Which three layers of OSI model is treated as "Media Layers"?

Question

It is generally said that BGP protocol belongs to Application Layer of OSI model but how is this possible ?

It is made use of by border routers which belongs to level-3 then will that mean border routers are special kind of routers which act at application layer?

How do they differ from general routers?

Answer 1

Media Layers: Used whenever any message passes through the host device

1. Physical Layer :
   Data Unit: Bit
   Functions: Transmission and reception of raw bit streams over a physical medium

2. Data Link Layer
   Data Unit: Bit/Frame
   Functions: Reliable transmission of data frames between two nodes connected by a physical layer

3. Network Layer
   Data Unit: Packet/Datagram
   Functions: Structuring and managing a multi-node network, including addressing, routing and traffic control