9000 is the standard "customer" jumbo IP MTU for reasons outlined in the article but ISP backbone links are necessarily larger than that to accommodate other he

9000 is the standard "customer" jumbo IP MTU for reasons outlined in the article but ISP backbone links are necessarily larger than that to accommodate other headers, typically MPLS and/or Vxlan. Ipsec also has layer3/4 overhead but that is usually used by the customer not the ISP in their infrastructure. In addition to the IP MTU you also have layer2 headers, almost always Ethernet these days. Ethernet can optionally have one or more 802.1Q headers, among other things. Networks that use only one vendor sometimes just configure their internal links at the maximum supported by that vendor/platform. In multi-vendor networks this is problematic because each vendor has different maximums. In that case it is best to set an internal standard like 9100 IP MTU that is within every vendor's limits and leaves plenty of room for all the overhead (layer 2 and 3) listed above. It's important to note that some protocols like OSPF require all participants on a segment to have the exact same IP MTU configuration. EDIT: clarified 9000 is the standard customer jumbo IP MTU size, not standard customer IP MTU size which is of course 1500.