Planning to replace my existing iSCSI switches with two 3750E switches. My iSCSN SAN supports jumbo frame (9000 bytes) and so does these two 3750E. I will turn on jumbo frame (9000 bytes) on all ports (except the FE) and two ports that will be configured as routed ports. These two routed ports will use the default 1500 bytes. Questions:
(1) Some of my hosts on these switches will use jumbo frame and some will not. For those that cannot run jumbo frame, will the switches negotiate the MTU back down to the default 1500 byes?
(2) My iSCSI SAn will utilitize the routed ports to replicate itself to another SAN. At the other end of the routed ports, the MTU is 1500 so they do match. Now since the iSCSI SAN will be at 9000 bytes and the routed ports are at 1500 bytes, what will happen? Will I see lots of dropped packets or is this workable at all?
All depends on how big frames are sent by hosts. If they send frames above 1500 bytes they will be dropped on the 1500 bytes MTU links. If you really want to use jumbo then you have to enable it through the whole path the frames will take.
Thank you for the input. Maybe I need to ask in this way. On the 3750E, packets come in at 9000 bytes from one of the ports, and destine to go out a routed port at 1500, will the switch frament the packets and then transmit them?
I think in that case the fragment will be dropped. If it comes as a 9000 byte fragment (not packet) the switch just change the MAC addresses (if it goes out an L3 port, as the router does) but will not fragment it. Another thing if the host tries to send a 9000 bytes IP packet through an 1500 bytes MTU interface. In that case the packet (and not the fragment) will be fragmented. I hope it make sense.
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