I learned that on Multilayer switches, I mean that run legacy IOS, when you configure an interface as “no switchport” and then assign it an IP address, the switch, behind the scenes, creates a vlan in the extended range + an SVI and stick the IP address to these freshly created SVI
I can see the internal vlan picked by the switch with the show vlan internal usage command.
(I don’t talk here about the policy (ascending/descending); doesn’t matter here)
Now: when I try the same on new Cat9K platforms running IOS-XE, the show vlan internal usage command doesn’t exist anymore and I’ve read on some forums that “we don’t need it anymore”
My question is :
Why wouldn’t we need it anymore? I mean is the way a multilayer switch handles a native routed interface different than before?
I mean does the switch still go through the process of creating an internal vlan + svi or not at all?
If not, does it mean that nowadays Multilayer switches handle native routed ports like routers do?
Since in the legacy ios platforms you couldn’t create a vlan with the same number than those used internally by the switch, I think it is worth knowing, in terms of design, how it is handled nowadays.
And I don’t find the answer in any documentation so if anybody knows, Thank you for sharing.
In legacy IOS with older switches, when you configured an interface with the no switchport command and assigned it an IP address, the switch would internally create a VLAN (typically in the extended range) and an SVI associated with that VLAN. This was necessary because the switch’s architecture still fundamentally relied on the concept of VLANs and SVIs to handle Layer 3 routing, even though the interface was acting as a routed port. The show vlan internal usage command would show which VLANs were internally allocated by the switch for these purposes. The reason this command exists is so that you can check it before assigning new VLAN IDs, just to make sure that they don’t conflict with these internal allocations.
With the Catalyst 9000 series and IOS-XE, the architecture has evolved to better integrate Layer 2 and Layer 3 functionality. The new architecture no longer requires the creation of an internal VLAN and SVI when an interface is configured as a routed port. Instead, the routed port is handled more like a traditional router interface, which does not rely on VLANs or SVIs for Layer 3 functionality.
So since Layer 3 functionality is implemented differently (i.e. without the need to allocate VLAN IDs to an SVI), the show vlan internal usage command is obsolete.
