03-04-2017 11:07 AM - edited 03-08-2019 09:36 AM
Hi everyone,
I have a question regarding Cut through switching and IP fragmentation.
From what I read: "a cut-through switch engages in the forwarding process soon after it has examined the destination MAC (DMAC) address of an incoming frame"
So what would happen if I receive a 9000B packet on a switchport set to MTU 1500 when using Cut through switching:
1. Would the packet be forwarded? Would the switch fragment it? What if the DF bit is set?
2. Would the packet be discarded and the jumbo frame counter increased? This is what I think would happen but since the forwarding take place before the switchport finds out the packet size exceed the switchport MTU I don't know if the behavior would be different.
Thanks!
Matthieu
Solved! Go to Solution.
03-06-2017 05:55 AM
NB: in your questions, you're mixing L2 and L3. L2 doesn't care about IP fragments or the DF bit setting. L2 doesn't fragment, although L2 MTU, at a L3 interface, may cause L3 to fragment.
#1 Will the packet be forwarded? Maybe. Although you note the receiving port is configured for a MTU of 1500, much depends on whether the interface can physically process jumbo frames. If it can, I understand Cisco switches will often accept the frame, even when larger than the configured MTU. If the interface cannot physically accept the jumbo frame, it will be truncated. However, normally for egress, interface will enforce its MTU, i.e. won't transmit a too large frame. Yours in an interesting question, as cut through begins to forward before the whole frame is received. Without research further, I suspect its might be logically dealt with similar to the situation that a corrupted frame is received.
Would the switch fragment it? It shouldn't if you mean a L3 IP fragment. Likely it's sent truncated, and is seen as a corrupt frame by other devices on the same "wire".
What if DF bit is set? If DF bit were to come in play, you would need to be egressing a L3 interface and for those a new frame should be constructed. I.e. don't believe cut through applies.
#2 I'm not really sure, but again, suspect the frame might be truncated if you have two ports, for the same VLAN, on the same switch, with different sized MTUs.
03-05-2017 06:46 AM
Hi Matthieu,
Normally , As per Cisco documentation Cut-through switching mode is enabled by default. Switches operating in cut-through switching mode start forwarding the frame as soon as the switch has read the destination details in the packet header. A switch in cut-through mode forwards the data before it has completed receiving the entire frame.
The switching speed in cut-through mode is faster than the switching speed in store-and-forward switching mode.
Have a look on the operation of Cut through Switching mode...
http://thenetworksherpa.com/cut-through-corruption-and-crc-stomping/
Hope it Helps..
-GI
03-06-2017 05:43 AM
Hi Ganesh,
Thanks for your reply. I understand the scenario regarding CRC errors with Cut through switching but this does not answer the question regarding IP fragmentation when using Cut through switching.
Regards,
Matthieu
03-06-2017 05:55 AM
NB: in your questions, you're mixing L2 and L3. L2 doesn't care about IP fragments or the DF bit setting. L2 doesn't fragment, although L2 MTU, at a L3 interface, may cause L3 to fragment.
#1 Will the packet be forwarded? Maybe. Although you note the receiving port is configured for a MTU of 1500, much depends on whether the interface can physically process jumbo frames. If it can, I understand Cisco switches will often accept the frame, even when larger than the configured MTU. If the interface cannot physically accept the jumbo frame, it will be truncated. However, normally for egress, interface will enforce its MTU, i.e. won't transmit a too large frame. Yours in an interesting question, as cut through begins to forward before the whole frame is received. Without research further, I suspect its might be logically dealt with similar to the situation that a corrupted frame is received.
Would the switch fragment it? It shouldn't if you mean a L3 IP fragment. Likely it's sent truncated, and is seen as a corrupt frame by other devices on the same "wire".
What if DF bit is set? If DF bit were to come in play, you would need to be egressing a L3 interface and for those a new frame should be constructed. I.e. don't believe cut through applies.
#2 I'm not really sure, but again, suspect the frame might be truncated if you have two ports, for the same VLAN, on the same switch, with different sized MTUs.
03-06-2017 06:13 AM
Thanks Joseph - I am going to read about the difference between L2 MTU and IP MTU as you mentioned I am mixed both.
Thanks for clarifying this and for taking the time to answer this!
Regards,
Matthieu
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide