Danger! Data Loops; Edsger Dijkstra's Graph Theory - Cisco RJ-45-to-AUX Brochure

and Electronics Engineers (IEEE) a protocol similar to STP to become a networking standard. However, after
the IEEE 802 committee revised it into what is now known as the IEEE 802.1D standard (Spanning Tree
Protocol), the protocol differed just enough from DEC's version that they were incompatible.

Danger! Data Loops!

Data loops can easily become a network disaster. A transparent bridge always likes to retransmit a broadcast it
receives and never mark the frame with a time to live (TTL) or an identifier that says "Hey I started here!" or
"I've been around here a few times already." The result? Your transparent bridge keeps re−creating broadcasts
in an expanding fashion. The bridge actually begins rebroadcasting broadcasts. Think this situation will last
forever? Not likely—the bridges will eventually cause a broadcast storm and bring down the entire network.
STP fixes this problem and forces all the redundant data paths into a blocked state. By blocking the paths to
destinations other than the given root path, STP creates only one path through the network. If any of the
forwarding paths through one of the network segments in the spanning tree become unreachable, STP will
reconfigure the spanning−tree topology and use the once blocked links in the network. STP calculates the
network topology using the Spanning−Tree Algorithm (STA).
To avoid confusion, let me clarify that the Spanning−Tree Protocol and the Spanning−Tree Algorithm are two
separate entities. STA chooses a reference point in the network and calculates the redundant paths to that
reference point. If the STA finds a redundant path, it will choose one path to forward and the redundant paths
to block. Using this process, STP and the STA effectively sever all the redundant links within the network.
STA is based on the graph theory developed by Edsger Dijkstra to construct a loop−free subset of the network
topology. Let's take a look at this theory.

Edsger Dijkstra's Graph Theory

The STA uses solutions obtained by a graph theory also known as the Shortest Path Algorithm. As I
mentioned earlier, this algorithm is used to construct a loop−free subset of the network's topology. The same
theory is also used in other link state protocols, such as Open Shortest Path First (OSPF), to calculate routing
solutions. The theory states that for a connected graph consisting of interfaces and edges connecting pairs of
interfaces, a spanning tree of the edges maintains the connectivity of the graph while containing no loops.
The algorithm provides a directed graph where each link is represented by vertices and weighted edges, as
shown in Figure 10.2. Each link represents a cost. The weighted edges, which usually have more hops in the
link than do the straight−through points, are assigned higher values. Each link in the path has a value, and the
total of the values to a given point or destination is the total weighted value of the path. The lowest total
weighted value represents the most efficient path from one point to another point.
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