Either in a new deployment or after an environment migration from other switch platform to another Juniper QFX or EX, devices may initially pass connectivity validation (such as small ping tests). However, as production traffic scales
for a Storage Area Network (SAN/NAS) server(s), connectivity across the Layer 2 domain becomes highly intermittent or fails entirely.
This could be symptom of an MTU mismatch between Storage networks (SAN/NAS) and Juniper switches .These specialized storage environments are commonly configured to use Jumbo frames to maximize throughput and reduce CPU overhead on the storage controllers. I
If the newly migrated or entirely new switch ports are left at default settings, they will drop this heavy storage traffic.
If you are experiencing this issue, you will observe the following symptoms:
Initial Success: Basic network validation (pinging with default packet sizes) succeeds because small control/handshake packets pass through seamlessly.
Oversized Frame Errors: The switch port connected to the SAN server or storage shows an increasing count of input errors specifically categorized as "Oversized frames.
Silent Drops: There are no Spanning Tree Protocol (STP) topology changes and no relevant error logs generated in syslog regarding the affected storage VLAN
Failure Under Load: Network access drops packets heavily or fail completely as soon as hosts are attempt to be added and to initiate large data transfers, while initial validation of connectivity are successful,
as more devices are added or migrated, connectivity across VLANs became increasingly intermittent.
Taking xe-0/0/0 as an example name , display he interface info "show interfaces xe-0/0/0" look for interface errors and oversized frames
MTU: 1514 Input errors: Errors: 30602, Drops: 0, Framing errors: 0, Runts: 0, Policed drops: 0, Resource errors: 0 Oversized frames: 30602
If the Oversized frames count matches or closely mirrors the Input errors count, the switch is likely dropping packets because the connected storage device is sending Jumbo Frames that exceed the switch port's default limit.
Because the switch port cannot accept frames larger than 1514 bytes, the hardware classifies the storage payloads as invalid and silently drops them at Layer 2.
.
Confirm the MTU needs to match the hosts and SAN/NAS, for instance 9000.
Set the MTU to 9216 (Juniper's standard Jumbo Frame setting to accommodate a 9000 payload plus headers)
#set interfaces xe-0/0/0 mtu 9216
#commit
clear interfaces statistics xe-0/0/0
show interfaces xe-0/0/0 extensive