OmniSwitch OS6860/OS6900/OS10K Troubleshooting Guide Part No.032996-00 Rev.A
AOS Release 7.X and 8.X January 2015
Alcatel-Lucent Page 42 of 148
7. Troubleshooting Spanning Tree
Summary of the commands in this chapter is listed here:
_________________________________________________________________
show spantree
show spantree vlan <vlan-id>
show spantree ports
show spantree ports active
show interfaces <slot/port>
show vlan xx members
debug stp bpdu-stats 1 start
debug stp bpdu-stats show 1
debug $(pidof stpNi) "p stpniFlushCount"
debug $(pidof stpNi) "call stpni_printStats(1,1)"
dump chg vlan <vlan-id>
stg stp 5
__________________________________________________________________
7.1. Basic troubleshooting
To troubleshoot STP issue it is necessary to have a network diagram that depicts both the physical (cables) and
logical (VLANs) configurations. It also very useful to know which ports are normally in blocking/forwarding
prior to any problem.
A failure of the Spanning Tree Protocol (STP) will usually cause either a bridging loop on one or more LANs
or constant reconvergence of STP. Each of these scenarios will cause several problems.
• If there is a bridging loop on the LAN there can be a broadcast storm because broadcast packets will
continuously loop within the network. In addition, when a unicast address is learned on a port and toggling
from one port to another in a very short period, the unicast traffic will be affected.
• If STP is constantly reconverging temporary network outages can occur as ports can reach a state where they
are cycling through the 30 seconds of listening and learning as defined by 802.1D. If STP is constantly
reconverging the LAN can be perpetually down.
To determine the cause of an STP problem, it is useful to first verify the configuration, especially if the
network having problems has recently been installed or reconfigured.
Use the show spantree command to verify that STP is enabled and that all devices participating in spanning
tree are running the same STP protocol.
OS6860-> show spantree
Spanning Tree Path Cost Mode : AUTO
Vlan STP Status Protocol Priority
-----+----------+--------+--------------
1 ON RSTP 32768 (0x8000)
4094 OFF RSTP 32768 (0x8000)
Use the show spantree command and specify a VLAN to verify the correct mode, designated root ID, root
port, and configurable timers. The timers need to be consistent across a physical link running STP. It is also
useful to note the number of Topology changes and Topology age. If topology changes are incrementing
quickly, the devices pariticipating in spanning tree cannot agree who is the root bridge. This can be caused by
dropped BPDUs (to be discussed later), a bridge that insists it is root regardless of received BPDUs, or a
physical link going in and out of service.
To Next Page
Loading...
