Based on the provided interface output and your description, the occasional reception of two jumbo packets is not related to the optical circulator or transceiver (Layer 1). Here's a detailed explanation:
1. Jumbo Packets are a Layer 2 Issue
·Jumbo packets (or jumbo frames) are Ethernet frames exceeding the standard MTU size of 1,500 bytes (typically up to 9,000 bytes). They are Layer 2 constructs (frames), not Layer 3 packets, as they involve frame size violations rather than IP packet issues.
The show interface output shows:
·2 jumbo packets: Frames larger than the configured MTU (1,500 bytes in your output) but within the system's jumbo MTU limit (if enabled).
·2 input errors and 2 giants: These counters align with jumbo frames being discarded as errors due to MTU mismatch or misconfiguration.
2. Optical Circulator is a Passive Component
·The circulator is a passive optical device that directs light unidirectionally (e.g., Port 1 → Port 2, Port 2 → Port 3) but cannot generate, modify, or interpret data frames. It operates purely at the physical layer (Light/Layer 1) and has no awareness of frame sizes or content.
·Similarly, the transceiver (e.g., 400G QSFP-DD) converts electrical signals to optical signals and vice versa but does not create or alter frame sizes. Issues like jumbo frames originate from higher layers.
3. Likely Causes (Layer 2 or Configuration)
·MTU Mismatch: A connected device (e.g., server, storage array) might be sending frames larger than the interface's MTU (1,500 bytes). This is common in environments using jumbo frames for high-throughput applications like storage (iSCSI/NFS) or data backups.
·Misconfigured Jumbo Frame Support: If jumbo frames are partially enabled (e.g., system jumbo MTU set but not applied consistently), oversized frames may be counted as jumbo packets and dropped.
·Software/Hardware Anomaly: Rarely, a brief glitch in NIC drivers, switching ASICs, or FEC (Forward Error Correction) processing could cause frame reassembly errors, leading to oversized frames. However, this is less common than configuration issues.
4. Why It’s Not Random Layer 3
Layer 3 (IP packets) issues would manifest as routing errors, TTL expires, or ACL drops, not jumbo packet counters. The term "packets" in the output is a misnomer; Cisco uses it interchangeably for frames in this context.
Recommended Actions:
1.Check Connected Devices: Verify the MTU configuration on devices linked to this interface. Ensure they are not sending frames >1,500 bytes unless jumbo frames are explicitly configured end-to-end.
2.Validate Jumbo Frame Settings: Use show system mtu to confirm the global jumbo MTU size. If needed, uniformly enable jumbo frames across all relevant interfaces.
3.Monitor Traffic Sources: Use packet captures or show counters to identify the source of oversized frames if the issue persists.
Conclusion: This is a Layer 2/configuration issue, not a Layer 1 hardware fault. The circulator and transceiver are innocent; focus on MTU consistency and endpoint configurations.
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Rockyw | If it solves your problem, please mark as answer. Thanks !
