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Community Member

Layer 3 VLAN Interface

What does the term "Layer 3 VLAN Interface" actually mean ? Is it the "interface VLAN" that is configured on switches with an IP address for management of the switch? Also, when the "interface VLAN" is configured on a switch, this is a logical interface, correct? Are there any physical interfaces mapped to this logical "interface VLAN"? I mean, what physical interfaces does the traffic pass thru for the "interface VLAN"?

5 REPLIES
Hall of Fame Super Blue

Re: Layer 3 VLAN Interface

Hi

Yes, Layer 3 vlan interface is the interface vlan you can create on switches eg.

int vlan 10

ip address 192.168.5.1 255.255.255.0

Note that on a layer 2 switch you can only have one L3 vlan interface active and it is indeed used for management as you say.

However on a L3 switch you can have multiple L3 vlan interfaces and the switch will route between them.

Yes they are a logical interface as opposed to the physical interfaces on the switch.

The physical interfaces are associated with the L2 vlan that you create on the switch. So if you have 10 interfaces in vlan 10 and one of the pc's attached to one of these interfaces wants to communicate with another pc in a different vlan it will send it's traffic to the L3 vlan interface.

HTH

Jon

Blue

Re: Layer 3 VLAN Interface

And just one more tidbit of information regarding VLAN interfaces to add to Jon's excellent post...

A L3 virtual, routed VLAN interface (Switched Virtual Interface) will only be in an "up, up" state if at least one of the end machines connected to the L2 physical ports is in an up state. If, for example, you disconnect all end machines connected to all the L2 phycial interfaces on VLAN 10, VLAN 10's SVI will go down.

HTH

Community Member

Re: Layer 3 VLAN Interface

Thank you. This makes sense. But, now when you are talking about the PC in VLAN 10 communicating with a PC in different VLAN, are you talking about the other PC being on that same switch, but a different vlan ? And is this why the traffic goes through L3 Interface VLAN? What if the other PC was on a different vlan on a different switch, then it would have to cross a Layer 3 WAN Interface, correct ?

Cisco Employee

Re: Layer 3 VLAN Interface

Just to keep it simple, what the other poster means is that on a particular switch if you have a SVI interface and you dont have any PC connect ed in the same vlan then the interface will be in down/down state. In order for a Vlan interface to be in up/up state you should have atleast one PC connected in the same layer-2 vlan or have atleast a trunk configured on the switch. Remember, a trunk port is part of every vlan and this will bring the SVI in up/up state.

HTH,

-amit singh

Hall of Fame Super Blue

Re: Layer 3 VLAN Interface

I was talking about PC on same switch but it applies to a PC on a different switch as well.

Whether the other PC is on a different switch in the same building or across a WAN traffic from the PC in vlan 10 will still get routed via the L3 vlan interface for vlan 10.

HTH

Jon

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