Layer 3 port to Layer 2 port...Behaviour?

[Aileron88]

Hi all,

I came across a setup yesterday whereas there was a Layer 3 Port connecting to a Layer 2 access port on another switch. The layer 2 port was a member of a VLAN that had a Layer 3 SVI.The switch with the Layer 2 port is still part of the EIGRP domain with the other device.

Question is, when sending from the Layer 2 Port to the Layer 3 Port on the other switch, I'm assuming the Layer 2 device just sends an IP Packet to the VLAN SVI and then out and works as normal when recieved on the L3 Port.

When the switch with the L3 Port recieves the packet, if destined for the switch with the Layer 2 port it just sends a normal IP Packet out and the Layer 2 port on the other switch just see's it as a normal packet that comes in on whatever access VLAN it's assigned to.

Does that all make sense so far!?

So my question is, what would be different on this when sending from L3 > L2 or L2 > L3 rather than L3 > L3 and why would you do this? Are there any benefits to doing this?

Thanks,

Adam

[Eugene Lau]

Hey Adam!

Let me have a go at this.

I think what you are asking is, why use a L2+L3SVI---L3 port vs L3---L3  vs L2+ L3SVI ---- L2+L3SVI?

From an IP routing and forwarding perspective, there is no difference. With multi-layer switches which switch traffic in hardware, there is also no difference in performance. It is a simply a choice of isolating the area from factors such as STP, problems associated to L2 broadcast domains.

Eg.

If this was a PE or provider edge device, I wouldn't want any influence from customer mis-config issues with STP or broadcast traffic if I was bridging multiple customers into the same switch, so I would use an L3 port.

HTH

Eugene

[Jon Marshall]

Adam

Just to add to Eugene's post.

An example where you might need to do this if one of the switches does not support L3 routed ports eg. a 6500 switch running in hybrid mode with CatOS  does not support L3 routed ports. So the only way to emulate this is to use a dedicated vlan for the link, put the actual switchport into that vlan and then create a L3 SVI for it.

Jon