Poorly configured Layer 2 designs usually result from a lack of knowledge of
the Spanning Tree Protocol, or a misunderstanding of its operation.
Examine the switched network topology in Figure
. At first
glance, it appears to be an efficient design. A few pings to devices in the
network reveals full connectivity. However, upon closer inspection the traffic
flow in this topology reveals suboptimal traffic patterns. To really analyze
the network topology it is necessary to look at the spanning tree states of
each of the ports. In Figure
it is
apparent that the network has unfortunately chosen the Marketing department
switch as the STP root.
Spanning tree has placed the Data Center port
leading to the Telephone Sales Center in a blocking state. Consequently the
majority of traffic must take the longest path to the Data Center. With the
Data Center at one end of the Spanning Tree, the link between the switch in
Reception and Data Center must carry the entire traffic load of the
organization.
Take the network administrator view of the network.
If complaints
are received from the Telephone Sales Center that network performance is
lacking, which link would he recommend be upgraded? Instinctively the
inexperienced administrator orders the 100 Mbps between the Telephone Sales
Center and the Data Center upgraded to Gigabit Ethernet. Embarrassingly, the
upgrade will have no affect on the performance of the network.
The
correct course of action is to set the Data Center switch as the STP root. This
will allow the traffic flow as shown in Figure
. It is possible
that an upgrade to the link is still required but at least the full benefits of
the upgrade will now be realized.
Tools do exist that allow the mapping of the Layer 2 topology. When dealing
with large complex networks or when required to work regularly with unfamiliar
networks, these tools can be of great benefit. For simpler, familiar, networks,
simply documenting the STP states within the LAN would be sufficient.
