Do MTU settings have to match?
You must ensure your MTU matches on both sides of an ethernet link; you should not have mismatched mtu values anywhere in the same ethernet broadcast domain. Note that the ethernet headers are not included in the MTU you are setting. Large incoming packets may be dropped based on the interface MTU size.
How do I find my MTU Mismatch?
One method to test and detect a reduced MTU size is to use a ping with a large packet size. Here are some examples of how to do this. On a Windows host you can also set the Do Not Fragment (DF) bit to 1 with the “-f” ping parameter. IPv4 routers fragment on behalf of the source node that is sending a larger packet.
Who decides MTU?
The maximum transfer unit (MTU) specifies the maximum transmission size of an interface. A different MTU value may be specified for each interface that TCP/IP uses. The MTU is usually determined by negotiating with the lower-level driver. However, this value may be overridden.
When do LACP MTUs need to be matched?
This isn’t critical for TCP services because of TCP MSS negotiation, but UDP services need matched MTUs if they send large packets. On other switch to switch / switch to router links, the data-link MTU must be at least large enough for your server MTUs + any other services added by your routers (such as IPSec, MPLS, etc…).
When do data link MTUs need to match?
On other switch to switch / switch to router links, the data-link MTU must be at least large enough for your server MTUs + any other services added by your routers (such as IPSec, MPLS, etc…). By way of example, if your transit switched MTUs are not large enough it can cause strange issues getting network file systems to work correctly.
How are IP MTUs and layer2 MTUs related?
IP MTUs are directly correlated to Layer2 MTUs (at least for Case 1, above). No matter what we do, we are always at the mercy of mitigating the problems from unintentional Layer2 MTU mismatches, since there is no Layer2 MTU discovery mechanism (unlike IP, which has ICMP error messages).
Do you have to match the MTUs of your router?
On router to router links The routers’ data-link MTUs should match your other routers, or you could wind up with strange IGP peering problems. The IP MTU on routed interfaces should be the same size as your server’s IP MTUs that must transit them.