[j-nsp] ip fragmentation, different mtu sizes

[snort bsd]

hi, all:

i have a genetic question regarding ip fragmentation. i have two routers; one is cisco and another is juniper. they connected back to back with default ethernet mtu (cisco 1522 and juniper 1518, of course with vlan on both ends). i understand that two vendors have different ways of calculating the overhead of headers.


when i send icmp pings, without specifying packets sizes (just default values) or specifying packet sizes smaller than the values (1472 on juniper side and 1500 on cisco side), everything is fine, but anything beyond thsoe two values on both ends, i got nothing.

i thought that, for ip mtu, anything bigger than ip mtu (or juniper term protocol mtu) would be fragmented into multiple packets. 

did i miss something or my understanding isn't correct?

thanks!