The stable active
spanning tree topology of a switched network is determined by the following:
(port priority and MAC address) associated with each Layer 2 interface.
path cost to the root bridge.
Unique bridge ID
(bridge priority and MAC address) associated with each VLAN on each switch.
The bridge protocol
data units (BPDUs) are transmitted in one direction from the root switch and
each switch sends configuration BPDUs to communicate and compute the spanning
tree topology. Each configuration BPDU contains the following minimal
Bridge ID of the
of the transmitting port
path cost to the root
ID of the switch that the transmitting switch believes to be the root switch
Values for the
hello, forward delay, and max-age protocol timers
When a switch
transmits a BPDU frame, all switches connected to the LAN on which the frame is
transmitted receive the BPDU. When a switch receives a BPDU, it does not
forward the frame but uses the information in the frame to calculate a BPDU,
and, if the topology changes, begin a BPDU transmission.
A BPDU exchange
results in the following:
bridge for each LAN segment is selected. This is the switch closest to the root
bridge through which frames are forwarded to the root.
A root port is
selected. This is the port providing the best path from the bridge to the root
One switch is
elected as the root switch.
in the spanning tree are selected.
distance to the root switch is calculated for each switch based on the path
For each VLAN, the
switch with the highest bridge 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 spanning tree
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 spanning tree blocking mode.
information about the transmitting bridge and its ports, including bridge and
MAC addresses, bridge priority, port priority, and path cost. Spanning tree
uses this information to elect the root bridge and root port for the switched
network, as well as the root port and designated port for each switched
MAC Address Allocation
The MAC address allocation manager has a pool
of MAC addresses that are used as bridge IDs for the VLAN spanning trees. In
the table below, you can view the number of VLANs allowed for each platform.
Table 2 Number of VLANs Allowed for
Number of VLANs Allowed
3640 or higher
MAC addresses are
allocated sequentially, with the first MAC address in the range assigned to
VLAN 1, the second MAC address in the range assigned to VLAN 2, and so forth.
For example, if the MAC address range is 00-e0-1e-9b-2e-00 to
00-e0-1e-9b-31-ff, the VLAN 1 bridge ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge
ID is 00-e0-1e-9b-2e-01, the VLAN 3 bridge ID is 00-e0-1e-9b-2e-02, and so
started when a root port or blocked port on a switch receives inferior bridge
protocol data units (BPDUs) from its designated bridge. An inferior BPDU
identifies one switch as both the root bridge and the designated bridge. When a
switch receives an inferior BPDU, it means that a link to which the switch is
not directly connected is failed. That is, the designated bridge has lost its
connection to the root switch. Under Spanning Tree Protocol (STP) rules, the
switch ignores inferior BPDUs for the configured maximum aging time specified
determines if it has an alternate path to the root switch. If the inferior BPDU
arrives on a blocked port, the root port and other blocked ports on the switch
become alternate paths to the root switch. If the inferior BPDU arrives on the
root port, all blocked ports become alternate paths to the root switch. If the
inferior BPDU arrives on the root port and there are no blocked ports, the
switch assumes that it lost connectivity to the root switch, causes the maximum
aging time on the root to expire, and becomes the root switch according to
normal STP rules.
ports are not considered as alternate paths to the root switch.
If the switch
possesses alternate paths to the root switch, it uses these alternate paths to
transmit the protocol data unit (PDU) that is called the root link query PDU.
The switch sends the root link query PDU on all alternate paths to the root
switch. If the switch determines that it has an alternate path to the root, it
causes the maximum aging time on ports on which it received the inferior BPDU
to expire. If all the alternate paths to the root switch indicate that the
switch has lost connectivity to the root switch, the switch causes the maximum
aging time on the ports on which it received an inferior BPDU to expire. If one
or more alternate paths connect to the root switch, the switch makes all ports
on which it received an inferior BPDU its designated ports and moves them out
of the blocking state (if they were in the blocking state), through the
listening and learning states, and into the forwarding state.
The figure below
shows an example topology with no link failures. Switch A, the root switch,
connects directly to Switch B over link L1 and to Switch C over link L2. The
interface on Switch C that connects directly to Switch B is in the blocking
Figure 1. BackboneFast Example Before
Indirect Link Failure
If link L1 fails,
Switch C cannot detect this failure because it is not connected directly to
link L1. However, Switch B is directly connected to the root switch over L1 and
it detects the failure, elects itself as the root switch, and begins sending
BPDUs to Switch C. When Switch C receives the inferior BPDUs from Switch B,
Switch C assumes that an indirect failure has occurred. At that point,
BackboneFast allows the blocked port on Switch C to move to the listening state
without waiting for the maximum aging time for the port to expire. BackboneFast
then changes the interface on Switch C to the forwarding state, providing a
path from Switch B to Switch A. This switchover takes 30 seconds, twice the
forward delay time, if the default forward delay time of 15 seconds is set. The
figure below shows how BackboneFast reconfigures the topology to account for
the failure of link L1.
Figure 2. BackboneFast Example After
Indirect Link Failure
If a new switch
is introduced into a shared-medium topology as shown in the figure below,
BackboneFast is not activated because inferior BPDUs did not come from the
designated bridge (Switch B). The new switch begins sending inferior BPDUs that
say it is the root switch. However, the other switches ignore these inferior
BPDUs, and the new switch learns that Switch B is the designated bridge to
Switch A, the root switch.
Figure 3. Adding a Switch in a