VLAN ifIndex mismatch - performance implications

Answered Question
Apr 10th, 2012

I have a Cisco Catalyst C6506-E running IOS Version 12.2(18)SXF13, and a Catalyst 3750 running Version 12.2(25r)SEC. 

I found an oddity looking at my VLANs configured between my 6500 and my 3750.  On the 6500, VLAN 223 has an ifIndex of 218.  The 3750's VLAN 223 has an ifIndex of 223.  The relevant running-config entry for both sides of the trunk connecting the two are identical, namely:



switchport trunk encapsulation dot1q

switchport trunk native vlan 223

switchport trunk allowed vlan 223

switchport mode trunk

The 3750 is operating in VTP transparent mode, and the 6500 is operating in vtp server mode.

Could this difference in ifIndex or the VTP mode cause any performance penalty for devices on VLAN 223?  Or is the ifindex attribute used solely for SNMP?

Thanks for your time,

Paul

I have this problem too.
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Correct Answer by phiharri about 2 years 3 months ago

Greetings Paul,

The mapping between interface and ifIndex is only relevant for SNMP and will have no bearing on the forwarding performance of your switches.

It's expected that similar interfaces will have different ifIndex values across devices, as product families often use different methods of allocating the indexes (Catalyst 3750 typically allocates a matching ifIndex value to the VLAN number for SVIs reserving 1-4096 for that purpose, while 6500 typically assigns sequentially based on the order that interfaces are brought up).

Hope this helps,

/Phil

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Correct Answer
phiharri Wed, 04/11/2012 - 04:56

Greetings Paul,

The mapping between interface and ifIndex is only relevant for SNMP and will have no bearing on the forwarding performance of your switches.

It's expected that similar interfaces will have different ifIndex values across devices, as product families often use different methods of allocating the indexes (Catalyst 3750 typically allocates a matching ifIndex value to the VLAN number for SVIs reserving 1-4096 for that purpose, while 6500 typically assigns sequentially based on the order that interfaces are brought up).

Hope this helps,

/Phil

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