This article explains the effects of setting multicast egress shared buffer to 0 percent on QFX5K Junos platforms.

Symptoms (when interface MTU is configured for jumbo, e.g., MTU 9192):

1. Ping failures may occur for packets above a certain size (could start as low as 1900 bytes).
2. TCP MSS is negotiated based on the configured MTU value and can cause:
   1. BGP issues—failure to establish/maintain sessions due to large BGP update messages.
   2. NETCONF operations like load-override that push large configuration payloads to fail.

CPU-generated traffic utilizes buffers from the multicast pool, so this allocation cannot be set to zero. These buffers are also used by BUM (Broadcast, Unknown unicast, and Multicast) traffic. A minimum of 5% must be allocated to each buffer type: lossless, lossy, and multicast. Refer to this documentation for details. 

Configuring the multicast egress shared buffer to 0% can result in packet handling issues for traffic destined to the host. 

Lab results for reference: 
QFX5200 [DUT] 192.168.1.1/30 <-- et-0/0/0 (MTU 9192) --> 192.168.1.2/30 [REF] MX10003

Non-working configuration example: 
user@qfx5200# show class-of-service | display set
set class-of-service shared-buffer egress buffer-partition lossless percent 0
set class-of-service shared-buffer egress buffer-partition lossy percent 100
set class-of-service shared-buffer egress buffer-partition multicast percent 0

{master:0}[edit]
user@qfx5200# run ping 192.168.1.2 size 2000 do-not-fragment count 5 rapid
PING 192.168.1.2 (192.168.1.2): 2000 data bytes
.....
--- 192.168.1.2 ping statistics ---
5 packets transmitted, 0 packets received, 100% packet loss

{master:0}[edit]
user@qfx5200# rollback 1
load complete

{master:0}[edit]
user@qfx5200# show | compare
[edit class-of-service shared-buffer egress buffer-partition lossless]
-   percent 0;
+   percent 5;
[edit class-of-service shared-buffer egress buffer-partition lossy]
-   percent 100;
+   percent 90;
[edit class-of-service shared-buffer egress buffer-partition multicast]
-   percent 0;
+   percent 5;

{master:0}[edit]
user@qfx5200# commit
configuration check succeeds
commit complete

Working Configuration Example: 
user@qfx5200# show class-of-service | display set
set class-of-service shared-buffer egress buffer-partition lossless percent 5
set class-of-service shared-buffer egress buffer-partition lossy percent 90
set class-of-service shared-buffer egress buffer-partition multicast percent 5

{master:0}[edit]
user@qfx5200# run ping 192.168.1.2 size 2000 do-not-fragment count 5 rapid
PING 192.168.1.2 (192.168.1.2): 2000 data bytes
!!!!!
--- 192.168.1.2 ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max/stddev = 8.203/8.533/9.581/0.526 ms