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C-15
Troubleshooting
Unusual Network Activity
Spanning-Tree Protocol (STP) and Fast-Uplink
Problems
Caution If you enable STP, it is recommended that you leave the remainder of the STP
parameter settings at their default values until you have had an opportunity
to evaluate STP performance in your network. Because incorrect STP settings
can adversely affect network performance, you should avoid making changes
without having a strong understanding of how STP operates. To learn the
details of STP operation, refer to the IEEE 802.1D standard.
Broadcast Storms Appearing in the Network. This can occur when
there are physical loops (redundant links) in the topology.Where this exists,
you should enable STP on all bridging devices in the topology in order for the
loop to be detected.
STP Blocks a Link in a VLAN Even Though There Are No Redundant
Links in that VLAN. In 802.1Q-compliant devices such as the switches cov-
ered by this guide, STP blocks redundant physical links even if they are in
separate VLANs. A solution is to use only one, multiple-VLAN (tagged) link
between the devices. Also, if ports are available, you can improve the band-
width in this situation by using a port trunk. See the chapter on VLANs in the
Advanced Traffic Management Guide.
Fast-Uplink Troubleshooting. Some of the problems that can result from
incorrect usage of Fast-Uplink STP include temporary loops and generation
of duplicate packets.
Problem sources can include:
■ Fast-Uplink is configured on a switch that is the STP root device.
■ Either the Hello Time or the Max Age setting (or both) is too long on one
or more switches. Return the Hello
Time and Max Age settings to their
default values (2 seconds and 20 seconds, respectively, on a switch).
■ A “downlink” port is connected to a switch that is further away (in hop
count) from the root device than the switch port on which fast-uplink STP
is configured.
■ Two edge switches are directly linked to each other with a fast-uplink
(Mode = Uplink) connection.
■ Fast uplink is configured on both ends of a link.
■ A switch serving as a backup STP root switch has ports configured for
fast-uplink STP and has become the root device due to a failure in the
original root device.