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New Member

VLAN questions

Hi,

I have a couple of questions about VLAN.

1. Based in the following scenario:

Switch A ---Trunk---- Switch B ----Trunk------ Switch C

If I create a VLAN in swithes A and C (for reference VLAN 100), but I dont create it in Switch B,    is it possible to communicate the hosts connected on Switch A in vlan 100 with the hosts connected to Vlan 100 in Switch B ?????

2. I know that the ethernet interfaces are broadcast. I supose that the VLAN interface are too, but  I would like a second opinion about that.

Thanks for your comments,

37 REPLIES
Hall of Fame Super Blue

Re: VLAN questions

dbarboza27 wrote:

Hi,

I have a couple of questions about VLAN.

1. Based in the following scenario:

Switch A ---Trunk---- Switch B ----Trunk------ Switch C

If I create a VLAN in swithes A and C (for reference VLAN 100), but I dont create it in Switch B,    is it possible to communicate the hosts connected on Switch A in vlan 100 with the hosts connected to Vlan 100 in Switch B ?????

2. I know that the ethernet interfaces are broadcast. I supose that the VLAN interface are too, but  I would like a second opinion about that.

Thanks for your comments,

1. No they won't be able to communicate ie. you need vlan 100 on all 3 switches. I'm assuming your question was can hosts on switch A in vlan 100 communicate with hosts on switch C in vlan 100 which is not quite what you wrote.

2. Not sure what you mean. L3 SVI's are like routed ethernet interfaces in that they do indeed understand broadcasts but will not by default route those broadcasts to another subnet.

Jon

New Member

Re: VLAN questions

I also assume that you want hosts configured in vlan 100 in switch A needs to communicate with hosts in vlan 100 in Switch C.

No problem. They can communicate with each other without any problem.

No need to create vlan 100 in switch B.

Make sure the trunks are configured properly between switches.

Regards,

Dasuntha

Hall of Fame Super Blue

Re: VLAN questions

Dasuntha_Dinesh wrote:

I also assume that you want hosts configured in vlan 100 in switch A needs to communicate with hosts in vlan 100 in Switch C.

No problem. They can communicate with each other without any problem.

No need to create vlan 100 in switch B.

Make sure the trunks are configured properly between switches.

Regards,

Dasuntha

Dasuntha

Have you tested this ?  If vlan 100 does not exist on switch B then you cannot get from switch A to switch C via L2 in vlan 100.

Jon

New Member

Re: VLAN questions

Jon,

I tested it & working fine.

And no need to configure vlan100 in switch B.

Only we need to configure trunk links properly.

Pls test. You'll also see it.

Regards,

Dasuntha

Hall of Fame Super Blue

Re: VLAN questions

Dasuntha_Dinesh wrote:

Jon,

I tested it & working fine.

And no need to configure vlan100 in switch B.

Only we need to configure trunk links properly.

Pls test. You'll also see it.

Regards,

Dasuntha

Dasuntha

Interesting. So when you say you need to configure the trunk links properly does that mean allowing vlan 100 on the trunk links ?

Jon

New Member

Re: VLAN questions

I didnt specify only vlan 100. I allowed all

I mean the trunk encapsulation should be properly configured.

I also had a doubt first, but I tested it & worked perfectly.

Hall of Fame Super Blue

Re: VLAN questions

Dasuntha_Dinesh wrote:

I didnt specify only vlan 100. I allowed all

I mean the trunk encapsulation should be properly configured.

I also had a doubt first, but I tested it & worked perfectly.

So a "sh vlan brief" on switch B does not show vlan 100 ?

Sorry to ask so many questions but i didn't think this would work so it's news to me

Jon

New Member

Re: VLAN questions

yes. of course.

vlan 100 cant be in switch B. Coz i didnt created it in Switch B & I m not using VTP.

And in this question, we are not trying inter vlan communication.

And both hosts are in the same vlan & same ip subnet & switches carry the vlan information if the trunks are properly configured.

Hall of Fame Super Blue

Re: VLAN questions

Dasuntha_Dinesh wrote:

yes. of course.

vlan 100 cant be in switch B. Coz i didnt created it in Switch B & I m not using VTP.

And in this question, we are not trying inter vlan communication.

And both hosts are in the same vlan & same ip subnet & switches carry the vlan information if the trunks are properly configured.

Then i stand corrected because i honestly thought that would not work. Thanks for the information and patience with my questions

Jon

Hall of Fame Super Blue

Re: VLAN questions

One more question - how have you set the switches up with VTP ie. are they transparent or VTP server/client ?

Jon

New Member

Re: VLAN questions

I didnt configure VTP in any switch

New Member

Re: VLAN questions

I tested it both L2 & L3 switches.

its working fine in both L2 & L3 switches.

Re: VLAN questions

Dear Dasuntha,

          Could you please post your configs.

          Because as Jon says, it doesnt work for me either!!! I couldnt make this intra VLAN communication without creating the vlan 100 in the SW-B, regardless of what VTP domain i stay in.

          The vlan i create in SW-A gets replicated to SW-C if i have SW-B in VTP mode transparent.   But thats not the problem / question here.

          The actual question is, Will the traffic from host A on SW-A's VLAN 100 reach SW-B's host B on vlan 100. ?

I would appreciate if you can post your configs. or atleast the output of  "show vlan brief, show int trunk" in all three switches..

Happy Networking !!!!

Narendrakumar B

Do rate the helpful posts !!

New Member

Re: VLAN questions

Switch A--- trunk link---- Switch B----trunk link ----Switch C

In example, you configured vlan 100 in Switch A and Switch c. But Switch B have configured in vlan 30 only

That time VTP help us to contact switch A(VTP SERVER MODE) and Switch C(VTP CLIENT MODE).

In Switch B has configured in VTP TRANSPARENT mode.

Re: VLAN questions

Jon,

Better to avoid too much of Cisco here (referring to VTP)

Once the trunk is up, it will by default carry all Vlans (whether or not its accessed at the switchport)

i.e SW1--SW2--SW3--SW4

Host on Vlan 10 at SW1 can contact host on SW4 in the same Vlan 10 if all the links between the switches trunk.

Just to simplify that VTP works over trunks and its for ease of administration and not required real communication.

Hall of Fame Super Blue

Re: VLAN questions

tharakabraham wrote:

Jon,

Better to avoid too much of Cisco here (referring to VTP)

Once the trunk is up, it will by default carry all Vlans (whether or not its accessed at the switchport)

i.e SW1--SW2--SW3--SW4

Host on Vlan 10 at SW1 can contact host on SW4 in the same Vlan 10 if all the links between the switches trunk.

Just to simplify that VTP works over trunks and its for ease of administration and not required real communication.

Tharak

This is where i am slightly confused.

If you look at the output from Dasuntha for all 3 switches you will see that switch B is not forwarding vlan 100 on it's trunk link to switch C. It can't be forwarding it because there is no vlan 100.

When the frame arrives from switch A at switch B on the trunk link the 802.1q tag is stripped from the frame and this tells switch B that the frame belongs to vlan 100. Switch B should then do a lookup in it's cam table and see that the destination mac-address is reachable via the trunk link connection to switch C and is in vlan 100. So the frame then has an 802.1q tag added for vlan 100 and forwarded to C. But if switch B doesn't have vlan 100 configured how does it know that it needs to add an 802.1q tag for vlan 100.

My understanding was that the mac-address table records not only the port but also the vlan ie. a switch has a mac-address table per vlan but if you haven't configured vlan 100 it won't have a mac-address table for this vlan and so switch B will not know that the frame should have a vlan 100 tag added.

So how does switch B -

1) know to add an 802.1q tag for vlan 100 before transmitting the frame to switch C

2) forward the frame in vlan 100 on the trunk link when vlan 100 is not active on the trunk link.

Note i'm not trying to prove anyone wrong here, i am simply trying to get a full understanding of how this works.

Jon

New Member

Re: VLAN questions

Hi Douglas,

Can u resend your question clearly?

New Member

Re: VLAN questions

HI Narendrakumar,

In the actual question, Douglas is not talking about inter vlan communication.

He wants to know if the traffic from switch A is going to switch C through switch B for vlan 100. And vlan 100 is not configured in switch B.

Pls find below the configs you requested. Note that VTP is not configured.

Switch A

A#sh vlan brief

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/2, Fa0/3, Fa0/4, Fa0/6
                                                Fa0/7, Fa0/8, Fa0/9, Fa0/10
                                                Fa0/11, Fa0/12, Fa0/13, Fa0/14
                                                Fa0/15, Fa0/16, Fa0/17, Fa0/18
                                                Fa0/19, Fa0/20, Fa0/21, Fa0/22
                                                Fa0/23, Fa0/24, Gig1/1, Gig1/2
100  VLAN0100                         active    Fa0/1
1002 fddi-default                     active   
1003 token-ring-default               active   
1004 fddinet-default                  active   
1005 trnet-default                    active   
A#
A#sh inter trunk
Port        Mode         Encapsulation  Status        Native vlan
Fa0/5       on           802.1q         trunking      1

Port        Vlans allowed on trunk
Fa0/5       1-1005

Port        Vlans allowed and active in management domain
Fa0/5       1,100

Port        Vlans in spanning tree forwarding state and not pruned
Fa0/5       1,100
A#

Switch B

B#sh vlan brief

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                Fa0/7, Fa0/8, Fa0/9, Fa0/10
                                                Fa0/11, Fa0/12, Fa0/13, Fa0/14
                                                Fa0/15, Fa0/16, Fa0/17, Fa0/18
                                                Fa0/19, Fa0/20, Fa0/21, Fa0/22
                                                Fa0/23, Fa0/24, Gig1/1, Gig1/2
1002 fddi-default                     active   
1003 token-ring-default               active   
1004 fddinet-default                  active   
1005 trnet-default                    active   
B#
B#sh inter trunk
Port        Mode         Encapsulation  Status        Native vlan
Fa0/5       on           802.1q         trunking      1
Fa0/6       on           802.1q         trunking      1

Port        Vlans allowed on trunk
Fa0/5       1-1005
Fa0/6       1-1005

Port        Vlans allowed and active in management domain
Fa0/5       1
Fa0/6       1

Port        Vlans in spanning tree forwarding state and not pruned
Fa0/5       1
Fa0/6       1
B#

Switch C

C#sh vlan brief

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/2, Fa0/3, Fa0/4, Fa0/5
                                                Fa0/7, Fa0/8, Fa0/9, Fa0/10
                                                Fa0/11, Fa0/12, Fa0/13, Fa0/14
                                                Fa0/15, Fa0/16, Fa0/17, Fa0/18
                                                Fa0/19, Fa0/20, Fa0/21, Fa0/22
                                                Fa0/23, Fa0/24, Gig1/1, Gig1/2
100  VLAN0100                         active    Fa0/1
1002 fddi-default                     active   
1003 token-ring-default               active   
1004 fddinet-default                  active   
1005 trnet-default                    active   
C#
C#sh inter tr
C#sh inter trunk
Port        Mode         Encapsulation  Status        Native vlan
Fa0/6       on           802.1q         trunking      1

Port        Vlans allowed on trunk
Fa0/6       1-1005

Port        Vlans allowed and active in management domain
Fa0/6       1,100

Port        Vlans in spanning tree forwarding state and not pruned
Fa0/6       1,100
C#

Hope this information will help you.

Pls let me know if you need any other information.

Regards,
Dasuntha






New Member

Re: VLAN questions

Hi Dasuntha,
According to your config logs,

VLAN 100 is not configured in SWITCH B, So Put in VTP mode as transparent, Switch A will be configured in VTP mode as Server, and Switch C is configured in VTP mode as Client.

New Member

Re: VLAN questions

Hi Thiruma,

I think you didnt understood the question.

No need to configure Vlan 100 in Switch B. That's what the main question asked about.

VLAN 100 traffic can pass without any problem to switch C.

So, clienst in VLAN 100 attached to Switch A can communicate with the clients in Vlan 100 in Switch C.

Hope you understand question & answar.

Regards,

Dasuntha

New Member

Re: VLAN questions

Hi Dhasundha,

I understood your question Properly,

In switch B, We don't need to configure Vlan 100, jus configure vtp mode as transparent, Switch A as vtp mode server, switch c as vtp mode client, Configure trunk links b/w Switch A & B & C.

Thanks,

Thiruma

Re: VLAN questions

Dear Dhasundha,

            All these work for me, yet Im not able to reach host  on SW-C from Host  on switch C. I have attached a file for your reference. I Would appreciate comments from others too..

Rgds,

Narendrakumar B

New Member

Re: VLAN questions

Hi Narendrakumar,

As I informed in my answars, we need to configure the trunk links properly.

I checked your file & the trunk links were not properly configured.

So, I configured it for you & it's working fine now.

Now both hosts can communicate with each other.

I attached the fixed file for you.

Then you can see that it's working fine.

Regards,

Dasuntha

Re: VLAN questions

Dear Dasuntha,

        Thankyou very much for your reply. I have checked the file you have sent me. Even in that the host arent able to reach eachother. I guess, there is some problem in the packettracer installed in my system (SUSE LINUX) itself. Not sure though.

        In the file that i have posted,  the trunks exists. Eventhough i have specified "switchport mode trunk"  only in one end of any switches, the trunks had formed sucessfully. Because in my file the switchports are in "Dynamic Auto" mode by default. So specifying this command in one end brings the trunk up.

         Anyway, I dont have real switches to check this.  I appreciate your efforts towards answering this question and i have rated your posts.

Happy Networking !!

Rgds,

Narendrakumar B

New Member

Re: VLAN questions

Hi Narendrakumar,

May be the problem is coming from the packet traser. a bug. I also noticed lots of bug  in the previous versions.

I m using Packet Traser 5.3.

I hope you're also using the same. Otherwise download 5.3 or I can give you a link to download.

Best Regards,

Dasuntha

Re: VLAN questions

Yes, That should be it. Because conceptually what u mean is right. I use PTv5.1I would appreciate if you could mail the link for PTv5.3 to narendrakumar1987in@gmail.com.

THanks in advance.

Happy Networking !!

Narendrakumar B

New Member

Re: VLAN questions

Hi All,

I hope you have heard about VTP Pruning:

"VTP Pruning allows switches to prevent broadcast and unknown unicast from flowing to switches that do not have any ports in that VLAN"

In the example: SwitchB does not have port for VLAN 100, that's why, there will not be communication between SwitchA and SwitchC.

Action: Create and assign VLAN100 on trunk ports on SwitchB.

Hope it will resolve the issue.

Thanks,

Sha

New Member

Re: VLAN questions

Hi Shahid,

No need to configure vlan 100 in switch B.

And switch A can communicate with Switch C.

And VTP Pruning we have to enable manually..

Regards,

Dasuntha

New Member

Re: VLAN questions

Hi Narendrakumar,

Download PT 5.3 from below link.

http://hotfile.com/dl/40746344/98501c6/PacketTracer53_setup.rar.html

Regards,

Dasuntha

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