Juniper EX4500 output discards troubleshooting.

By | 31.05.2019

Juniper EX4500 switches have lot of issues. One of them – packet buffer lack on microbursts when traffic flowing from 10G to 1G port .
This article describes Juniper EX4500 output discards troubleshooting on buffer lack.

The source of the problem is Marvel chip , which uses 4Mb buffer.
The EX4500 uses two PFE (ASIC) (0-19 ports / PFE # 1 and 20-39 ports / PFE # 2).

By default, Juniper uses a combination of dedicated and shared buffers. Dedicated buffer is allocated to each port and cannot be used by another port. If dedicated buffer is filled, then the buffer from the shared pool is allocated.

To reduce discards you should configure all buffer space as shared:

 chassis {
      fpc 0 {
         pic 0 {
 class-of-service {
     shared-buffer {
         percent 100;

With this settings enabled, the dedicated buffer is disabled completely and a larger amount is allocated to the shared pool.
This will help to improve the situation a bit, but completely, unfortunately, will not fix it.

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