MTU Troubleshooting on Cisco IOS

(Lazaros Agapides) #122

Hello John

I think the important thing here to note is that all of the MTU configs affect encapsulation. So if there is a segment with a size larger than the IP MTU, then that segment will be fragmented into multiple IP packets during encapsulation from L4 to L3. This would actually occur at the host that is sending the data. (routers do not de-encapsulate PDUs to layer 4 unless they are functioning as deep inspection firewalls, but I digress).

If a packet comes in to a router and is deencapsulated, its destination IP address is checked, and the exit interface is determined, then the packet will be reencapsulated from L3 to L2. It is at this point that the IP MTU and the Interface MTU are examined. If the interface MTU is too small for the size of the packet, it will be fragmented and encapsulated into two frames unless the DF bit is set. I don’t know of any other scenario in which fragmentation will not occur when the DF bit is not set.

I hope this has been helpful!


(Dominique R) #124

Hi Rene and staff,
this is a very good lesson for me because, before reading, this was a little confused in my memory: i don’t troubleshoot that type of issues every day
i am not sure that someone pointed out what follows because the forum is very long
With Ethernet, L2 does not fragment (it drops), L3 fragments when the ip mtu limit is reached
(i know one example of L2 fragmentation as PPP multilink)
So, in the lesson, Rene wants to demonstrate it in this way:
You decrease the L2 MTU with “mtu 1400” , this will let L3 MTU to 1500: so when you send an HTTP packet of 1500, L3 does not fragment and L2 drop: the things go wrong
But when i tried this in a lab, i watched that when L2 MTU is set to 1400 (or any possible value), L3 MTU is automaticaly set to the same value. And when you think about it, this is a logic behavior: why prevent L3 to fragment staying at 1500 while L2 is dropping at 1400 ?
So i think the lesson was made with old routers ? (i use CiscoIOSv15.6(1)T-1 in GNS3)

(Lazaros Agapides) #125

Hello Dominique

Keep in mind that when you change the MTU value with the mtu interface configuration command, the IP MTU value will also change to match. However, the opposite is not true, that is, if you change the IP MTU, this does not change the MTU of the interface. So it is possible to have a different value of the MTU of the interface and the IP MTU.

I hope this has been helpful!