Re: Query regarding VALN

rawatmohinder · ‎07-16-2006

Hello friends,

I had an interview recently with an R & D department of a company. They asked me some scenario related questions, in which i'm a bit confused. So, if u have got time, Please help me with this scenario...

The Scenario is like this:

They have two switches, both are L2 switches. First Switch, say S1 has VLAN "V-1" configured in all ports. and second switch, say S2 is configured with Vlan "V-2" in all the ports. Both these switches are connected to each other.

1. No trunk port is configured in any of the switches. They have two Nodes, both in the same subnet. Node A connected in Switch S1 and Node B connected to Switch S2. Will Node A ping Node B?

2. Now, Trunk Port is configured in the port, that is used to connect the switches. Will the nodes Ping each other? If No, then what should be done to make the nodes ping each other???? No additional network devices can be used.

Mahi

Roberto Salazar · ‎07-16-2006

1. No trunk port is configured in any of the switches. They have two Nodes, both in the same subnet. Node A connected in Switch S1 and Node B connected to Switch S2. Will Node A ping Node B?

Yes, both ports are access port and they are in the same subnet. packet come out of both switches with not vlan tagging, therefore, it will work. If pc in switch A send a ping to node in switch B, the packet sent form switch A will have no vlan tag, the switch B receives this packet and since all the ports belong to vlan 2 and the packet it receives on port connecting to switch A will be assumed that it belongs to vlan 2 as well. Same subnet so the ping will work.

2. Now, Trunk Port is configured in the port, that is used to connect the switches. Will the nodes Ping each other? If No, then what should be done to make the nodes ping each other???? No additional network devices can be used.

Yuo need to have both port trunk and with matching native vlan, see answer above on access ports sending packets with no vlan tag.

The question above is clearly for understanding the difference between trunk and access ports.

Please rate all posts.

rawatmohinder · ‎07-20-2006

thanks for ur reply..

rawatmohinder · ‎07-23-2006

Hi,

Can you explain the concept clearly...

you mean to say if the two VLAN,s are in same subnet then can send the data to each other..if no trunk port is there..

In the answer of second question...

you told that they can communicate with both trunks allowed the both vlan..r u considering the same subnet in this case also...

But can you explain the concept in brief..

Thanks

Mahi

Roberto Salazar · ‎07-23-2006

two switches (switch A and switch B) connected by access ports. switch A port connecting to switch B is in vlan 1, port of switch B connecting to switch A is in vlan 2. PC in both Switch A and B are in the same subnet will be able to communicate due to reason given in previous post.

Switch A and Switch B are now connected by trunk ports, given the native vlans are matching, and PC in vlan 1 of switch A and PC in vlan 2 of switch B are still in the same subnet, ip communication between PC's in switch A and B will not work. Reason: PC in switch A tried to ping IP address of PC in vlan 2 of Switch B, they are in same subnet. The ping goes to Switch B but this time there is vlan tag - vlan 1. That ping will be flooded or sent to ports belonging to vlan 1 because the source of the ping or any type of traffic is now tagged with a vlan id, the switch B which is the receiving end will have to send that packet to port belonging to vlan 1. The PC trying to contact in switch B is in vlan 2, so it fails.

I tried to make the expalanation in layman's term as best I could.

So, in summary, switches connected by access port in question 1 will be able to talk. on switch connected via trunk with the same condition as scenario one will not be able to talk, I'll leave at that. there is exception which have to do with misconfiguration and native vlan mismatch but that is another subjet for another time.

Please rate helpful post.

rawatmohinder · ‎07-27-2006

Hi Bosalza..

Thanks for replying..

So if want communication between pc inspite of trunk port...what we can do.. no aditional device can be used...scenario is same..

Thanks in advance..

Mahi

Roberto Salazar · ‎07-27-2006

If PC are in same subnet they must be in the same vlan. The trunk ports will have to have matching native vlan if using dot1q. If the PC are in different vlans that means they have to talk at layer 3 which is IP then you need a router.

I recommend a book titled Cisco LAN Switching by Kennedy Clark and Kevin Hamilton. It is a CCIE prep book but more LAN/Switch/L2 focus so all L2 technology is explained in a very clear manner.