Stp Root Bridges - Cisco RJ-45-to-AUX Brochure

Figure 10.2: An example of a directed graph.
STA assigns each switch in the network a unique identifier. This identifier is one of the switch's MAC
addresses, as well as an assigned priority (explained in more detail later in this chapter in "The Selection
Process"). After STA assigns each switch this unique identifier, it then assigns each port in every switch a
unique identifier. This port identifier is typically the port's own individual interface MAC address. Each
bridge port is then associated with a path cost, which represents the cost of transmitting a frame onto the
network through the port. Path costs are assigned by default, but can be assigned manually by a network
administrator.
As I stated in the last section, the graph theory chooses a reference point in the network and calculates the
redundant paths to that particular point in the network topology. After STP discovers all the links in the
network, the STA chooses a single path through the network and blocks the redundant links. It does so by
electing a root bridge. Let's take a look at root bridges in the next section.

STP Root Bridges

When STP initially comes online in a network, one of its first actions is to use the STA to select a root bridge
and a root port. The root bridge is the bridge with the lowest−value bridge identifier. Switches or bridges
using STP exchange multicast frames called Bridge Protocol Data Units (BPDUs) (discussed in the next
section). All the switches on the network use these BPDUs to broadcast their bridge IDs to the other switches
in the network. After the root bridge is selected, the root ports on all other bridges are determined.
Switch A in Figure 10.3 is acting as the root bridge, calculating the least−cost path to switch D. Notice the
numbers associated with the root bridge's path to each individual destination; the path with the lowest number
has the highest priority. The higher the number between individual segments, the higher the cost of
transmitting a frame between those two segments. The port through which the root bridge can be reached with
the least amount of hops or cost determines a bridge's root port; this is referred to as the least path cost.
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