Why do you assign a VLAN to a virtual router, and not an interface?

[jeronimo]

Hi,

I'm trying to understand why you assign a VLAN to a VR, instead of an interface, which is what I you would expect. Why does EXOS work that way?

How I would imagine it to work:
* Create user VR-A
* Go into VR-A and say "i want it to have an interface in VLAN 10 with IP x"
* Also say "i want it to have an interface in VLAN 20 with IP y" (so the VR will route between VLANs 10 and 20)
Further you could:
* Create a second user VR-B
* Go into VR-B and say "i want it to have an interface in VLAN 10 (same as above), this time with IP z (same IP range, or a secondary address)"

I have not tried if you can actually accomplish this... for now I am wondering why it works using VLANs and not interfaces/IPs...

This question arose because we were wondering whether you would be able to assign different secondary IP address of an interface to different VRs? Is it possible?

Thanks,
Marki

[André_Herkenrat]

You can configure secondary IP Adresses on a VLAN - It's possible - but bad style.
I recently had a customer with more that 10 secondary IPs on a subnet, that was really bad...

[jeronimo]

Even with ACLs you would not be as flexible as you would be with multiple VRs, because you would still only have one routing table. Unless of course you'd use PBR which would probably take you to hell very quickly.

I understand secondary addresses are bad. You must understand some migrations are permanent  You didn't say if you think this might be possible to configure or not?  Unfortunately I don't have any devices to try this on. Maybe I'll deploy a virtual ExOS to play with.

[JeremyClarkson]

you can have the VLAN on multiple VRs - they are just virtual and best to imagine the physical layout.

There is no point however creating 2 VLAN interfaces on the same layer 3 switch as you only need one for remote management. (unless of course you manage from different LANs)

[JeremyClarkson]

haha yes i know, this is true...its all depending what you need and use them for.

[jeronimo]

There certainly are more uses to VRs than switch mgmt 🙂