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Got a doubt while studying about LLC layer. From Network layer I came to know that IP fragmentation will be done, For L4 TCP will segment the data.
Here, the question is Protocols starting from Layer-2 (For eg : STP , RSTP), who will fragment the data.
Layer 2 does not do any fragmentation. It is up to Layer 3 to pass data to Layer 2 in a packet/frame size that will already match Layer 2's MTU. In fact, the reason Layer 3 does fragmentation in the first place is is because of the limitations of the Layer 2 Protocol.
IP doesn't care if the packet size is 1500 bytes of 9999 bytes, it just knows what its underlying Layer 2 protocol can handle. The "Packet Length" field in the IP header is a 16 bit value, which means IP can create a packet as big as 65535 bytes if necessary, but only if the underlying Layer 2 fabric can support it.
Now specifically speaking for "Layer 2 protocols" like CDP, STP, VTP, (etc...), those by design send very small sized frames that should never need to be broken up into smaller frames. So what you said is, in a way, correct, that the protocol itself handles the fragmentation (by means of not needing to send frames that would require any additional fragmentation).
Layer-2 (For eg : STP , RSTP) , who will fragment the data .
STP, RTSP, etc... are Ethernet payloads, and Ethernet does not support fragmentation.
Offhand, the only common layer2 protocol I can come up with that does support fragmentation is MLPPP; MLPPP fragmentation is commonly used to make L2 Frames small enough so voice traffic on E / T-carrier circuits doesn't see too much delay.
You can see the mlppp frame diagram at the tcpip guide. A quick glance at those frames shows that MLPPP (a layer-2 protocol) supports fragmentation at Layer-2.
Actually breaking up of PDUs takes place only at two layers:1)At transport layer called SEGMENTATION. 2)At network layer called FRAGMENTATION.
The need for fragmentation as mentioned in earlier answers are as per the restrictions upon on L-2 interfaces(E.g:for Ethernet it is 1500 bytes).This value is called MTU (Maximum Transmission Value).If Ethernet interfaces receive frames larger than this default MTU value they will drop those frames.Hence it has to be decided at L-3 level whether or not to fragment the IP datagram as per L-2 requirement.All the process of fragmentation and re-segmentation are done using multiple fields in IP HEADER. And to answer your question there is no fragmentation done at L-2 level for L-1 layer requirement.
Every interface has an MTU(MAx Transmision unit). For Ethernet it is 1500 bytes(usually). So the payload needs to be fragmanted (at Layer 3 ) so that it can fit into this max 1500 bytes. This 1500 bytes is the payload for the Layer 2.
