VLANSpanning Tree


VLAN/Spanning Tree

This section answers questions concerning VLANs and spanning tree on Cisco Catalyst IOS devices.

How Do I Collect VLAN Utilization?

There is no good way to collect VLAN utilization, for the following reasons:

  • Not all switch ports in a VLAN see all traffic in the VLAN.

  • Not all traffic for a VLAN traverses trunk ports.

  • Not all traffic for a VLAN hits a switch backplane.

Therefore, there is no single point at which you can measure traffic to accurately reflect the aggregate bandwidth consumed for a VLAN.

There are more useful methods for measuring capacity consumption. For instance, measuring broadcast and multicast traffic from a specific port can provide a benchmark for rising capacity needs. For a VLAN, this method is useful because all broadcast and multicast traffic is flooded to all ports in a VLAN.

For more information, please see "VLAN Utilization" in Chapter 15, "Monitoring VLANs."

How Can I Track Spanning Tree Topology Changes?

Watch for the topologyChange trap from RFC 1493. You will receive this trap from a switch whenever it detects a topology change.

For more information, please see "Error/Fault Data for VLANs" in Chapter 15.

Why Do I See the Spanning Tree for Only One VLAN with SNMP?

Catalyst 4000, 5000, and 6000 switches run one unique instance of spanning tree for each VLAN configured. Representing these multiple instances of spanning tree with SNMP can be difficult using normal SNMP structural mechanisms such as tables.

In order to view the spanning tree information for different VLANs, you must use a method called community string indexing. To specify which VLAN to view a table for, use a modified community string that contains the following format: community@vlan number. The use of this community string is shown in Example 19-9.

Example 19-9 Collecting spanning tree variables from VLAN 1 and VLAN 10 using community string indexing.
 ~% snmpwalk  c public bikini-atoll dot1dStpDesignatedRoot dot1dBridge.dot1dStp.dot1dStpDesignatedRoot.0 : OCTET STRING-   (hex): length =80: 00 09 00 90 92 e9 70 00 -- -- -- -- -- -- -- --     ......p......... ~% snmpwalk -c public@10 bikini-atoll dot1dStpDesignatedRoot dot1dBridge.dot1dStp.dot1dStpDesignatedRoot.0 : OCTET STRING-   (hex): length =80: 00 09 00 90 92 e9 70 09 -- -- -- -- -- -- -- --     ......p......... 

In Example 19-9, notice in the first snmpwalk that the community string without VLAN has been specified. By default, VLAN 1 is assumed. In the second snmpwalk, VLAN 10 was specified (public@10), and notice that the mac address for the root bridge is different.

For more information, please see "Error/Fault Data for VLANs" in Chapter 15.



Performance and Fault Management
Performance and Fault Management: A Practical Guide to Effectively Managing Cisco Network Devices (Cisco Press Core Series)
ISBN: 1578701805
EAN: 2147483647
Year: 2005
Pages: 200

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