Cisco VLAN interface connection

Posted on 2014-03-18
Last Modified: 2014-03-22
If I understand you can create Vlan for instance Vlan10 that might span 10 ports, then create interface Vlan10.

assuming there are other Vlans on the same switch and on other Cisco switches.. how is the interface Vlan helpful since you cannot connect a cable to it in one end another end to the other interface Vlan …

any explanation about interface Vlan will be helpful

Thank you
Question by:jskfan
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LVL 17

Assisted Solution

by:James H
James H earned 125 total points
ID: 39937769
VLAN's help you segregate one type of client from another.

Some examples:

VLAN 101 - DATA - PC's
VLAN 201 - VOICE/Phones
VLAN 50 - Servers ... etc
Each VLAN can either be route enabled or not (depending on need)

Typing Interface VLAN (Number) creates the ability to tag traffic. Now, this information is passed from swtich to switch and you can (and should) have ONE switch as the master and all others take their VLAN information from that switch. I've seen too many times how VLAN information is overwritten when introducing a switch which is misconfigured and assumes VTP server mode instead of client or transparent.

Author Comment

ID: 39937780
I believe you can create VLANs then create Trunk port to make VLANs on one switch talk to VLANs on another switch if allowed.
I am trying to understand VLAN interface…
LVL 15

Assisted Solution

WalkaboutTigger earned 125 total points
ID: 39937805
I would recommend you take a look at this video

For anyone else reading this who does not understand the concept of VLANs, let's provide a couple of definitions.

Virtual Local Area Networks (VLANs) are a mechanism to allow network administrators to create logical broadcast domains that can span across a single switch or multiple switches, regardless of physical proximity. This function is useful to reduce the size of broadcast domains or to allow groups or users to be logically grouped without the need to be physically located in the same place.

A Switch Virtual Interface (SVI) or Virtual Local Area Network Interface (VLAN Interface) is a VLAN of switch ports represented by one interface to a routing or bridging system. There is no physical interface for the VLAN and the SVI provides the Layer 3 processing for packets from all switch ports associated with the VLAN.

There is one-to-one mapping between a VLAN and SVI, thus only a single SVI can be mapped to a VLAN. By default, a SVI is created for the default VLAN (VLAN1) to permit remote switch administration. An SVI cannot be activated unless associated with a physical port.

SVIs are generally configured for a VLAN for the following reasons:
* Allow traffic to be routed between VLANs by providing a default gateway for the VLAN.
* Provide fallback bridging (if required for non-routable protocols).
* Provide Layer 3 IP connectivity to the switch.
* Support bridging configurations and routing protocol.
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LVL 46

Assisted Solution

by:Craig Beck
Craig Beck earned 250 total points
ID: 39937897
The VLAN 'interface' (often called SVI) is a logical interface on the switch which is on that VLAN.  So you could have a logical interface on the switch for VLAN1, another logical interface for VLAN10, etc...

The configuration would look like this...

interface Vlan1
 ip address
interface Vlan10
 ip address

Open in new window

You don't need VLAN interfaces to be able to put ports on the switch into those VLANs.  In fact, in standard Cisco layer-2 switches you can have only one Vlan interface at a time.  If you create multiple Vlan interfaces they all stay down apart from one.

In a layer-2 switch the Vlan interface IP address is the management address.

However, in a layer-3 switch it's a bit different.  Layer-3 allows routing, therefore the switch must have more than one IP address if it is to be used in layer-3 mode (otherwise routing would be pointless).  You don't have to use a layer-3 switch as a router though so you don't 'have' to create more than one SVI if it's not routing.

You can create as many Vlan interfaces as the layer-3 switch allows and they will all be 'UP' at the same time as long as a port is connected and assigned to the VLAN.

Author Comment

ID: 39938940
On a Single switch, it makes sense to me to create SVIs for each VLAN, so that when you you type the IP ROUTING command, it will make each VLAN able to talk to the other VLAN without creating a Trunk Port( Obviously all ports are on the same switch).

However, if you have multiple switches, I believe Trunk port to the Router will do the same thing.

Author Comment

ID: 39938985
If more than one VLAN is configured on a switch , I believe it is helpful to configure SVIs on that Switch, for instance if we have VLAN10 and 11 on the same switch and we have PC10 plugged to VLAN10 and PC11 plugged to VLAN11, then with SVIs,  PC10 can talk to PC11 without going through  Trunk port to  the Router then back to Trunk Port then to the other VLAN on the same switch
LVL 46

Accepted Solution

Craig Beck earned 250 total points
ID: 39939180
If you require routing on that switch then yes, SVIs are helpful so that you don't need a dedicated router, but only as long as the switch is layer-3 capable.  This will work on a 3750, for example, but not on a 2950.

However, for the switch (forgetting routing) SVIs aren't necessary to allow clients on the same VLAN to see each-other.

Try not to consider trunks and routing as the same thing - they're very different.

Author Closing Comment

ID: 39947446
Thank you

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