?– If two or more ports in a single bridge are connected to the same LAN, and one of
those ports is a designated port or a root port, then the others are backup ports and
are blocked.
?– If the selection of the root bridge, designated bridge, or root port results in a tie, e.g.,
if two candidates for designated bridge are equidistant from the root bridge, then
tiebreakers are used. The first tiebreaker is a priority configured on the bridge or
port by the system administrator, followed by the MAC address assigned to the
bridge or port. Because MAC addresses are globally unique, all ties can be broken.
380 Chapter 13
?– Whenever a port first appears, the bridge waits a certain amount of time, transmitting
Spanning Tree Protocol packets, before it forwards traffic to or from that port. This
prevents temporary loops in case the port is connected to another bridge and should be
blocked. Other timers are invoked in certain situations to block ports for a short period
of time to make it impossible to have even a temporary forwarding loop.
In-band Signaling
In order to pass the control protocols that run the STP and protocols that are particular
to single LANs, and in order to provide for future expansion, 16 multicast destination
MAC addresses have been reserved.
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