Which layer is BGP?

[gazwoz (Gareth W)]

Hi Guys - simple question - which layer is BGP? Thanks - Gareth.

[rickyccnp (RICHARD A)]

BGP is a layer 4 protocol

[gazwoz (Gareth W)]

TCP is a layer 4 protocol - is BGP also layer 4 because it is encapsulated in TCP?

[rickyccnp (RICHARD A)]

BGP is a Layer 4 protocol that sits on top of TCP. . Peers that have been manually configured to exchange routing information will form a TCP connection and begin speaking BGP. There is no discovery in BGP

https://tools.ietf.org/html/rfc4271

[lagapidis (Lazarus Agapidis)]

Hello Gareth

@rickyccnp is correct in that BGP uses TCP in order to operate and exchange information and thus it cannot be a Layer 3 protocol. However, BGP is actually considered an Application layer protocol. In the TCP/IP stack, any entity that functions above the Transport layer is considered an Application Layer protocol. BGP’s intelligence operates within a process in routing devices that sits on top of the Transport layer, and uses TCP to communicate its information between peers.

Take a look at this post for a bit more info:

BGP Communities Explained

Hello Mohamed Even though it is used to exchange routing and reachability information, which is a Layer 3 functionality, BGP itself does indeed operate at the Application layer. Because it uses TCP to actually exchange information, it is the Application layer entity of BGP that provides the data for the TCP segments. So BGP is indeed an Application layer protocol. Compare this with OPSF. It doesn’t use any functionalities above Layer 3, so OSPF is only a Layer 3 protocol. I hope this has b…

I hope this has been helpful!

Laz

[gazwoz (Gareth W)]

Hi Laz,

That’s very interesting. So all of the BGP message types (OPEN, UPDATE, KEEPALIVE etc) are actually the payload of the TCP header?

Would BGP also be considered an application layer protocol in the OSI model?

Thanks,

Gareth.

[lagapidis (Lazarus Agapidis)]

Hello Gareth

Yes, BGP messages are the payload of TCP segments. You can verify this by looking at any wireshark output of BGP messages. Here’s an example I found online of a BGP OPEN message. Notice that BGP rides on top of TCP:

BGP like many modern protocols adhere to the TCP/IP stack rather than the OSI model. In essence, the Session and Presentation layers of the OSI model are actually contained within the Application layer of the TCP/IP model. So to answer your question, yes, BGP, in the context of the OSI model, would still be considered an Application layer protocol.

I hope this has been helpful!

Laz

[deensr456 (viru p)]

It is application layer.

[gazwoz (Gareth W)]

Thanks Laz! Interesting to know that there are actually elements of the application layer that interest us as network engineers!