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Layer 3 switching question


I am connecting 2 buildings together over a Laser line of site link.On both sides of the Link are 2 3550 Layer 3 switches.I just need someone to check if what I am trying to do is correct.

I am setting up a routed hop between the 2 switches by setting up the ports on both sides in the same vlan and IP subnet.

this link is actually connecting 2 different companies together because of an aquisition.

The one side has 11 vlans and the other side has everything in 1 Vlan which is the default.

Do i need to setup the ports that the laser device is connecting to as a 802.1q trunk or ISL trunk in order for All the Vlans to communicate between each other.I will also configure EIGRP on both layer 3 switches in order to advertise the networks to us and vice versa.

The laser link has got a wireless backup link which I would like to let eigrp pickup the primary route and if that fails the wireless will become the prefered route to the destination networks on the other side.

I was going to use rapid spanning tree , but I decided not to in order to avoid one huge spanning tree.Much safer going for the Layer 3 routing or routed hop option with the switches.

Can someone point me in the right direction and just confirm that I am on the right track of getting this Laser link working with 3550 Layer 3 switches on both sides of the link.

Thanks so Much for you help....


Re: Layer 3 switching question

If you are running the link as a routed link, then don't set it up as a trunk of any type. The 3550's should learn about the networks (vlans) on the other side, and route to them through the laser link.


Re: Layer 3 switching question

Set the ports up as a Layer-3 routed link:

!Switch #1

interface fastethernet0/1

no switchport

ip address


!Switch #2

interface fastethernet0/1

no switchport

ip address

Obviously use your own IP addressing scheme and make sure you are either running a routing protocol or have static routes covering the networks available either side of the link (run a routing protocol...)


New Member

Re: Layer 3 switching question

Thanks guys

I appreciate your help.

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