On 2021-03-05 05:35, James Dougherty wrote:
Technically, the MTU should be 1496 of you use VLANs, or else it's
actually already a jumbo frame. The total size according to the ethernet
spec is really 1518. Larger are not allowed. VLAN information is
actually a part of the payload.
Yes, 1518 is the size, true. But VLANS are not part of the payload. An
untagged frame which enters a VLAN
switch will have the 802.1Q tag inserted after the DA/SA. When that happens
the CRC has to be recomputed
so therefore *it is not* just part of the payload (e.g. like a UDP
datagram). Larger are allowed, we typically use 2K
sizes for additional link layer encapsulation (most of the switching
hardware today does at least).
The CRC covers the whole packet, so of course the CRC have to be
recomputed if you add a VLAN tag.
The VLAN tags are not added after the DA/SA, but actually after the
length field.
When you add VLANs, you normally no longer use ethernet, but instead
802.3. In which this is not considered a part of the payload, I know.
But the controller receiving the packet might have no clue about this.
It's all at a higher level of protocol handling. It's nice that 802.3
have defined so that larger frames are allowed. Older ethernet
controllers will still drop such packets because they are over 1500
bytes of payload. Even if they understand 802.3. And even if software
would understand VLANs.
Not that the thing you quoted states "MAC Client Data Field". And that
is 1500 (the normal standard), 1504 with Q-tag, and so on. That's the
MAC Client *Data* field. It's part of the data.
But I suspect you know this. :-)
I feel I should stop now anyway. Feel free to mail me in private though.
But I don't want to pollute the mailbox of everyone with my nitpickings.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
