1-3
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring STP
Understanding Spanning-Tree Features
Spanning-Tree Topology and BPDUs
The stable, active spanning-tree topology of a switched network is controlled by these elements:
•
The unique bridge ID (switch priority and MAC address) associated with each VLAN on each
switch. In a switch stack, all switches use the same bridge ID for a given spanning-tree instance.
•
The spanning-tree path cost to the root switch.
•
The port identifier (port priority and MAC address) associated with each Layer 2 interface.
When the switches in a network are powered up, each functions as the root switch. Each switch sends a
configuration BPDU through all of its ports. The BPDUs communicate and compute the spanning-tree
topology. Each configuration BPDU contains this information:
•
The unique bridge ID of the switch that the sending switch identifies as the root switch
•
The spanning-tree path cost to the root
•
The bridge ID of the sending switch
•
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
information (lower bridge ID,
lower path cost, and so forth), it stores the information for that port. If this BPDU is received on 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 switch receives a configuration BPDU that contains
inferior
information to that currently stored for
that port, it discards the BPDU. If the switch is a designated switch 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 this way, inferior information is discarded, and superior information is propagated on the
network.
A BPDU exchange results in these actions:
•
One switch in the network is elected as the root switch (the logical center of the spanning-tree
topology in a switched network). In a switch stack, one stack member is elected as the stack root
switch. The stack root switch contains the outgoing root port (Switch 1), as shown in
.
For each VLAN, the switch with the highest switch priority (the lowest numerical priority value) is
elected as the root switch. If all switches are configured with the default 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 bridge ID, as shown in
.
•
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.
When selecting the root port on a switch stack, spanning tree follows this sequence:
–
Selects the lowest root bridge ID
–
Selects the lowest path cost to the root switch
–
Selects the lowest designated bridge ID
–
Selects the lowest designated path cost
–
Selects the lowest port ID
Summary of Contents for Catalyst 3560-X Series
Page 12: ...Contents 10 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 13: ...Contents 11 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 14: ...Contents 12 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 15: ...Contents 13 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 16: ...Contents 14 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 17: ...Contents 15 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 18: ...Contents 16 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 19: ...Contents 17 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 20: ...Contents 18 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 21: ...Contents 19 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 22: ...Contents 20 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 23: ...Contents 21 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 24: ...Contents 22 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 25: ...Contents 23 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 26: ...Contents 24 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 27: ...Contents 25 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 28: ...Contents 26 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 29: ...Contents 27 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 30: ...Contents 28 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 31: ...Contents 29 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 32: ...Contents 30 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 33: ...Contents 31 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 34: ...Contents 32 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 35: ...Contents 33 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 36: ...Contents 34 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 37: ...Contents 35 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 38: ...Contents 36 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...
Page 42: ...56 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 Preface ...
Page 1538: ...Index IN 58 Catalyst 3750 X and 3560 X Switch Software Configuration Guide OL 25303 03 ...