What is the command ? ... change the don't fragment packet on Nexus9k

kornman666 · ‎05-10-2017

Dear all

we have a problem on packet set df bit to 1 (Don't fragment) pass through n9k (customer need to change df bit and no need to change interface support jumbo frame on n9k)

On Router i can change don't fragment by route map+set ip df,

On Nexus Os i can't find command ???

Georg Pauwen · ‎05-10-2017

Hello,

as far as I recall, the Nexus switches do not support fragmentation and reassembly, which means the DF bit is always set to 1, and you cannot change that.

sbhadrav@cisco.com · ‎04-19-2018

Fragmentation occurs at L3 not a L2 (unless we deal with FR or ATM or other technologies). For sure it does not apply to ethernet.

Other important concept is that the MTU MUST be configured with the same value on each L3 link between 2 routers.

router1 1500 --------- 1500 router2 9000 ------- 9000 router3 9000 ---------- 9000 router4 1500 --------- 1500 router5

fragmentation will occur between router4 and router5 assuming that router4 gets a frame with size 9000 from router3 and will fragment in at least 6 smaller 1500 frames to be sent to router5 (this is a scholastic excercise as since our ip packet most likely is coming from router1 its size won't be larger than 1500).

if you have a MTU mismatch between 2 ends of a link you will have drops in the larger-to-smaller direction

router1 9000 ------- 1500 router2

router2 will drop all the frames bigger than 1500 (well a little bit more) coming from router1

this scenario is clearly a configuration mistake!!!

if you enable jumbo support on one link you need to be sure that on the other side of it there is a l3 device which also supports jumbo frames

L2 switches simply drop frames bigger than the port mtu.. but if this happen you clearly configured something wrong.

Say that you set a L2 port of a L2 switch to 9000, since the mtu is calculated at l3 the real frame size is the one calculated by the L3 device (host or router) which sends the frame, therefore most likely will be 1500. So there is no risk to drop anything on the switch.