7-25
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Layer 2 PDU Rate Limiting Works
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
“Configuring the Hello Time” section on page 7-50
) to detect
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 network convergence 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:
•
Generated 50 percent of the maximum age time (see the
“Configuring the Maximum Aging Time”
section on page 7-51
)
•
Rate limited at one for every 60 seconds
Understanding How Layer 2 PDU Rate Limiting Works
You can use rate limiters to prevent receiving an unwanted number of protocol data units (PDUs) or more
than a certain number of PDUs from a neighboring switch. The Layer 2 PDU rate limiters are supported
in the hardware on the Catalyst 6500 series switches. They rate limit traffic on the Local Target Logic
(LTL) index.
You can configure up to four rate limiters. You can configure rate limters to limit the following PDU
types globally on the switch:
•
Spanning-tree BPDUs—IEEE and SSTP, CDP, UDLD, VTP, and PAgP
•
Layer 2 protocol tunnel-encapsulated PDUs
•
802.1X port security
These restrictions apply if you want to enable rate limiting:
•
Hardware-based rate limiters are supported on Catalyst 6500 series switches that are configured
with a PFC3A or later PFC.
•
The Catalyst 6500 series switch cannot be in truncated mode. If you attempt to enable rate limiting
and you are in truncated mode, an error message is displayed.
•
If the rate limiter is enabled and some events cause the system to go from nontruncated mode to
truncated mode, rate limiting is disabled and an informational message is displayed.
