Interconnecting switches NOT using a trunk link

Answered Question
Sep 28th, 2006
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Hi,


Let's assume we have 2 switches, switch A and Switch B. We interconnect both switches using FastEthernet0/1 interface on both of them. The fastethernet0/1 port on Switch A is configured as belonging to VLAN 24. Same interface on Switch B is configured as belonging to default VLAN (VLAN 1). For connecting both switches I use a cross-over cable, and I DON'T configure trunk on the link.


The question is: would the traffic sourced on a workstation connected to switch B (vlan 1 traffic) traverse the fastethernet0/1 port in Switch A and will it reach a workstation connected to any other port of Switch A?

I believe not, because there is a VLAN mismatch at the 2 inter-connecting ports.


Am I right?


Thanks in advance.

Correct Answer by hdecreis about 10 years 10 months ago

If interconnection between Switch A and Switch B is an access link (no trunking) belonging to vlan 25 on Switch A and vlan 50 on Switch B, the Ethernet continuity is OK, switches will forward frames without bothering about vlans.


Since L2 connectivity is OK, you can consider it is equivalent as having both PC on the same VLAN. in fact, the "VLAN" is the same from end to end, it is just numbered differently on the switches.


Correct Answer by dabels about 10 years 10 months ago

you will see there is a vlan mismatch when you do a show CDP nei on these ports. This is because CDP passes the vlan number and trys to help you out. but as stated before if the ports are not designated as trunks there is no tagging.

Correct Answer by Bobby Thekkekandam about 10 years 10 months ago

The concept of a Native VLAN only exists when there is a trunk. When there is no trunk, traffic between the two devices is not tagged, so the frames that move across the link are not tagged with any VLAN information. So even if the port on switch A belongs to VLAN x and the port on switch B belongs to VLAN y, if the link is not configured as a trunk, then VLAN x and VLAN y are effectively bridged.


HTH,


Bobby


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Correct Answer by farkascsgy about 10 years 10 months ago

Hi,


Both ports are configured as access port? If yes the frames are not tagged, and traffic can be exchanged between the two equipments. But only L2 traffic, L3 can also when the subnet is the same for the two vlans.


bye

FCS


Please rate me if I helped.

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Correct Answer
farkascsgy Thu, 09/28/2006 - 07:12
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Hi,


Both ports are configured as access port? If yes the frames are not tagged, and traffic can be exchanged between the two equipments. But only L2 traffic, L3 can also when the subnet is the same for the two vlans.


bye

FCS


Please rate me if I helped.

s.debenito Thu, 09/28/2006 - 07:33
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Hi farkascsqy,


First at all, thanks for your answer.


But I don't understand why it should works.


When Switch B would send a frame destinated to any machine connected to Switch A, Switch A assume that the frame belongs to Native VLAN ...which is VLAN 1 by default. Right?


So, why should a workstation connected to a port on a VLAN other than 1 receive the frame from Switch B?


Thanks.

Correct Answer
Bobby Thekkekandam Thu, 09/28/2006 - 08:46
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The concept of a Native VLAN only exists when there is a trunk. When there is no trunk, traffic between the two devices is not tagged, so the frames that move across the link are not tagged with any VLAN information. So even if the port on switch A belongs to VLAN x and the port on switch B belongs to VLAN y, if the link is not configured as a trunk, then VLAN x and VLAN y are effectively bridged.


HTH,


Bobby


*Please rate helpful posts.

Correct Answer
dabels Thu, 09/28/2006 - 11:29
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you will see there is a vlan mismatch when you do a show CDP nei on these ports. This is because CDP passes the vlan number and trys to help you out. but as stated before if the ports are not designated as trunks there is no tagging.

hdecreis Thu, 09/28/2006 - 23:45
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This is what I often saw called "manual vlan mapping", particularly useful in some cases when you want to map sw1-vlanA to sw2-vlanB but sw2-vlanA exists and should be kept alive. Example:


If you need to pass vlan800 from one switching cloud to another, using a third switching cloud inbetween, which switching cloud already have a vlan800 up&running, which should not be bridged with yours.


classic vlan mapping make vlan800 unavailable in the intermediate switching cloud.


QinQ is not really straightforward, and not available on all devices.


Interconnecting the switching clouds with access ports (part of a new vlan, say vlan 200 in the intermediate switching domain, and part of vlan 800 in the others two) is the most simple and effective way to achieve this. Only problem is that you can't have more than one vlan per physical interconnection... and CDP whinning in logs or disabled on those ports.


Of course, having a proper vlan numbering scheme from the start is a lot cleaner...


PS: if there is another way to do this, cleaner than that, I would be happy to hear it ?

s.debenito Fri, 09/29/2006 - 00:36
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And what would happen if there is one PC connected into a port of Switch A belonging to VLAN 25 and there is another PC connected into a port of Switch B belonging to VLAN 50 and they are both on the same IP Subnet ? would the 2 PCs be able to communicate between them even if they are on different VLANs ??


Thank you so much !!


Correct Answer
hdecreis Fri, 09/29/2006 - 00:57
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If interconnection between Switch A and Switch B is an access link (no trunking) belonging to vlan 25 on Switch A and vlan 50 on Switch B, the Ethernet continuity is OK, switches will forward frames without bothering about vlans.


Since L2 connectivity is OK, you can consider it is equivalent as having both PC on the same VLAN. in fact, the "VLAN" is the same from end to end, it is just numbered differently on the switches.


jackyoung Thu, 09/28/2006 - 23:57
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Due to no trunking, the VLAN configuration is configured locally and not affect other devices. So it will be no problem, sw A won't care and not know what VLAN is sw B using. Unless it is trunked connection, you have to match the VLAN to make it works.


Hope this helps.

s.debenito Fri, 09/29/2006 - 01:11
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Sorry Jackyoung , but I don't understand your last phrase:


"Unless it is trunked connection, you have to match the VLAN to make it works."


Do I have to match the VLAN numbers if it is NOT a trunked connection?


Thanks.



glen.grant Fri, 09/29/2006 - 04:42
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No you don't have to match the vlan numbers on each side to make it work . You will get cdp mismatch errors but traffic will still flow . It is still best practice to keep them the same to eliminate confusion and from getting the cdp errors...

jackyoung Fri, 09/29/2006 - 05:57
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No, as Glen said. VLAN setting is take place locally if it is not a trunk. You can imagine that if the a switch w/ VLAN setting and the port connect to a hub or router, does the connected hub or router required to configure the VLAN to make it work ? The ans. is no., because they are independently.


Hope this clarify.

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