As we know IS-IS is layer-2 routing protocol, so it doesn't have native support for fragmentation right! So IS-IS use Padding to maximum MTU size to find out the MTU requirement on the link! now again using Padding Router will come to know the connected link MTU! what if we have scenario like: R1---R2---R3, now how R1 will come to know the total path MTU from R1 to R3 if we have different MTU between R1 & R2 as well as R2 & R3? so in short how Fragmentation will work for the IS-IS update packet?