How is ifSpeed value of a L3 VLAN determined?

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Dec 29th, 2009

It seems all the VLANs report back via ifSpeed they're 1G links, without explicit bandwidth statement on the IOS. Why aren't they "set" to some other arbitrary value? What does it mean when a VLAN is reportedly nearing 100% utilization of its "1G" bandwidth.

I have this problem too.
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Joe Clarke Tue, 12/29/2009 - 12:10

The value of ifSpeed is generaly the backplane speed of the device, but may simply be arbitrarily set to 1 Gbps (depending on the platform).  Trying to calculate utilization of a VLAN is difficult since the packets appearing on the SVI may not relfect the actual L3 packets being switched.  If you want an idea of backplane utilization, and this is a 6500 or 7600, consider using cc6kxbarStatisticsInUtil and cc6kxbarStatisticsOutUtil to get an idea of the individual module channel utilizations.

If you want to know the amount of L3 traffic being switched, use the cseL3VlanStatsTable from the CISCO-SWITCH-ENGINE-MIB.

yjdabear Tue, 12/29/2009 - 12:38

Thanks. Yes, it's concerning the cat6k platform. Given the reporting tool's calculation is based on ifHCInOctets (Bits In) and the arbitrary 1G ifSpeed, is there any correlation of these numbers to the cseL3VlanStatsEntry numbers? Are the formers valid indications of BW util at all?

Here're some sample numbers for VLAN 900

ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifHCInOctets.268 : Counter64: 758975309159539

cisco.ciscoMgmt.ciscoSwitchEngineMIB.cseMIBObjects.cseL3Objects.cseL3VlanStatsTable.cseL3VlanStatsEntry.5.9008.900 : Counter64: 0

cisco.ciscoMgmt.ciscoSwitchEngineMIB.cseMIBObjects.cseL3Objects.cseL3VlanStatsTable.cseL3VlanStatsEntry.6.9008.900 : Counter64: 0

cisco.ciscoMgmt.ciscoSwitchEngineMIB.cseMIBObjects.cseL3Objects.cseL3VlanStatsTable.cseL3VlanStatsEntry.9.6021.900 : Counter64: 1175076358594201

cisco.ciscoMgmt.ciscoSwitchEngineMIB.cseMIBObjects.cseL3Objects.cseL3VlanStatsTable.cseL3VlanStatsEntry.9.7021.900 : Counter64: 0

Joe Clarke Tue, 12/29/2009 - 23:18

No, the latter is a better indication of packets actually being switched (i.e. the cseL3VlanStatsTable stats).  Packets switched in hardware will not appear in the ifXTable counters.

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