Connecting two sites with dark fiber

Unanswered Question
Jan 16th, 2009

I have presently two sites connected with two LAN-X (100Mb i 10Mb).

LAN-X is provided by telco. I will be replacing these links with two dark fiber links.

Here shows how I will connect both sites with dark fiber.

Site 1 (vlan150) 3750<----->3750-E--------fiber1---------3750-E<------>4506 (vlan100) Site 2

Site 1 (vlan150) 3750<----->3750-E--------fiber2---------3750-E<------>4506 (vlan100) Site 2

What would be the best practice to configure these 3750-E to pass vlans between both sites?

I have this problem too.
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Giuseppe Larosa Fri, 01/16/2009 - 09:01

Hello Zbigniew,

>> What would be the best practice to configure these 3750-E to pass vlans between both sites?

L2 connection using trunk ports and allowing all vlans on both sides

if you are using VTP and VTP domain names don't match you need to manually configure the trunk with

swithport trunk enc dot1q

switchport mode trunk

if you instead just want to connect the two sites you can think of a L3 link with a routing protocol over it.

this has the advantage of a better control of broadcasts the fiber will be used only when needed.

Hope to help


zbigniewkozyra Sat, 01/17/2009 - 06:00

Thank you for a reply.

I need to use l2 because I need to have same vlans available on both sides. Which ports I put in trunk mode? The one between 3750-E? What about ports between 4506 and 3750-E and between 3750 and 3750-E ? Should I create etherchannel using both fiber?

mahmoodmkl Sat, 01/17/2009 - 06:59


U need to have all the ports which u r using to connected all the devices as trunk links.

Yeh u can make a ether-channel if u want.



wesleyterry Sun, 01/18/2009 - 21:15

If you don't use etherchannel, aren't you going to run into STP issues? I'm not an expert, but default properties of spanning tree will disable one of your links. If you do etherchannel, I think you'll basically have a redundant physical connection and twice the bandwidth...

As far as the trunking question, if it is all L2, you're going to need to trunk between all of the links you have in your picture above.


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