Troubleshooting

Unusual Network Activity

 

Spanning-Tree Protocol (MSTP) and Fast-Uplink

 

Problems

 

 

C a u t i o n

If you enable MSTP, it is recommended that you leave the remainder of the

 

MSTP parameter settings at their default values until you have had an oppor-

 

tunity to evaluate MSTP performance in your network. Because incorrect

 

MSTP settings can adversely affect network performance, you should avoid

 

making changes without having a strong understanding of how MSTP oper-

 

ates. To learn the details of MSTP operation, refer to the IEEE 802.1s 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 MSTP 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 MSTP 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 bandwidth in this situation by using a port trunk. Refer

 

to “Spanning Tree Operation with VLANs” in the chapter titled “Static Virtual

 

LANs (VLANs)” in the Advanced Traffic Management Guide for your switch.

 

Fast-Uplink Troubleshooting. Some of the problems that can result from

 

incorrect usage of Fast-Uplink MSTP include temporary loops and generation

 

of duplicate packets.

Problem sources can include:

■Fast-Uplink is configured on a switch that is the MSTP 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 MSTP 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 MSTP root switch has ports configured for fast-uplink MSTP and has become the root device due to a failure in the original root device.

C-16