Identifying A Data Loop; Avoiding Trouble - D-Link DGS-3308 Series User Manual

8-port Gigabit Ethernet Switch User's Guide
converge. If the CPU is over-utilized, it is possible that BPDUs may not be sent in a timely fashion. STP is generally not
very CPU intensive and is given priority over other processes, so this type of error is rare.
It can be seen that very low values for the MAX AGE and the FORWARD DELAY can result in an unstable spanning tree.
The loss of BPDUs can lead to data loops. The diameter of the network can also cause problems. The default values for
STP give a maximum network diameter of about seven. This means that two switches in the network cannot be more than
seven hops apart. Part of this diameter restriction is the BPDU age field. As BPDUs are propagated from the root bridge
to the leaves of the spanning tree, each bridge increments the age field. When this field is beyond the maximum age, the
packet is discarded. For large diameter networks, STP convergence can be very slow.

Identifying a Data Loop

Broadcast storms have a very similar effect on the network to data loops, but broadcast storm controls in modern switches
have (along with subnetting and other network practices) have been very effective in controlling broadcast storms. The best
way to determine if a data loop exists is to capture traffic on a saturated link and check if similar packets are seen
multiple times.
Generally, if all the users of a given domain are having trouble connecting to the network at the same time, a data loop can
be suspected. The port utilization data in the switch's console will give unusually high values in this case.
The priority for most cases is to restore connectivity as soon as possible. The simplest remedy is to manually disable all of
the ports that provide redundant links. Disabling ports one at a time, and then checking for a restoration of the user's
connectivity will identify the link that is causing the problem, if time allows. Connectivity will be restored immediately
after disabling a data loop.

Avoiding Trouble

Know where the root is located.
Although the STP can elect a root bridge, a well-designed network will have an identifiable root for each VLAN. Careful
setup of the STP parameters will lead to the selection of this best switch as the root for each VLAN. Redundant links can
then be built into the network. STP is well suited to maintaining connectivity in the event of a device failure or removal,
but is poorly suited to designing networks.
Know which links are redundant.
Organize the redundant links and tune the port cost parameter of STP to force those ports to be in the blocking state.
For each VLAN, know which ports should be blocking in a stable network. A network diagram that shows each physical
loop in the network and which ports break which loops is extremely helpful.
Minimize the number of ports in the blocking state.
A single blocking port transitioning to the forwarding state at an inappropriate time can cause a large part of a network to
fail. Limiting the number of blocked ports help to limit the risk of an inappropriate transition.
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