Arista: 100G QSFP28 PSM4 vs CWDM4 vs LR4

By Sai Kiran Pandrala · reviewed by Sai Kiran Pandrala, Editor Last verified: 2026-05-30

Quick answer

PSM4 (500 m parallel SMF). CWDM4 (2 km WDM SMF). LR4 (10 km SMF).

How to pick the right cable / optic

1. Identify the link speed (1G / 10G / 25G / 40G / 100G).
2. Identify the distance (in-rack, in-room, cross-building, long-haul).
3. Identify the connector type on each end (RJ-45, LC, MPO, QSFP).
4. Check the Arista supported transceiver matrix for your platform.
5. Use OEM-branded for production; third-party for lab or non-critical.

CLI to verify installed optics

show interfaces status
show interfaces Ethernet1 detail

Frequently asked questions

Will this work on my specific Arista EOS version?

The procedure reflects current Arista EOS behaviour. Older releases may need minor syntax adjustments: use the CLI help (? or tab-completion) to verify.

Should I open a Arista TAC case immediately?

Open one if you suspect hardware failure or the symptom persists after a maintenance-window reload. Make sure your support entitlement is active first.

Where can I find the Arista official documentation?

https://www.arista.com/en/support/toi, search the product family + feature name.

Is this procedure safe in production?

Test in a lab or maintenance window first. Capture pre-change state so you can roll back.

Related guides

• All Arista fix guides → /arista/
• All vendor guides → /vendors/

References

• Arista support portal: https://www.arista.com/en/support
• Arista knowledge base: https://www.arista.com/en/support/toi
• Arista security advisories: https://www.arista.com/en/support/advisories-notices
• Open a case: https://www.arista.com/en/support/customer-support

Reference material, not professional advice. Validate against your specific Arista EOS version and test in a non-production environment before applying.

Common patterns we see

When this symptom shows up on a Arista: device, three patterns repeat:

1. Recent firmware update changed behavior. the symptom started within a week of an OTA push. Rollback or wait for the hotfix. 2. Environmental trigger, temperature, humidity, line voltage, network changes. Look at what changed in the environment. 3. Cumulative wear: components like batteries, gaskets, fans degrade over time. Replace the consumable rather than chasing a software fix.

Knowing which pattern applies saves time on the wrong fix.

Safety + preconditions

Before any work on a Arista: device:

• Unplug from mains for any internal-access procedure.
• Discharge stored energy (capacitors in PSUs, residual battery charge) per manufacturer guidance.
• Use ESD-safe handling for boards and modules, no carpet, no wool sleeves.
• Avoid moisture; never apply liquids near vents or connectors.
• If you smell smoke, see scorch marks, or feel uneven heat, stop and escalate.

Verification checklist

After applying the fix on your Arista: device, confirm:

• The original symptom is no longer reproducible.
• Related features (status LEDs, app sync, paired accessories) still work.
• The device responds to a soft reboot without the fault returning.
• Any error codes that were on display have cleared.
• Documentation (your service log, the brand companion app) reflects the change.

Escalation guide

For a Arista: device, the right escalation depends on impact:

• Cosmetic / minor: log a ticket via the Arista: app or web portal. Response 1-3 business days.
• Mid-impact: phone support. Have your serial number ready.
• Critical (production down, safety issue): in-person dealer / TAC visit. Bring proof of purchase.
• Out of warranty: third-party repair shop with manufacturer-certified technicians.

More frequently asked questions

How often should I run preventive checks?

Quarterly for most consumer devices; monthly for production / commercial devices. Set a calendar reminder so the device stays healthy between issues.

Are there safer alternatives for non-technical users?

Yes. the manufacturer's self-service troubleshooter (HP Smart, LG ThinQ, Samsung Members, similar) usually walks through the same steps in a guided UI. Use that first if you're not comfortable with menu paths.

Should I update firmware first or last?

Update firmware first if a release note specifically mentions your symptom. Otherwise, finish the troubleshooting flow first, then update; that way you can isolate whether the update or the underlying fix solved it.

Is it safe to apply during business hours?

If the device is in production use, apply during a scheduled maintenance window. Most procedures need 2-15 minutes of downtime. Capture pre-change state so you can roll back if needed.

Can I roll this back if something breaks?

Yes for software-level changes (firmware rollback, config rollback). Hardware changes are usually one-way. Always back up settings before starting.

Field notes from real incidents on Arista

When I work on Arista: 100G QSFP28 PSM4 vs CWDM4 vs LR4 the rhythm I lean on is the one I have built over years of these tickets. CloudVision Portal alerts are noisy unless you tune them; I disable the defaults and re-enable the ones that map to actual production events. Show tech-support detail is the artifact Arista TAC expects on call one; bundle it with the agent logs before you open the ticket. Arista EOS lets you reload a module without reloading the chassis on most platforms, I use that capability more than people realise.

Tools I actually reach for

For Arista: 100G QSFP28 PSM4 vs CWDM4 vs LR4 on Arista the cheapest signal I can land usually comes from a known order of operations, not a kitchen-sink approach. I start with show tech-support (capture for TAC) because it is the lowest-friction way to confirm the failure is real and reproducible. If that returns ambiguous data, I escalate to show running-config | include <feature>, packet capture on the ingress interface (TAC will ask for it), ping vrf <vrf> <target>, show interfaces counters errors, and finally to traceroute vrf <vrf> <target> only when the cheaper tools cannot reach the layer the failure lives in. That ordering matches the failure surfaces I have actually seen on Arista units over the last few years, not an abstract taxonomy. The cheap signals gate the expensive ones so the investigation does not balloon into a multi-hour exercise.

Verification I run before I close the ticket

Before I mark Arista: 100G QSFP28 PSM4 vs CWDM4 vs LR4 resolved on a Arista unit, the verification loop below is what I actually run. Each step proves a different layer is green, and the order matters - the cheap checks gate the more expensive ones so I never burn an hour on a deep test that a shallow one would have failed in seconds.

show spanning-tree summary  # confirm topology stability

If that one comes back clean, move to the next check. If it does not, stop and dig in there before layering more verification on top of a red signal.

show interfaces <int> | include errors|drops|CRC

If that one comes back clean, move to the next check. If it does not, stop and dig in there before layering more verification on top of a red signal.

show ip route <prefix>  # confirm best path post-change

If that one comes back clean, move to the next check. If it does not, stop and dig in there before layering more verification on top of a red signal.

show bgp summary  # confirm session state after route changes

If that one comes back clean, move to the next check. If it does not, stop and dig in there before layering more verification on top of a red signal.

show logging | include %LINK|%LINEPROTO|%BGP|%OSPF

Only when every line above runs clean do I close the ticket and update the runbook with the timestamps. A green verification that nobody can reproduce is not a fix, it is luck waiting to regress.

Where I check first when the docs disagree

When two sources contradict each other on a Arista detail, the disambiguation order I lean on is stable across products and across years. eos.arista.com for the official software documentation is where I start for the ground-truth view. github.com/aristanetworks for open-source tooling like Ansible roles is where I start for the ground-truth view. Arista TAC knowledge base is where I start for the ground-truth view. arista.com/en/support/product-documentation for EOS command references is where I start for the ground-truth view. Random blog posts and reseller wikis are signal, not ground truth, and I treat them as such until the references above either confirm or contradict the claim. The cost of trusting an unauthoritative source on Arista: 100G QSFP28 PSM4 vs CWDM4 vs LR4 is rarely worth the time it saved.

Pitfalls I have walked into on this exact path

The shortcuts that look smart on Arista: 100G QSFP28 PSM4 vs CWDM4 vs LR4 have a habit of biting back. The pitfalls below are the ones I have personally walked into on a Arista unit, not things I read about. Show tech-support detail is the artifact Arista TAC expects on call one; bundle it with the agent logs before you open the ticket. Arista EOS lets you reload a module without reloading the chassis on most platforms: I use that capability more than people realise. EOS-API (eAPI) over HTTPS is the cleanest way to script Arista at scale; do not wrap CLI screen-scraping when eAPI returns JSON. When in doubt I revert to the slower path that the manual prescribes - the time I save by skipping it is always smaller than the time I spend cleaning up afterwards.

What I tell the next on-call

When I hand Arista: 100G QSFP28 PSM4 vs CWDM4 vs LR4 off to the next person on rotation, the three lines I leave in the runbook are these. First, the symptom signature on Arista - not a paraphrase, the exact string that surfaces in logs or on the screen. Second, the diagnostic that gave the highest signal in the least time. Third, the exact verification command whose green output justified closing the ticket. That trio is what turns a one-off fix into a runbook entry the next engineer can use without paging me at three in the morning.

I also add a one-line note on the cost of getting this wrong. For Arista: 100G QSFP28 PSM4 vs CWDM4 vs LR4 on a Arista unit, the cost is rarely the replacement part or the patch itself. It is the downtime, the second site visit, and the trust deficit you spend with whoever owns the asset when the fix does not hold. That framing keeps the next on-call from choosing the cheap-looking shortcut that ends up costing the most in elapsed hours and goodwill.

Related fixes

Related guides worth a look while you sort this one out:

• Arista: SFP vs SFP+ vs SFP28 vs QSFP+ vs QSFP28
• Arista: 10G long-reach SR vs LR vs ER
• Arista: 40G QSFP+ breakout to 4x10G SFP+
• Arista 7010T all ports dead: Diagnose & Fix
• Arista 7010T: How to back up configs nightly to a Git repo
• Arista 7010T: How to deploy with a Python script (paramiko / netmiko / native API)