vlans & L3

Unanswered Question
Aug 15th, 2007

Hello Folks,

1. The vlans main advantage is to increase broadcast domains ok/not? where the L2 switches (2950) have only one broadcast domain for all ports coz only we are creating vlans in L2 am i right?

2. Now my in L3 switch each port have one broadcast domain ok? now my qualm is there is no need to use vlan's in L3 switch yes/not.

Any body can explain about on this or I am misconstrue. plz. gentlemens help me.

Regards,

Naidu.

I have this problem too.
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mohammedmahmoud Wed, 08/15/2007 - 01:18

Hi,

VLAN = Broadcast domain > Any broadcasts received on a switch port (layer2 port) are flooded to all switch ports (except the port that received the frames) that belongs to the same VLAN.

On a single layer 2 switch you can have multiple VLANs = multiple broadcast domains, and assign your ports to the desired VLANs (broadcast domains).

As for layer3 switch it depends upon the configuration of the interface itself and not on the fact that it is a layer 3 switch, if the interface is configured as layer 2 interface then it is similar to the layer 2 switch case, you can assign this interface to a certain VLAN (broadcast domain), while if this interface is a layer 3 interface then it is by default its own broadcast domain.

I hope that i've been informative.

HTH,

Mohammed Mahmoud.

Richard Burts Wed, 08/15/2007 - 02:47

Naidu

It is not necessarily true that a 2950 has a single broadcast domain. If the 2950 were configured with only a single VLAN then yes the 2950 would have a single broadcast domain. But if the 2950 were configured with 5 VLANs then the 2950 would have 5 broadcast domains.

On a layer 3 switch if you configure a port as a layer 3 (routed) port then yes that port is a single broadcast domain. But if you configure some ports as layer 2 ports, then the port belongs to a VLAN and the broadcast domain is all of the ports in that VLAN.

HTH

Rick

ilnaiduccna Wed, 08/15/2007 - 03:10

Hi Rick,

I am so blissful with your reply and sorry my disregard.

I mean to say that only one broadcast domain before configure vlans in 2950, I had knew that we will configure vlans for increase broadcast domains see my prior post.

As you told why we should go to configure L3 switch ports as L2 ports is there any advantages. And we can't use all L3 ports as L3 routed ports?

Plz. tell me how we can configure L3 ports as L2 ports what are the steps & commands. plz. lemme it would be appreciated.

Regards,

Naidu

Hello,

You didn't tell us what your L3 switch is.

These are the commands needed to set an interface (GigabitEthernet 4/26) as a L2 port (a switchport) on a Catalyst 6500:

Router(config)#interface GigabitEthernet 4/26

Router(config-if)#switchport

Router(config-if)#switchport mode access

Router(config-if)#switchport access vlan 26

Router(config-if)#no shutdown

Router(config-if)#end

These commands will also set the interface in access mode in vlan 26.

If you don't have a Catalyst 6500 running Cisco IOS commands may be different :-).

Richard Burts Wed, 08/15/2007 - 06:29

Naidu

Perhaps there are parts of your questions that I do not understand yet. Here is my answer and if it does not fully address your question then perhaps you can clarify for me.

I think that we agree that on a layer 2 switch we configure VLANs and part of what VLANs accomplish is to manage the size of the broadcast domain. I believe that the question now has to do with layer 3 switches.

So let me explain it this way. A layer 3 switch is a switch whose ports are able to operate at either layer 2 or layer 3 depending on how they are configured. If the port is configured as a layer 2 port then it can be configured as either switchport access and then end stations (PCs or servers) can be connected to it or it can be configured as a trunk. Or the port can be configured as a layer 3 port which is a routed port. If it is a layer 3 routed port you might connect routers or other layer 3 devices into that port.

On many of the layer 3 switches the configuration command to make a port into a layer 3 port is:

no switchport

ip address

the layer 2 commands include:

switchport mode access

switchport access vlan

or

switchport trunk

depending on the particular layer 3 switch that you have the syntax might be slightly different.

HTH

Rick

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