Function encode_frame 

pub fn encode_frame(
    destination: [u8; 6],
    source: [u8; 6],
    bpdu: &[u8],
) -> Result<Vec<u8>>

Wrap a BPDU body in an 802.3 + LLC frame, padded to the 60-octet minimum.

The length field carries the MAC client data length — LLC header plus BPDU — and deliberately does not include the padding. A configuration BPDU therefore always shows 00 26 (38) and an RST BPDU 00 27 (39).

Refuses a body that would not fit the length field as a length. Every BPDU this codec produces is 4, 35 or 36 octets, so no caller here can trip it — but this is a pub fn taking an arbitrary slice, and narrowing with a bare as u16 would let a future one write a value of 1536 or more, which IEEE 802.3 reserves for EtherType. The frame would then stop being 802.3 at all and every receiver would read it as Ethernet II, the BPDU never parsed. decode_frame already rejects exactly this in the other direction.