Version—Contains 1 byte and the value of zero.• Message Type—Contains 1 byte and the value of zero.• Flag—Contains 1 byte; only the first 2 bits are used. The topology change (TC) bit signals that there
has been a topology change. The topology change acknowledgment (TCA) bit is then set to
acknowledge receipt of a configuration message with the TC signal bit set.
•
Root ID—Contains 8 bytes that identify the root bridge by listing a 2−byte priority followed by a
6−byte ID.
•
Root Path Cost—Contains 4 bytes containing the cost of the path from the bridge sending the
configuration message to the root bridge.
•
Bridge ID—Contains 8 bytes identifying the priority and ID of the bridge sending the message.• Port ID—Contains 2 bytes identifying the port from which the configuration message was sent. This
field allows loops created by multiple attached bridges to be detected immediately by STP.
•
Message Age—Contains 2 bytes specifying the amount of time since the root sent the configuration
message on which the current configuration message is based.
•
MaxAge—Contains 2 bytes indicating when the current configuration message should be discarded.• Hello Time—Contains 2 bytes indicating the time period between root bridge configuration messages.• FwdDelay—Contains 2 bytes indicating the length of time that the bridge should wait before
transitioning to a new state following a topology change in the network.
•
Timer’s affect the way BPDUs operate in a network and converge in the event of a data loop or network
topology change. Let’s take a look at these timers.
BPDU Timers
STP uses timers to prevent data loops and to determine how long it will take STP to converge after a link
failure or change in the network topology. As frames and packets travel through the switched network, this
data faces propagation delays. Propagation delays occur due to such things as bandwidth utilization, packet
length, switch processing, or any other port−to−port delay encountered as data traverses the network.
As a result of propagation delays, BPDUs can be late to their destinations, making the switch think that a
network topology change has occurred. Because propagation delays can occur at any time in the network,
when a switch port converts from a blocked state to a forwarding state, the port can inadvertently create
temporary data loops because it has not received a complete picture of the network topology.
To overcome network propagation delays, STP members use timers to force the ports to wait for the correct
topology information. The timers are set by default on the switch. Table 10.2 shows the three different STP
timers and the default timer settings based on a default setting of 2 for the Hello Time and 7 for the switch
diameter. Based on these assumptions, the network should always form a stable topology.
Table 10.2: The default STP timers and their default settings.
STP Timer Variable Description Default
Hello Time Determines how often the switch
will broadcast hello messages to
other switches
2 seconds
Maximum Time/MaxAge Determines how long protocol
information received on a port is
stored by the switch
20 seconds
FwdDelay Determines how long listening and
learning will last before the port
begins forwarding
15 seconds
Selecting the root bridge is important in calculating the port cost in the network. The port cost from each child
switch in the network is calculated from the network’s root bridge. In the next section, we’ll take a look at
how the BPDUs are used in the process of selecting a root bridge.
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