[j-nsp] [c-nsp] ip fragmentation, different mtu sizes

Amjad Ul Hasnain Qasmi zhqasmi at cyber.net.pk
Tue Dec 17 23:25:39 EST 2013

can you paste the output of "show ip interface <interface>" from cisco and
show interface <interface> from Juniper.

Amjad Ul Hasnain Qasmi

-----Original Message-----
From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
snort bsd
Sent: Wednesday, December 18, 2013 6:03 AM
To: juniper-nsp; cisco-nsp
Subject: [c-nsp] ip fragmentation, different mtu sizes

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?

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