For a pilot phase I've set up a new Juniper infrastructure including two mixed VC (2x4500, 2x4200) as core switches and two access switches (EX2200, Cisco 3560). We fixed the MTU to 1500 on all Uplinks and AEs because the customer wants to do a 1:1 replacement.
When we connected the mentioned setup to the existing Cisco core switch (6500 trunk port) everything was working fine with small sized frames. When I was connected on the Juniper access switch, I was able to ping something routed on the Cisco core with the maximum frame size of 1450 bytes.
On the existing infrastructure pings are working upto 65535 bytes. The are fragmented of course. But why does this not work through the Juniper infrastructure? I guess the EX devices are able to fragment the traffic automatically. Is there a mismatch between the Cisco MTU 1500 and the Juniper MTU 1500?