Digital Audio Interfacing and Networking 1497port that eventually returns to the same switch. If it
finds no such path, it will activate the port. The delay
might be on the order of 30 seconds to a minute. If it
finds a path back to itself on the port, it will disable that
port. Whenever a connection to a port is made or lost,
and the port has spanning tree active, the switch will
reexamine all the ports for loops and activate those
where loops are not found.
In any network, damage to the cabling is one of the
more common causes of network failures. Spanning tree
can be used to reduce the impact of such failures on
network operation, Fig. 39-35.
When managed switches with spanning tree capa-
bility are used it is common to deliberately build the
network with loops. The switches will find the loops
and disable enough of the links between switches to
insure the network topology is a star of stars and stable.
If one of the active links is later disabled, perhaps due to
physical damage to the cable or the failure of another
switch, then one or more of the currently disabled links
will automatically be restored to operation. This
provides an inexpensive way to increase the reliability
of a network.
Fig. 39-36 shows one possible network topology that
can be stable if spanning tree is used. Such a network
design can be quite robust, and accommodate multiple
failures while maintaining operation.
One difficulty with designing a network that uses
spanning tree is that we can’t know which links will be
disabled and which will stay active. This makes it diffi-cult to predict the amount of traffic a given link will
carry.
While spanning tree used with the correct network
topology can increase system reliability, it does not
respond instantly to failures or changes in network
topology. At times it may take several minutes for oper-
ation to be restored after a failure.39.11.12.3 MeshingAt this time meshing is only available on some Hewlett
Packard (HP) Procurve switches. Meshing is an attempt
to combine the best portions of trunking and spanning
tree into a new protocol. Unlike spanning tree, meshing
does not disable any links. Instead it keeps track of
packets and prevents them from being recirculated
around loops. When there are multiple possible routes
for a packet to take to its destination, meshing attempts
to send the packet by the most direct route.
One of the most significant advantages of meshing is
that recovery from failures of links or switches is far
faster than spanning tree, and may be accomplished in
seconds rather than minutes.39.11.12.4 Rapid Spanning TreeMore recently a new protocol has brought much of the
advantages of meshing and other proprietary technolo-
gies into the general market. It allows restoration of a
network typically in seconds rather than minutes.Figure 39-35. A loop around three switches. Spanning tree would disable one of the three links between the switches, and
allow the network to be stable.
nodenodenodehub nodenodenodenodehub nodenodenodenodehub node