User's Manual

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Chapter 34 Configuring STP
34.1 STP Introduction
The standard Spanning Tree Protocol (STP) is based on the IEEE 802.1D standard. An SWITCH stack appears
as a single spanning-tree node to the rest of the network, and all stack members use the same bridge ID.
The spanning-tree algorithm and the spanning-tree protocol can set any bridge LAN to be a simply connected
mobile topology. In the mobile topology, some bridge ports can forward frames, while other ports are blocked
and cannot forward data. A port in blocked state can also be contained in the mobile topology. When some
network device is out of effect, added or removed, the port in blocked state will enter the forwarding state.
In the spanning-tree topology, a bridge is regarded as a root or a root bridge. Each LAN segment has a bridge
port to take in charge of data forwarding from this network segment to the root. This bridge port is regarded as
the designated port of this LAN segment, while the bridge where the bridge port locates is regarded as the
designated bridge of LAN The root is the designated bridge of each LAN segment that connects this root. In
each bridge port, the port that is nearest to the root bridge is the root port of this bridge and only the root port
and the designated port are in forwarding state; another kind of ports are open, but they are not root ports or
designated ports but standby ports.
The following parameters decide the structure of the stabile mobile topology:
(1) Each unique bridge identifier
(2) path cost of each port
(3) ID of each bridge port
The bridge with the highest priority (the identifier value is the smallest) will be chosen as the root bridge. The
ports of each bridge in the network all have root path cost, that is, the root path cost is the smallest value of
the path cost sum of all ports between the root bridge and the bridge. The designated port of each LAN segment
means the port that connect this LAN segment and has the smallest root path cost; if several ports have the
same root path cost, their bridge identifiers will first be compared and then their port identifiers. According to
this method, each LAN segment has only one designated port and each bridge has only one root port.
The spanning tree topology makes the loop inexistent in a network, guaranteeing the stability and fault recovery
of the network. With the wide spread of Ethernet switch, STP plays a more and more important role. Therefore,
STP is provided as a basic function of switches.
Rapid Spanning-Tree Protocol (RSTP) is an important update of 802.1D STP. When faults occur in the bridge,
bridge port or LAN segment in a network, RSTP will realize the rapid convergence of the network topology. In
this case, the new root port on the bridge will enter the forwarding state promptly, and at the same time the
direct acceptance between bridges can make a designated port to forward data immediately. Please refer to
Chapter 2 for RSTP protocolConfiguring RSTP.
This chapter describes how to configure the standard STP of the switch.
Note:
802.1D STP and 802.1D RSTP mentioned in this text are simplified as SSTP and RSTP
respectively. SSTP here is short for Single Spanning-Tree Protocol.