
have not detected the topology change will still forward the data through the old
path. If the new root port and designated port begin to forward data immediately
after they are elected, an occasional loop may still occur. In RSTP, a transitional state
mechanism is thus adopted to ensure the new configuration BPDU has been
propagated throughout the network before the root port and designated port begin
to send data again. That is, the root port and designated port should undergo a
transitional state for a period of Forward Delay before they enter the forwarding
state.
throughout the network right away, so the old root ports and designated ports that
have not detected the topology change will still forward the data through the old
path. If the new root port and designated port begin to forward data immediately
after they are elected, an occasional loop may still occur. In RSTP, a transitional state
mechanism is thus adopted to ensure the new configuration BPDU has been
propagated throughout the network before the root port and designated port begin
to send data again. That is, the root port and designated port should undergo a
transitional state for a period of Forward Delay before they enter the forwarding
state.
Implement RSTP on the
Switch
The Switch implements the Rapid Spanning Tree Protocol (RSTP), an enhanced form
of STP. The Forward Delay for the root ports and designated ports to enter forwarding
state is greatly reduced in certain conditions, thereby shortening the time period for
stabilizing the network topology.
of STP. The Forward Delay for the root ports and designated ports to enter forwarding
state is greatly reduced in certain conditions, thereby shortening the time period for
stabilizing the network topology.
To achieve the rapid transition of the root port state, the following requirement
should be met: The old root port on this Switch has stopped data forwarding and the
designated port in the upstream has begun forwarding data.
should be met: The old root port on this Switch has stopped data forwarding and the
designated port in the upstream has begun forwarding data.
The conditions for rapid state transition of the designated port are:
■
The port is an edge port that does not connect with any Switch directly or
indirectly. If the designated port is an edge port, it can Switch to forwarding state
directly without immediately forwarding data.
indirectly. If the designated port is an edge port, it can Switch to forwarding state
directly without immediately forwarding data.
■
The port is connected with the point-to-point link, that is, it is the master port in
aggregation ports or full duplex port. It is feasible to configure a point-to-point
connection. However, errors may occur and therefore this configuration is not
recommended. If the designated port is connected with the point-to-point link, it
can enter the forwarding state right after handshaking with the downstream
Switch and receiving the response.
aggregation ports or full duplex port. It is feasible to configure a point-to-point
connection. However, errors may occur and therefore this configuration is not
recommended. If the designated port is connected with the point-to-point link, it
can enter the forwarding state right after handshaking with the downstream
Switch and receiving the response.
The Switch that uses RSTP is compatible with the one using STP. Both protocol
packets can be identified by the Switch running RSTP and used in spanning tree
calculation.
packets can be identified by the Switch running RSTP and used in spanning tree
calculation.