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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 11 Configuring STP Understanding Spanning-Tree Features
•Message age
•The identifier of the sending interface
•Values for the hello, forward delay, and max-age protocol timers
When a switch receives a configuration BPDU that contains superior informat ion (lower bridg e ID,
lo wer pat h cost, a nd so forth), it stores the in formatio n for th at port. I f this BPDU is r ecei v ed o n the root
port of the switch, the switch also forwards it with an updated message to all attached LANs for which
it is the designated switch.
If a sw itch receives a co n fig ur at ion B PDU th at co n tains inferior informa tio n to that cu r rentl y stor ed for
that port , it discards the BPDU. If the switc h is a designated switc h for the LAN from which the inferior
BPDU was received, it sends that LAN a BPDU containing the up-to-date information stored for that
port. In thi s way, infe rior infor matio n is discarde d, and super ior informa tion is propa gated on the
network.
A BPDU exchange results in these actions:
•One switch is elected as the root switch.
•A root port is selected for each switch (except the root switch). This port provides the best path
(lowest cost) when the switch forwards packets to the root switch.
•The short est distan ce to the root sw itch is calcul ated for ea ch switch base d on the path co st.
•A des ignated switc h for eac h LAN se gment is se lected. The d esignated swi tch incur s the lo west path
cost w hen for wardi ng pac kets f rom tha t L A N to th e ro ot swi tch . Th e por t thr ough whi ch th e
designated switch is attached to the LAN is called the desig nated port.
•Interfaces included in the spanning-tree instance are s elected. Root ports and designated ports are
put in the for warding state.
•All interfaces not included in the spanning tree are blocked.

Election of the Root Switch

All sw itche s in t he L ay er 2 net work p art icipa tin g in sp an ning tre e gat her i nform at ion abou t othe r
switches in the network through an exchange of BPDU data messages. This exchange of messages results
in these actions:
•The ele ctio n of a unique ro ot switch for each spanning -tree instanc e
•The election of a designated switch for every switched LAN segment
•The removal of loops in the switched network by blocking Layer 2 interfaces connected to redundant
links
For each VLAN, th e switch with the highest switch priority (the lowest numerical priority value) is
elected as the root switc h. If all swit ches are conf igured with the defaul t priority (32768), the switch with
the lowest MAC address in the VLAN becomes the root switch. The switch priority value occupies the
most significant bits of the bri dge ID.
When you change the switch priority value, you change the probability that the switch will be elected as
the root switch. Configuring a higher value decreases the probability; a lower value increases the
probability.
The root switch is the logical center of the spanning-tree topology in a switched network. All paths that
are not needed to reach the root switch from anywhere in the switched network are placed in the
spanning-tree blocking mode.