Why do we need L3 in Transmission?

Dear friend！

I think there are two explanations:

The first explanation

NG WDM uses the L0+L1+L2 three-layer architecture. 

The L0 optical layer supports wavelength multiplexing/demultiplexing and DWDM optical signal adding/dropping. 

The L1 electrical layer supports the cross-connection of ODUKk/VC services. 

The L2 layer implements Ethernet/MPLS-TP switching

In some literature, the L0 layer is defined as L1, and then L2 is labeled as L3 layer.

The second explanation

In the packet transmission network, the equipment can provide flexible routing links for users. Therefore, the L3 layer must be used. The Layer 3 protocol is widely used on Huawei PTN devices. More information.

PTN equipment can be considered as a combination of optical transmission equipment and routers. According to Huawei's product classification table, PTN devices are classified into metro routers.

Thank you!