EtherType Meaning in Layer 2 Frame

[karenmelkonyanstu]

Hello,

I understand what is EtherType - It is used to indicate which protocol  is encapsulated the payload of the frame.
However I don't understand why for example L3 Packet doesn't have similar field?
Why only L2 Frame needs this kind of field which would indicate upper protocol? 

[Joseph W. Doherty]

Ah, but an IPv4 packet does have a protocol field.

As to why an Ethernet frame needs a type field, it doesn't!  Suggest you look up non-Ethernet II frames (when the type field is considered a length field).

[karenmelkonyanstu]

Yeah Indeed IP Header has protocol field - thanks for spotting it BUT for example MPLS doesn't have though upper header also as in case of Ethernet II can be either IPv4 or IPv6.
I am aware of Ethernet 802.3 Standard Frame which field is only about field and it confused actually me more.
So are you saying that technically there is no need for this field and it just historically was left there?
So let's say if we had removed this particular information things would still work?

[Joseph W. Doherty · Accepted solution]

"So are you saying that technically there is no need for this field and it just historically was left there?
So let's say if we had removed this particular information things would still work?"

I'm saying "need" is sometime really for "convenience".

Consider, strictly speaking, should L2 be involved with carrying any hint about what its L3 payload is?  Once you do that, you've bound two distinct layers.  But by doing so, you've likely reduced L3 workload, often an important consideration especially in the early days of networking (much the logic behind Classful IP, I suspect).

You mention MPLS, which is what logical layer?  Study its history, and you should see its primary original purpose was fast L3 forwarding.  Excluding performance, and its later evolution, was it (logically) needed?

[MHM Cisco World]

Ether type need to make sw port know header length to read it Or remove and punt to higher level.

This what I know about ether type.

MHM

[M02@rt37]

Hello @karenmelkonyanstu 

In L2, devices communicate using frames. The EtherType field in an Ethernet frame indicates the type of protocol or payload encapsulated within the frame. It helps the receiving device to understand how to interpret the data in the frame.

In L3, devices communicate using packets. The primary protocol at this layer is IP. In the context of IP, the equivalent of the EtherType field is the Protocol field found in the IP header. The Protocol field in the IP header identifies the specific protocol (such as TCP or UDP) that is encapsulated in the IP packet. It serves a similar purpose to the EtherType field but operates at the Network Layer.

The reason Layer 3 packets don't have a field equivalent to EtherType is rooted in the design of the OSI model and the division of responsibilities between layers:

Layer 2  is concerned with local delivery of frames between devices on the same network. EtherType is used to identify the type of payload in the Ethernet frame, allowing devices on the local network to understand how to process the data.

Layer 3 is responsible for end-to-end communication across multiple networks. IP is the dominant protocol at this layer, and the Protocol field in the IP header is used to identify the type of payload (e.g., TCP, UDP) within the IP packet. Since IP is a network-layer protocol, the Protocol field is specific to IP packets.