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Chapter 2
Layer-2 Switching
bridge, then a port cost is determined by checking the bandwidth of each
link. The lowest cost port becomes the root port.
Designated port
Either a root port or a port that has been determined
as having the best (lower) cost--a
designated port
will be marked as a
forwarding port.
Port cost
Determined when multiple links are used between two
switches and none are root ports. The cost of a link is determined by
the bandwidth of a link.
Nondesignated port
Port with a higher cost than the designated port
that will be put in blocking mode--a
nondesignated port is not a forward-
ing port.
Forwarding port Port that forwards frames.
Blocked port Port that will not forward frames, in order to prevent
loops. However, a blocked port will always listen to frames.
Spanning-Tree Operations
As I've said before, STP's job is to find all links in the network and shut down
any redundant ones, thereby preventing network loops from occurring. STP
does this by first electing a root bridge that will preside over network topology
decisions. Those decisions include determining which "roads" are the best
ones for frames to travel on normally, and which ones should be reserved as
backup routes if one of the primary "roads" fail.
Things tend to go a lot more smoothly when you don't have more than
one person making a navigational decision, and so, there can only be one
root bridge in any given network. I'll discuss the root bridge election process
more completely in the next section.
Selecting the Root Bridge
The bridge ID is used to elect the root bridge in the network as well as to
determine the root port. This ID is 8 bytes long, and includes both the pri-
ority and the MAC address of the device. The default priority on all devices
running the IEEE STP version is 32,768.
To determine the root bridge, the priorities of the bridge and the MAC
address are combined. If two switches or bridges happen to have the same pri-
ority value, then the MAC address becomes the tie breaker for figuring out
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