L2 trunking versus L3 switching

[Horacio Gutierrez]

Hello,

Quick question some of our switches have the switchport mode trunk command configured between the 3750 switches but other 3750 switches connected to our 6509 core switch do not have the switchport mode trunk command to permit Vlans from going across the swtiches instead it has an ip address and says no switchport what is the difference between does two. Is trunking used only for Layer 2 and L3 is used to route  interface vlans?

Thanks.

[Somasundaram Jayaraman]

Hi,

++ Trunking allows us to extend the vlans to other switch but still for routing between two vlans the packets needs to hit the svi.

++ If there is an ip address configured between the switch and the router, then the l3 header gets added to the packet.

But the l3 information is stripped off when it goes out of the l3 interface and reaches the l2 part.

++ The advantage of this is layer 2 protocols such as stp, will not come into picture when the packet is in L3 path.

++ In both the cases if the packet needs to get routed between two vlans , the packet needs to hit the SVI.

Hope this answers your questions.

Cheers

Somu

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[Edwin Summers]

Horacio,

You are on the right path.  Trunking is a layer 2 function that carries multiple (tagged) VLANs across a link.  The "no switchport" command (on supporting switches) configures the port as a routed (layer 3) port.  The port will have an IP address assigned and route traffic between destinations.

What you are seeing is a common design practice.  Access switches (switches in comm closets that connect directly to the end-user devices) will connect to distribution switches via trunk links.  This allows the use of VLANs on the access switches to segregate traffic, and backhauling of all traffic back to distribution layer.  At the distribution layer, the distribution switches connect to the core via layer 3 links.  The VLANs terminate here, and the traffic is routed to its final destination. 

As you suspected, the VLANs "terminate" at the distribution switch, and the traffic is routed from there.