GE T1000, Техническое руководство

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GE Reason Switches Chapter 4
– Functions
REASON SWITCHES-TM-EN-3 67
Each port can also be configured as point-to-point or shared link type. Full duplex
ports are considered directly point-to-point links, which makes them change their
state to forwarding directly if they are designated port. On the other hand, half-
duplex ports are considered shared link ports.
RSTP protocol has changed the way the “flags” field at the BPDU frame is used, when
compared to STP protocol. In RSTP the bits are used as follows:
Figure 27: BPDU flag field at RSTP protocol
The change on bits usage was done because the mechanism of transmission of
BPDU packets changed. In the RSTP protocol, each bridge in the network can send
BPDU
packets, and they can also receive BPDU packets from a ‘less important’ bridge
in the topology. Thus, as all of the bridges can send BPDU, it is faster to detect a link
failure. In case of receiving a BPDU packet from an inferior bridge, it assumes that
the connection to the root has been lost and a reconfiguration of the topology is
needed.
Finally, in the RSTP protocol each bridge that is in the topology starts sending BPDU
packets based on the ‘hello time’ of the frame’s transmission. RSTP aware bridges
can send BPDU without receiving it from a root bridge. If the bridge stops receiving
BPDU packets from designated or root bridges three times in a row, it will assume it
has lost the connection to the bridge and then a reconfiguration at the ports should
b
e started. By default, Reason Switches send a ‘hello time’ message every 2 seconds.
6.4 MSTP Protocol
Multiple Spanning Tree protocol is an enhancement of the RSTP protocol, developed
to be used in environments with VLANs. When using this protocol, all spanning tree
information is contained in single BPDU packet, thus reducing traffic and ensuring
that the MSTP protocol is compatible with other spanning tree protocols.
The main improvement related to RSTP is the possibility to create regions of spanning
tree, mapped to defined VLANs, making spanning tree convergence faster.