Chapter 8
Spanning Tree
STP (Spanning Tree Protocol), subject to IEEE 802.1D standard, is to disbranch a ring network in
the Data Link layer in a local network. Devices running STP discover loops in the network and
block ports by exchanging information, in that way, a ring network can be disbranched to form a
tree-topological ring-free network to prevent packets from being duplicated and forwarded
endlessly in the network.
BPDU (Bridge Protocol Data Unit) is the protocol data that STP and RSTP use. Enough
information is carried in BPDU to ensure the spanning tree generation. STP is to determine the
topology of the network via transferring BPDUs between devices.
To implement spanning tree function, the switches in the network transfer BPDUs between
each other to exchange information and all the switches supporting STP receive and process
the received BPDUs. BPDUs carry the information that is needed for switches to figure out the
spanning tree.
STP Elements
Bridge ID
(Bridge Identifier):
The value of the priority and MAC address of the switch. It is used
to select the root bridge. The bridge ID is composed of a 2-byte priority and a 6-byte MAC
address. The priority is allowed to be configured manually on the switch, and the switch with
the lowest priority value will be elected as the root bridge. If the priority of all the switches are
the same, the switch with the lowest MAC address is selected as the root bridge.
Root Bridge
:
The root of a spanning tree. There is only one root bridge in each spanning tree,
and the root bridge has the lowest bridge ID. Configure the switch with the best performance in
the ring network as the root bridge to ensure best network performance and reliability.
Designated Bridge
: Indicates the switch has the lowest path cost from the switch to the root
bridge in each LAN segment. BPDUs are forwarded to the network segment through the
designated bridge.
Path Cost
: The path cost reflects the link speed of the port. The smaller the value, the higher
link speed the port has.
The path cost can be manually configured on each port. If not, the path cost value is
automatically calculated according to the link speed as shown below:
Link Speed
Path Cost Value
10Mb/s
2,000,000
100Mb/s
200,000
1Gb/s
2,0000
10Gb/s
2000
Table 8-1 Default path cost value
Root Path Cost
:
The root path cost is the accumulated path costs from the root bridge to the
other switches. When the root bridge sends its BPDU, the root path cost value is 0. When a
connected switch receives this BPDU, it increments the path cost of its local incoming port.
Then it forwards this BPDU to the downstream switch, with the updated root path cost. The
value of the accumulated root path cost increases as the BPDU propagates further.
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