Cisco WS-C4003 Software Guide - Page 130

Understanding How BPDU Skewing Works Chapter 7 Configuring Spanning Tree 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: • Spanning tree timers lapse • Expected BPDUs are not received • Spanning tree detects topology changes. 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 non-root switches receive and process one BPDU during each configured time period. A VLAN might 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 "Configuring Hello Time" section on page 31) 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 non-root 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: • Generated 50 percent of the maximum age time (see the "Configuring Maximum Aging Time" section on page 32) • Rate limited at one for every 60 seconds Configuring Spanning Tree BPDU Skewing Commands that support the spanning tree BPDU skewing feature perform these functions: • Allow you to enable or disable BPDU skewing. The default is disabled. • Modify the show spantree summary output to show if the skew detection is enabled and for which VLANs or PVST+ or MISTP instances the skew was detected. • Provide a display of the VLAN or PVST+ or MISTP instance and the port affected by the skew including this information: - The duration (in absolute time) of the last skew - The duration (in absolute time) of the worst skew - The date and time of the worst duration To change how spanning tree performs BPDU skewing statistics gathering, enter the set spantree bpdu-skewing command. The bpdu-skewing command is disabled by default. 7-34 Software Configuration Guide-Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4 78-12647-02