Juniper Configuration Hints

Juniper QFX buffering:

There is some good documentation of buffer configuration on the Juniper QFX here:


Juniper MX960 2-port 100GE MPC4E line cards vs. SCBE fabric cards:

(This showed up during SC14 and SC15 - many thanks to Dave Diller of Max Gigapop for sharing the information!)

In an MX960, if you have MPC4E cards and also have SCBE fabric cards, then you need "increased-bandwidth" mode on the fabric to leverage all three fabric cards at once to be able to hit line rate on the MPC4E 100GE ports. By default, the chassis runs the fabric in redundancy mode, which keeps the third SCBE card as a hot spare, and does not make the fabric capacity of the third SCBE available for packet forwarding. To check for this, see if drops start accumulating in the output of "show pfe statistics traffic" when load gets too high.

The configuration to place the fabric in increased-bandwidth mode is as follows: 

[edit chassis]
+   fabric {
+       redundancy-mode {
+           increased-bandwidth;
+       }
+   }

Each fabric card has 80G of capacity to provide 160G/line card, but the MPC4E has two PFEs, so each PFE only sees 40G per fabric card, and thus it peaks at 80G of fabric availability on each "half slot" in the default mode, rather than "160G to the slot to be used as needed." Running the fabric in increased-bandwidth mode provides sufficient bandwidth to support the MPC4E card.

This is not an issue with the older MPC3E line card, because it will only accept one 100GE port, and the whole line card only has one PFE. This means one PFE sees the full 80G x 2 when the fabric is in redundancy mode by default and therefore the single 100GE port is not oversubscribed toward the fabric.  It's only a problem with the newer card which has a second port and a second PFE to handle it - the newer card needs more fabric bandwidth than the default configuration provides in the presence of the older fabric cards.

If you have the latest SCBE2 fabric cards, then running the fabric in redundant mode (the third card is maintained as a hot spare) is fine as the newer fabric cards have sufficient capacity to support both 100GE ports on the MPC4E line card.

Juniper documentation links: