GE T1000, Technical Manual

Page: 59 / 302

GE Reason Switches Chapter 4
– Functions
REASON SWITCHES-TM-EN-3 59
6 Spanning Tree Protocol
Spanning Tree protocol is a mechanism created to solve the problems that arise
when a loop is inserted in a LAN. As shown in the Loop fundamentals section,
Ethernet networks were not developed to work in loop topologies. As redundant
paths are generally required for most of network applications, several protocols have
been developed to solve these problems.
The most common protocol to identify loops is the Spanning Tree Protocol, defined
by IEEE 802.1D-2004 Standard. IEC 61850-90-4 Technical Report specifies that, for
substation networks, Rapid Spanning Tree Protocol (RSTP) shall be used when looped
topologies, such as ring topology, are required at the station level. This chapter
contains the following sections:
This chapter contains the following sections:
Spanning Tree fundamentals;

STP protocol;

RSTP protocol;

MSTP protocol.
6.1 Spanning Fundamentals
The need of a protocol to solve problem of loops in Ethernet networks started at the
beginning of the commercial usage. As loops were required for better reliability of
networks and loop-free topologies are difficult to maintain, automatic loop detection
became a necessity. Therefore, the Spanning Tree protocol was created.
The protocol operates sending and receiving specific packets over the network, to
map actual topology and act when required. The packets are the BPDU (Bridge
Protocol Data Units) packets, and they are structured as shown below.
Figure 15: BPDU Packet
By sending these packets over the network, switches can map physical topology to
search for loops and disable them. Thus, the resulting logical topology will be a loop
free tree topology. The following figure shows the possible paths for data traffic from
IED A to IED B.