Understanding How Bpdu Skewing Works - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Chapter 8 Configuring Spanning Tree
To configure the spanning tree maximum aging time for a VLAN or an instance, perform this task in
privileged mode:
Task
Step 1 Configure the maximum aging time for a VLAN
or an MISTP instance.
Step 2 Verify the configuration.
This example shows how to configure the spanning tree maximum aging time for VLAN 100 to
36 seconds:
Console> (enable) set spantree maxage 36 100
Spantree 100 max aging time set to 36 seconds.
Console> (enable)
This example shows how to set the maximum aging time for an instance to 25 seconds:
Console> (enable) set spantree maxage 25 mistp-instance 1
Instance 1 max aging time set to 25 seconds.
Console> (enable)

Understanding How BPDU Skewing Works

BPDU skewing is the difference between when the BPDUs are expected to be received and the time
BPDUs are actually received. Skewing occurs when the following occurs:
•
•
•
The skew causes BPDUs to reflood the network to keep the spanning tree topology database current.
The root switch advertises its presence by sending out BPDUs for the configured Hello time interval.
The nonroot switches receive and process one BPDU during each configured time period. A VLAN may
not receive the BPDU as scheduled. If the BPDU is not received on a VLAN at the configured time
interval, the BPDU is skewed.
Spanning tree uses the Hello Time (see the
when a connection to the root switch exists through a port and when that connection is lost. This feature
applies to both PVST+ and MISTP. In MISTP, the skew detection is on a per-instance basis.
BPDU skewing detects BPDUs that are not processed in a regular time frame on the nonroot switches in
the network. If BPDU skewing occurs, a syslog message is displayed. The syslog applies to both PVST+
and MISTP.
The number of syslog messages that are generated may impact the convergence of the network and the
CPU utilization of the switch. New syslog messages are not generated as individual messages for every
VLAN because the higher the number of syslog messages that are reported, the slower the switching
process will be. To reduce the impact on the switch, the syslog messages are as follows:
•
•
78-13315-02
Spanning tree timers lapse.
Expected BPDUs are not received.
Spanning tree detects topology changes.
Generated 50 percent of the maximum age time (see the
section on page 8-36)
Rate limited at one for every 60 seconds
Command
set spantree maxage agingtime [vlans]
mistp-instance instances
show spantree [mod/port] mistp-instance
[instances] [active]
"Configuring the Hello Time" section on page
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
Understanding How BPDU Skewing Works
"Configuring the Maximum Aging Time"
8-35) to detect
8-37