VLANs & Interfaces

[zyang]

A quick (possible dumb) question.

If I have a layer 3 switch and let's say that FA0/1 is set to access vlan 1. 

If I change it so that it is no longer a switchport but an IP port by typing:

" ... conf t

int fa0/1

no switchport

ip add 192.168.1.100 255.255.255.0"

This becomes an  IP port.  If so, is it still part of VLAN 1?  Meaning does it still get broadcasts from VLAN 1?  If so, why doesn't Cisco let me modify which VLAN it is part of?  (You can't change its VLAN membership unless you change it back to a regular switchport and then assign it to another VLAN).

However if it doesn't belong to VLAN 1 and doesn't get any broadcasts from VLAN1, does that mean the only broadcasts that can be sent to it, is if someone is in the 192.168.1.0 network and sends a broadcast?

Thanks.

[Dan-Ciprian Cicioiu · Accepted solution]

Hi Zhi,

A layer 3 switch has 2 types of ports :

  - layer 2 ports

  - layer 3 ports

Layer 2 ports are configured using switchport commands and they have to have "switchport" enabled. This way you can assign a vlan on that port

Layer 3 ports which have the IP configured direcly on the interface. Those ports are also running on vlans - because this is the architecture of the switch, but they are using internal vlans : "show vlan internal usage" in order to see your internal vlans used for layer 3 ports.

Regards

Dan

[zyang]

Thank you.

I checked it out and it seems they start at 1006+ vlan.

I wasn't able to confirm it (it's a production layer3 switch, and I don't have any ready test switch), but does each IP port get it's own internal vlan?  And they would be all different?  And I don't suppose that's configurable?

[Dan-Ciprian Cicioiu]

Zhi ,

Internal ports are used only in case of Layer 3 ports - when you configured the IP directly on the port + no switchport - and yes , they get different internal vlans. The switch allocates automaticaly the internal vlans.

In order to map two different ports to a vlan you can use Layer 2 ports :

switchport

switchport mode access

switchport access vlan 2

For example

Regards

Dan

[zyang]

Thanks for the answers!

Been very helpful and illuminating.