Managed switch - selecting jumbo frame size?
From my understanding, in order to get jumbo frames to work correctly, the same value has to be set on all devices that support it.
I've set my PC and NAS to 7K. My current switch supports up to 9K but as its unmanaged, I have no control or web interface.
I've been looking at managed switches and it seems that jumbo frames can only be enabled or disabled through the web management. I'm confused as to how a jumbo framed network can function correctly if the switch isn't set to a matching size.
Do the switches automatically detect the frame size on the network and switch accordingly?
jumbo frames are Ethernet frames with more than 1500 bytes of payload. Conventionally, jumbo frames can carry up to 9000 bytes of payload, but variations exist and some care must be taken when using the term. Many Gigabit Ethernet switches and Gigabit Ethernet network interface cards support jumbo frames. Some Fast Ethernet switches and Fast Ethernet network interface cards also support jumbo frames
Ethernet has used 1500 byte frame sizes since it was created (around 1980). To maintain backward compatibility, 100 Mbps ethernet used the same size, and today "standard" gigabit ethernet is also using 1500 byte frames. This is so a packet to/from any combination of 10/100/1000 Mbps ethernet devices can be handled without any layer two fragmentation or reassembly.
"Jumbo frames" extends ethernet to 9000 bytes. Why 9000? First because ethernet uses a 32 bit CRC that loses its effectiveness above about 12000 bytes. And secondly, 9000 was large enough to carry an 8 KB application datagram (e.g. NFS) plus packet header overhead. Is 9000 bytes enough? It's a lot better than 1500, but for pure performance reasons there is little reason to stop there. At 64 KB we reach the limit of an IPv4 datagram, while IPv6 allows for packets up to 4 GB in size. For ethernet however, the 32 bit CRC limit is hard to change, so don't expect to see ethernet frame sizes above 9000 bytes anytime soon.
Two basic approaches exist:
On a port by port basis, where everything "downstream" from a given port is known to support jumbo frames.
Using 802.1q Virtual LANs, where jumbo frame and non-jumbo frame devices are segregated to different VLANs.