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Beginner

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

4 REPLIES 4
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VIP Advisor

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Beginner

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.

https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-software-releases-122-mainline/15096-idb-limit.html#t1

 

Regards

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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 ***

 

Sebastian

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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

Giuseppe