Ip vlan on a switch.

[speculor_cisco]

Suppose you have a switch with many vlans configured and that there is not inter-vlan routing configured.

An access port on the switch is linked to a router.

I imagine that the vlan of this access port become necessarily the only vlan through with I can access the switch from outside the router.

I have set an ip address for every vlan configured on the switch.

At this point I should ping without problem an ip vlan on the switch from a pc belonging to the same vlan on the switch, and I should ping also the vlan attached to the router from outside the router.

In this case all ip vlan on the switch are working, even if the "inside" ip vlans are not very useful.

Why someone says that only an IP vlan can work at a time?

With boson netsim simulator they can work at the same time.

Thanks.

[Jon Marshall]

On a L2 switch only one L3 vlan interface with an IP address can be active at any one time.

On a L3 switch you can have multiple L3 vlan interfaces with IP addresses active at the same time.

So it all depends on whether the switch is a L2 only or a L3 switch.

Jon

[speculor_cisco]

Ok, if I think I have to manage the switch from the outside.

But what about the non useful ping that I can do from a pc that belongs to a port on the same vlan, say vlan 10.

If the ping works then that vlan interface works.

It is not so important, I know, but the switch from the inside knows that it has multiple vlan interfaces.

Do you think boson netsim simulator do not work very well?

[vinod.agrahari]

Hi Specular,

Ping from inside of any vlan happens due to single broadcast domain and would be pingable from one PC Connected to another in the same vlan in case of any switch either in L2 Switch or L3 Switch.

In case of L2 Switch ,to manage it we give one ip address to there native vlan and access from out side via differnt ways and to access router from outside connect it interface in the native valn.

Please let me know if i did not explain/ clarify any thing in it.

Regards

Vinod Agrahari

[speculor_cisco]

I have added an image with an example.

With Boson NetSim simulator, switch 2950, I can ping both Vlan 10 and Vlan 20 interfaces on the switch from any inside Pc.

This seems to me right.

Why does someone say that only one interface is active at a time?

May be are there some problems with pings from the outside Pc_5?

Suppose that Pc_5 is the computer through with I manage the switch.

I could imagine that, as I can set only a default gateway on the switch, at this point I must decide definitely which is my management Vlan.

For instance, if I set the default gateway on the switch to 192.168.10.1, than Vlan 10 become my management Vlan, but I could choose also Vlan 20.

When someone says that only one Vlan interface is active on the switch, they consider the only Vlan interface that is accessible from the outside, Pc_5 for example?

And about this limitation, it seems to me that this limitation is due only to the fact that I can not set a different default gateway for every Vlan interface.

I tried with the simulator, with an only stupid default gateway 1.1.1.1, and all pings worked from Pc_5, that means that the simulator is a little strange.

What do you think about this scenario?

May be I am in error, but I am quite sure about the logic of the discussion.

[speculor_cisco]

I have added an image with an example.

With Boson NetSim simulator, switch 2950, I can ping both Vlan 10 and Vlan 20 interfaces on the switch from any inside Pc.

This seems to me right.

Why does someone say that only one interface is active at a time?

May be are there some problems with pings from the outside Pc_5?

Suppose that Pc_5 is the computer through with I manage the switch.

I could imagine that, as I can set only a default gateway on the switch, at this point I must decide definitely which is my management Vlan.

For instance, if I set the default gateway on the switch to 192.168.10.1, than Vlan 10 become my management Vlan, but I could choose also Vlan 20.

When someone says that only one Vlan interface is active on the switch, they consider the only Vlan interface that is accessible from the outside, Pc_5 for example?

And about this limitation, it seems to me that this limitation is due only to the fact that I can not set a different default gateway for every Vlan interface.

I tried with the simulator, with an only stupid default gateway 1.1.1.1, and all pings worked from Pc_5, that means that the simulator is a little strange.

What do you think about this scenario?

May be I am in error, but I am quite sure about the logic of the discussion.

[Nagaraja Thanthry]

Hello,

The simulator does not depict the actual hardware. Hardware platforms like 2950/2900XL/3500XL are pure layer 2 switches with management capabilities. In these platforms, the Layer 3 interface (VLAN SVI) is used for management purposes only. On any device, we need a single management interface to manage the device. Hence, these platforms support only one SVI at any given point of time (it is hardware limitation). Also, these SVI's are not designed for any default gateway purposes. If you go to higher end platforms like 2960/3550/3560/3750, then you can have multiple Layer 3 interfaces active at the same time. A

Hope this helps.

Regards,

NT

[speculor_cisco]

Ok. I suppose that the default gateway that I can set on the switch is used by the switch for the management traffic, in order to find

a way for the outbond traffic outside the local network. I have noted that you use the term SVI. In a layer 3 scenario, the IP of the SVI

is the default gateway of the PCs in that vlan, so they are two different concepts. Are you sure that Cisco gives them the same name?

In a layer 2 scenario I have read that there is a Vlan interface, virtual of course, that represents the end point of the management packets.

Is it important to do this difference, or it is obvious?

[Nagaraja Thanthry]

Hello,

In both scenarios, the interface is still considered SVI (Switch Virtual

Interface) as they are virtual in nature. But the functionality will be

different based on the type of switch you are using.

Hope this helps.

Regards,

NT

[speculor_cisco]

At this point, what do you think about Boson NetSim simulator?

I have already tested an incredible mac-table logical error in normal routing forwarding, error not present in GNS obviously.

How can I use the simulator? The answers that it gives are not real world answers.

Thanks.

[Nagaraja Thanthry]

Hello,

I don't think Boson simulator uses actual IOS for simulating Cisco devices

(GNS does). So, the answers Boson gives is more of theoretical (not

practical) in nature. It is a good tool to learn but not a good tool to

simulate and test a network.

Hope this helps.

Regards,

NT

[Jon Marshall]

It's very difficult to write a simulator for switches because most of what they do is in hardware. GNS3 is primarily for routers and because they carry out their operations in software that is why it is a lot more accurate.

Cisco also do something called packet tracer but i don't know how accurate that is.

Jon