4GB of RAM on a Juniper J-Series router (J6350)

Officially the Juniper J-series routers such as the J6350 only supports a maximum 2GB of RAM. The J6350 comes with 1GB of RAM installed and if you want to add more then you have to buy your RAM from Juniper at inflated prices, however as the J-series are standard x86 machines with Netburst era Celeron and Pentium 4 processors you can simply install standard DDR400 RAM if you aren’t worried about being covered by JTAC support.

Not only does installing your own RAM save you a shedload of money compared to the Juniper equivalent part (JXX50-MEM-512M-S), but you can also exceed the “supported” 2GB maximum 4x512MB configuration, which provides additional memory to the control plane. The four memory slots present on a J6350 can take a 1GB DIMM each, however thanks to a 32bit architecture and mapping of the PCI bus into the same address space, you can only actually use 3.5GB of that 4GB.

The J-series routers are picky about what RAM they use however, so I like to stick to Crucial sticks; partly as that’s what Juniper use but mostly because I use Crucial elsewhere for their great customer service and a genuinely quality product. If you want to stick with a 2GB maximum then you can use Crucial part number CT2KIT6464Z40B, which provides 2x512MB sticks. Alternatively, Crucial part number CT2KIT12864Z40B provides 2x1GB sticks, allowing you to max the chassis out at 4GB across all four slots.

Once you’ve got all 4GB of memory installed in the chasis, boot the router up and from the JUNOS operational mode CLI issue the command “show chassis routing-engine” – you should see a total of 3584MB of memory split between 3008MB of control plane memory and 576MB of data plane memory. If you only opted for 2GB of memory then the 2048MB total will be split between 1472MB of control plane memory and 576MB of data plane memory.

Remember, you replace the RAM in your routers at your own risk and doing so will likely void your warranty as well as rending your system unspported by JTAC.