YAGT - Jumbo and std frames on the same subnet

[cross6]

What happens in the case of there being mixed devices on the same subnet? I'm assuming that since there isn't a router or L3 switch to fragment that you have to rely on the gigabit hosts to do MTU discovery?

 

[spidey07]

Since there is no router PMTUD does not come into play. Just to add PMTUD uses ICMP messages and the fragment bit.

you're hosed on UDP, TCP is fine. In the intitial TCP 3-way handshake the max segment size is sent (NOT negotiated) - essentially the payload above layer4. It is sent by both hosts and the lower value is choosen by both.

 

[cross6]

Originally posted by: spidey07
Since there is no router PMTUD does not come into play. Just to add PMTUD uses ICMP messages and the fragment bit.

you're hosed on UDP, TCP is fine. In the intitial TCP 3-way handshake the max segment size is sent (NOT negotiated) - essentially the payload above layer4. It is sent by both hosts and the lower value is choosen by both.

Doh I should have known that about the handshake 😱 Been studying for Cisco certs haha.


Thanks spidey that's what I needed to know!