Troubleshooting

 

Unusual Network Activity

 

Spanning-Tree Protocol (STP) and Fast-Uplink

 

Problems

 

 

C a u t i o n

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 switches such as the Series

 

5300XL switch, 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 “Spanning Tree Operation

 

with VLANs” on page 11-31.

 

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.

C-15