User's Manual

Key Concepts 97
discovered by the previous detecting devices operation are physically
connected together.
The topology process starts when all discovery operations have
completed. The topology process is applied to one subnet (or more
specifically multinet) at a time.
3Com Network Director uses the term multinet to describe a situation
where more than one subnet is attached to a single router port. This can
result in devices on different subnets sharing the same switched
infrastructure. Consequently, although devices may logically be on
different subnets they may learn the MAC addresses of the devices on the
other subnet(s) in their bridge tables. Switches learning the MAC
addresses of other switches is an important concept in determining the
network topology.
The topology algorithm uses Bridge Forwarding Database (FDB) tables to
determine on which ports a switch has learned the MAC addresses of
other switches in the network. It uses the information of switch ports that
have learned the addresses of other switches in order to build a tree of
network devices representing the physically connectivity. In reality, each
subnet/multinet may not physically appear to be a tree, for instance if
spanning tree is used to employ resilient paths. The algorithm used to
deduce the topology takes such scenarios into consideration.
Before the tree-building process can take place 3Com Network Director
needs to obtain the list of learned addresses for each switch. In practice
this involves more than just reading the FDB tables, as different device
types have different port and unit numbering schemes. For efficiency
purposes an initial attempt is made to determine the positions of most of
the end stations. This is done by checking for ports that have only learned
the address of a single device that is not a switch. Because the positions
of many end stations are resolved here, the amount of information
required to be read from the switches is reduced. A summary of the main
topology stages is given below.
Unlike the discovery stages described in the previous section these stages
are all represented on a single progress bar in the Network Discovery
Progress dialog box.
Sizing stage
This stage determines the units and ports that belong to each switch. It
establishes which ports have a link present so that it knows which ports