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First encounter with IP Fragmentation and MTU problems

About a week ago, I was called to  a client to investigate issues they were having as it pertains to browsing the internet. To my surprise when I arrived on site and tested the sites they were having issues with, I was able to browse these sites just fine. At first I was told that there were only 2 users having the problem so I decided to check these two laptops to ascertain whether there was some virus on the system that was causing the problem. This of course proved to not be the case, as both laptops were fully protected and virus definitions were up to date. I did some further investigations on the systems, only to discover that all users on the network were experiencing the problem. This puzzled me even more. To prove whether is was a problem with the internet connection, one of the users informed me that he had connected to a near by wifi connection and was able to browse these same sites just fine. This meant that the problem had to lie within the network.

Upon doing some initial research off site on what possible causes for the problem could be, I was advised that it could be a problem with MTU. Given that I never experienced this issue before, I had to dig deeper to get further details. The customer was using Cisco equipment for their switching infrastructure and Juniper for their firewall. I proceeded to take a ASA 5505 on my next visit to the client and swapped out the juniper to test. The tests were successful. All users were able to browse the websites they were having issues with. At this stage I figured we must have had a defective Junos box on our hands so I swapped it out for a replacement that was already on site. Alright, so now we’ve got a brand new spare Juniper SRX in to replace the old one. All should be fine now, right? Wrong! Users began to experience the same issue again. This of course boggled my mind even further. I mean, the internet worked fine with the ASA. I swapped out the old Junos for a replacement, and we’re back with the same problem? What could be the cause?

Back at the office, I was digging deeper, trying to learn as much as I can about the symptoms that can arise whenever you experience MTU issues. Cisco had a great article about this, which totally went in depth  about the causes and possible solutions to the problem. If you’d like to read it for  yourself, you can have a look here. I am by no means familiar with Juniper configurations and the Junos box was managed by another team in Holland. Given how technology works, and how all vendors have basically their own way of achieving the same results, I knew that Juniper must have had an option similar to Cisco’s “ip tcp adjust-mss” command and after some research, I came across this Knowlege Base article about how to achieve the same result in Junos. Working along with the team in Holland, I instructed them of the changes to make and also provided the link to the KB. Users were able to browse all sites just fine once again after the changes were made to the Junos box.

For those of you who are wondering how I was able to browse just fine on the network before the Junos box was configured  and other users weren’t; my laptop had the Cisco VPN Client software installed. This application usually reduces your MTU to 1300. By default Windows uses a MTU of 1500, so if somewhere along the path to the destination, a device was configured for a smaller mtu you could experience the problems as mentioned above.

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