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Ali Hazim

Sub-Interface Number Range

Hi All,
When Configuring a Sub-Interface In a Router, The Number That Can Be Assigned To It Will Be Within The Range (0 - 4294967296), Which Means That The Range Is Represented By a 32 Bits, So My Questions Are:
1- Where Are Those 32 Bits (Ethernet Header, IP Header, Or Some Sort Of Registry)?
2- The Range Is Waaaay To Big For a Real Case Scenario, Since Sub-Interfaces Are Used To Create a Router On a Stick To Provide Connectivity Between Different VLANs, And Since There Are Only 4096 VLNAs, Shouldn't The Number Of Sub-Interfaces Be No More Than (4096)?

Thanks In Advance

VIP Advisor

Hi, for question 1, the image attach to this should be help you.


For question 2, exactly no more than 4096 vlans.Also the following links show a table with IOS and platform type.



Hi @Ali Hazim 


As other provided the document  hope they make sense. Just to add to what others posted, the range available for the sub-interface is the range that is based on the 32bit value, but the total number of interface that a platform can support is based on the maximum idb(s) supported. 


*** Please mark this post helpful if it contributed to your issue ***



Giuseppe Larosa
Hall of Fame Master

Hello Ali,

about your first question the sub-interface identifier never exits the router, so it is not something to be mapped inside the ethernet header or 802.1Q tag. It is just a logical element in 32 bit integer. The mapping of the subinterface with a Vlan id is made with encapsulation dot1q XX command for ethernet based interfaces.

It is best practice to have the subinterface-id = vlan-id in specified in Vlan command but you are free to use a non matching subinterface identifier.

As noted by other colleagues each platform has a maximum number of logical interfaces (the IDB limit).

The choice of a 32 bit field for interface identifier has been made by IOS developers to make it indipendent from current L2 technologies limitations and to be ready for future.


I add a personal note / experience: many years ago in a MPLS L3 VPN deployment we had to use a loopback address with the loopback number that followed a numbering convention to represent the customer, the site and other info. There was a specific value of the loopback number 1XXXXXX that caused the device (C7500) to crash and reboot. We had hit a SW bug.


Hope to help