MTU Troubleshooting on Cisco IOS

Hello Gareth

Remember that even though they work closely together, Ethernet and IP as standards have developed separately and independently. The definitions of their internal mechanisms are their own, so any definitions of particular features can be defined within their own layer. The only mechanisms in which they should function together is where encapsulation of the IP packet into the Ethernet frame takes place.

Now I don’t have any documentation to support this (and I invite anyone to support or refute this for the sake of constructive conversation), but my feeling is that the reason layer 2 doesn’t include the header while layer 3 does, is so that there is a common definition of the specific MTU mechanism on both layers for the purpose of encapsulation. What I mean is, by doing so, the definition of MTU measures the same thing, from the perspective of either layer. So an MTU of 1500 for IP will work perfectly for an MTU of 1500 at Ethernet. Why? Because the payload of Ethernet (which is L2 definition of MTU) is the whole IP packet including the header (which is L3 definition of MTU). So if you have 1500 for IP MTU and 1500 for Ethernet MTU, you’re good to go.

We’ve been spending time talking about the MTU for Ethernet, but remember, there are other L2 technologies with other MTU needs that are not as “in sync” as Ethernet and IP. For example, PPPoE uses 1492, IEEE802.11 Wi-Fi can be up to 2304 bytes, while Token Ring and FDDI are 4464 and 4352 respectively. (I know, I know, those technologies are rare today, but just adding it in there to make the point).

I hope this discussion has been helpful!