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Worknotes

:bulb: A step-by-step test for verifying that newly purchased 25G NICs are compatible with an Arista switch.
Topology: Server A (25G NIC) ── Arista switch (25G port) ── Server B (25G NIC)
Key checks: link speed (25G), transceiver detection, FEC mode match, error counters, measured throughput

[00] Overall Architecture



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Server A                       Arista switch                       Server B
┌──────────────┐              ┌──────────────────┐              ┌──────────────┐
│ 25G NIC      │  SFP28 DAC   │ Ethernet1 (25G)  │  SFP28 DAC   │ 25G NIC      │
│ ens1f0       ├──────────────┤                  ├──────────────┤ ens1f0       │
│ 10.0.0.1/24  │              │ Ethernet2 (25G)  │              │ 10.0.0.2/24  │
└──────────────┘              └──────────────────┘              └──────────────┘
       │                              │                                │
   ethtool / iperf3            show interfaces             ethtool / iperf3

:warning: Most 25G link compatibility problems come from one of three things: ① transceiver/cable not detected, ② FEC mode mismatch, ③ speed auto-negotiation failure. This guide focuses on those three.

[01] Test Goals and Pass Criteria

Item Pass criteria
Link speed 25 Gbps on both ends, up
Transceiver detection Switch/server identify the SFP28 transceiver; vendor/serial shown
FEC mode Same mode on both switch and server (RS-FEC recommended), uncorrected = 0
Error counters No increase in CRC/symbol/FEC-uncorrected errors
Throughput ≥ 23 Gbps with multiple streams (accounting for overhead)
Latency Tens of µs RTT through the same switch, 0% loss

[02] Prerequisites and Information Gathering

Item Details
Server A/B OS (e.g. Ubuntu 22.04 / RHEL 9), 25G NIC model, slot (PCIe Gen3 x8+)
25G NIC e.g. Mellanox ConnectX-4 Lx / ConnectX-5, Intel E810/XXV710, Broadcom
Cable SFP28 DAC (≤3m) or AOC/optical — confirm it’s 25G (not 10G SFP+)
Switch Arista EOS with 25G-capable ports (e.g. 7050X3/7060X)
Tools ethtool, iperf3, ping, (optional) sockperf, lspci 


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# Install tools on the servers (Ubuntu)
sudo apt-get install -y ethtool iperf3 pciutils
# RHEL/Rocky
sudo dnf install -y ethtool iperf3 pciutils

[03] Physical Cabling and Interface Identification

Connect Server A’s NIC to switch Ethernet1 and Server B’s NIC to Ethernet2 with SFP28 cables. First identify the 25G interface name on each server.



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# Interface list with state
ip -br link

Sample output:



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lo               UNKNOWN        00:00:00:00:00:00
eno1             UP             a0:36:9f:xx:xx:xx
ens1f0           UP             b8:ce:f6:xx:xx:xx





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# NIC model and PCIe location
lspci | grep -i ethernet

Sample output:



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01:00.0 Ethernet controller: Mellanox Technologies MT27710 Family [ConnectX-4 Lx]

ens1f0 is the 25G interface under test. Confirm the same on both servers.

[04] Switch Side — Link/Speed Check

From the Arista console (or SSH), check port status.



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sw-core-01# show interfaces Ethernet1 status

Sample output:



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Port       Name   Status       Vlan     Duplex Speed  Type            Flags
Et1               connected    1        full   25G    25GBASE-CR
Field Meaning
Status: connected Physical link up (notconnect = not connected / negotiation failed)
Speed: 25G Negotiated at 25 Gbps (dropping to 10G = cable/port config issue)
Type: 25GBASE-CR DAC copper. Optical would be 25GBASE-SR, etc.

To see multiple ports at once:



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sw-core-01# show interfaces status | include 25G

Confirm the port is “configured” for 25G (negotiated ≠ configured):

The Speed in show interfaces ... status is the negotiated result. What speed is actually configured on the port has to be checked separately in the running-config. In environments where auto-negotiation is flaky, it’s safer to force the port to 25G.



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sw-core-01# show running-config interfaces Ethernet1

Sample output (default — auto-negotiation):



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interface Ethernet1
   ! no speed line means auto (auto-negotiation)

Sample output (forced 25G):



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interface Ethernet1
   speed forced 25gfull

To see it alongside the current operating speed:



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sw-core-01# show interfaces Ethernet1 status | include 25G
sw-core-01# show interfaces Ethernet1 | include -i -E 'Speed|BW|bandwidth'
Check Meaning
No speed line in running-config Auto-negotiation — negotiates to 25G if the transceiver is 25G
speed forced 25gfull Forced to 25G (use when auto-neg fails / platform compatibility issues)
Speed is 25G in status Actually linked up at 25G

Force 25G (if needed):



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sw-core-01(config)# interface Ethernet1
sw-core-01(config-if-Et1)# speed forced 25gfull

:bulb: On some platforms a port belongs to a 40G/100G breakout group (e.g. 4x10G / 4x25G) and won’t come up as a standalone 25G port. In that case you must first split the port group into 25G breakout mode so that 25G subports like Ethernet1/1 appear.

[05] Switch Side — Transceiver Detection (Compatibility Core)

Check that the switch correctly identifies the new transceiver/cable. Arista may flag unsupported/third-party transceivers.



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sw-core-01# show interfaces Ethernet1 transceiver

Sample output:



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Port    Vendor          Part Number    Serial Number    Type
Et1     Arista Networks CAB-D-D-25G-3M  XYZ123456789    25GBASE-CR

Detailed diagnostics (optical power for optics):



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sw-core-01# show interfaces Ethernet1 transceiver detail
Check Meaning
Vendor/Part/Serial shown Transceiver EEPROM read correctly
Type is 25GBASE-* 25G-specific cable (10G shown = wrong cable)
Rx Power in range (optical) Optical signal level healthy

:warning: On Arista, third-party transceivers may be blocked by default policy. If needed, check rejection logs with show logging | include transceiver and confirm the model is policy-allowed.

[06] FEC Mode Check and Match (Most Important)

At 25G, if the FEC (Forward Error Correction) mode differs between the two ends, the link may not come up — or come up but accumulate errors. This is the core step of new-card compatibility verification.

  • RS-FEC (CL91, Reed-Solomon): generally required for 25GBASE-CR (DAC > 3m) and SR optics
  • FC-FEC (CL74, Fire-code / BASE-R): usable on short DACs
  • None (disabled): very short passive cables only

Switch-side FEC check:



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sw-core-01# show interfaces Ethernet1 phy detail | include -i fec

Or from the interface config:



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sw-core-01# show running-config interfaces Ethernet1

Sample output:



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interface Ethernet1
   error-correction encoding reed-solomon

Switch-side FEC config (if needed):



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sw-core-01(config)# interface Ethernet1
sw-core-01(config-if-Et1)# error-correction encoding reed-solomon

Options: reed-solomon (RS-FEC), fire-code (FC-FEC), disabled. Match the server side exactly.

Server-side FEC check:



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ethtool --show-fec ens1f0

Sample output:



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FEC parameters for ens1f0:
Configured FEC encodings: Auto
Active FEC encoding: RS
Field Meaning
Active FEC encoding: RS RS-FEC active now (must match the switch)
Auto Auto-negotiated. If one side is fixed, fix both

Force server-side FEC (on mismatch):



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# Fix to RS-FEC
sudo ethtool --set-fec ens1f0 encoding rs
# Options: rs | baser (= FC-FEC) | off | auto

:bulb: Fixing both ends to the same mode is the most stable. Mixing one auto and one rs can cause intermittent link flaps.

[07] Server Side — Link/Speed Check



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ethtool ens1f0

Sample output (excerpt):



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Settings for ens1f0:
        Supported link modes:   25000baseCR/Full
        Speed: 25000Mb/s
        Duplex: Full
        Auto-negotiation: on
        Link detected: yes
Field Meaning
Speed: 25000Mb/s 25 Gbps link OK
Link detected: yes Physical link up
Auto-negotiation: on Auto-neg in use (policy must match both ends)

Both servers (A/B) must show 25000Mb/s, Link detected: yes.

[08] Server Side — Driver/Firmware Check

For a new card, driver/firmware versions directly affect compatibility.



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ethtool -i ens1f0

Sample output:



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driver: mlx5_core
version: 5.15.0-xx
firmware-version: 14.32.1010 (MT_0000000080)
bus-info: 0000:01:00.0
Field Meaning
driver Kernel NIC driver (e.g. mlx5_core, ice, i40e, bnxt_en)
firmware-version NIC firmware — confirm it’s the vendor-recommended latest

Old firmware can carry 25G/FEC negotiation bugs, so cross-check vendor release notes.

[09] PCIe Bandwidth Check (Bottleneck)

To push 25G (≈3.125 GB/s) you need at least PCIe Gen3 x8.



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sudo lspci -vv -s 01:00.0 | grep -i -E 'LnkCap|LnkSta'

Sample output:



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LnkCap: ... Speed 8GT/s, Width x8
LnkSta: ... Speed 8GT/s, Width x8
Check Meaning
LnkSta Width x8 Slot operating at x8 (dropping to x4 throttles throughput)
Speed 8GT/s (Gen3)+ Can carry 25G line rate

[10] IP Setup and Basic Connectivity



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# Server A
sudo ip addr add 10.0.0.1/24 dev ens1f0
sudo ip link set ens1f0 up
# Server B
sudo ip addr add 10.0.0.2/24 dev ens1f0
sudo ip link set ens1f0 up

Connectivity check:



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# From A to B
ping -c 4 10.0.0.2

Sample output:



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64 bytes from 10.0.0.2: icmp_seq=1 ttl=64 time=0.045 ms
...
4 packets transmitted, 4 received, 0% packet loss

0% packet loss with µs-level RTT means the L2/L3 path is healthy.

[11] MTU / Jumbo Frame Setup and Verification

Before high-throughput tests, set Jumbo Frames (9000) to lower CPU load and raise throughput. Both servers and the switch port/VLAN must use the same MTU.



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# Both servers
sudo ip link set ens1f0 mtu 9000





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# Switch (Arista) — port L2 MTU
sw-core-01(config)# interface Ethernet1
sw-core-01(config-if-Et1)# l2 mtu 9214

Verify the jumbo path (9000B without fragmentation):



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ping -M do -s 8972 -c 4 10.0.0.2
Result Meaning
Success (0% loss) 9000 MTU path OK
Message too long / 100% loss MTU mismatch somewhere → recheck switch/server MTU

[12] Throughput Test — iperf3

A single TCP stream may not saturate 25G due to single-core limits. Use multiple streams (-P) to approach line rate.

Server B (receiver / server role):



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iperf3 -s

Server A (sender / client):



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# 8 parallel streams, 30 seconds
iperf3 -c 10.0.0.2 -P 8 -t 30

Sample output (excerpt):



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[SUM]   0.00-30.00  sec  82.1 GBytes  23.5 Gbits/sec   0   sender
[SUM]   0.00-30.00  sec  82.0 GBytes  23.5 Gbits/sec       receiver
Field Meaning
23.5 Gbits/sec Measured throughput — ~94% of 25G, normal given TCP/header overhead
Retr (retransmits) 0 or small Closer to 0 is better. A spike suggests FEC/congestion/buffer issues

Test the reverse direction (B→A) too:



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iperf3 -c 10.0.0.2 -P 8 -t 30 -R

If both directions exceed 23 Gbps, compatibility/performance pass. If only one direction is low, inspect that sender’s NIC/cable.

To see line-rate loss with UDP:



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iperf3 -c 10.0.0.2 -u -b 25G -t 20

Lost/Total Datagrams should be near 0%.

[13] End-to-End 25G Verification (PC A → Switch → PC B)

So far each segment (Server A, switch port, Server B) was checked individually. The final step confirms, end-to-end, that when PC A sends at 25G, PC B receives at the same 25G. If this test passes, it proves in one shot that every element on the path (Server A NIC → switch forwarding → Server B NIC) satisfies 25G.

Decision logic:

Observation Conclusion
PC A sends ≈ 25G and PC B receives ≈ 25G TX NIC · switch forwarding · RX NIC all OK at 25G
PC A sends 25G, PC B receives < 25G Switch queue drops or RX-side NIC/PCIe bottleneck
PC A can’t even send at 25G TX-side NIC/cable/port negotiation problem

Key idea: if the line rate injected at one end comes out at the other end intact, it simultaneously proves the switch in the middle and both NICs all handle 25G.

1) Measure the sender — PC A:



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# PC A → PC B, 8 streams to approach line rate
iperf3 -c 10.0.0.2 -P 8 -t 30

2) Measure the receiver — PC B (at the same time):

In PC B’s iperf3 -s output, check that the receiver line’s Gbits/sec matches PC A’s send value. To watch the live receive rate on the interface separately:



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# On PC B: per-second received bytes → receive line rate
sar -n DEV 1
# or
ifstat -i ens1f0 1

Interpreting the output:



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# PC A (sender) iperf3
[SUM]   0.00-30.00  sec  82.1 GBytes  23.5 Gbits/sec        sender
# PC B (receiver) iperf3 server
[SUM]   0.00-30.00  sec  82.0 GBytes  23.5 Gbits/sec        receiver
# PC B (sar -n DEV)
ens1f0  ...  rxkB/s ≈ 2.9e6   (≈ 23.5 Gbps received)
Check Meaning
Send ≈ Receive ≈ 23 Gbps+ End-to-end 25G verification passed — switch + both NICs all pass
Receive far below send Switch drops / RX NIC bottleneck → check show interfaces Et2 counters, ethtool -S for drops

3) Bidirectional (full-duplex) check:

25G is full-duplex, so each direction should sustain 25G simultaneously.



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# PC A: simultaneous TX + RX load
iperf3 -c 10.0.0.2 -P 8 -t 30 --bidir

Sample output (excerpt):



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[SUM][TX]  0.00-30.00  sec  81.9 GBytes  23.4 Gbits/sec        sender
[SUM][RX]  0.00-30.00  sec  81.8 GBytes  23.4 Gbits/sec        receiver

If both directions sustain ~23 Gbps, full-duplex 25G is verified too. This empirically confirms that “traffic PC A sent at 25G is received by PC B at 25G,” so you can conclude that both the switch and both PCs’ NICs satisfy 25G.

[14] Latency Test



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# RTT distribution at a fast interval
sudo ping -i 0.2 -c 50 10.0.0.2

Sample output:



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rtt min/avg/max/mdev = 0.038/0.046/0.071/0.008 ms
Metric Meaning
avg tens of µs Normal through the same switch
Small mdev (jitter) Stable. Large spikes suggest FEC retries/buffer issues

[15] Error Counter Check (Before/After Comparison)

Snapshot counters before and after the throughput test and look at the delta. Compatibility problems show up here.

Server side:



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ethtool -S ens1f0 | grep -i -E 'err|drop|fec|crc'

Sample output:



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     rx_crc_errors_phy: 0
     rx_corrected_bits_phy: 1024
     rx_symbol_err_phy: 0
     fec_uncorrectable_blocks: 0
Counter Meaning
rx_crc_errors_phy CRC errors — must not increase (cable/FEC issue)
fec_corrected_* Bits FEC corrected — small increase is normal (evidence FEC is working)
fec_uncorrectable_* Correction failures — must be 0. Increase = link quality / FEC mismatch
rx_symbol_err_phy Symbol errors — 0 preferred

Switch side:



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sw-core-01# show interfaces Ethernet1 counters errors

Sample output:



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Port    FCS Err  Align Err  Symbol Err  Rx Err  Tx Err
Et1     0        0          0           0       0





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# FEC stats (corrected/uncorrected codewords)
sw-core-01# show interfaces Ethernet1 phy detail | include -i -A2 fec
Check Meaning
FCS/Symbol Err stay 0 Link quality good
FEC uncorrected = 0 Required pass condition

[16] Compatibility Troubleshooting

Symptom Cause / action
Link won’t come up (notconnect / Link detected: no) FEC mismatch → fix both switch and server to the same mode (RS-FEC). Confirm the cable is 25G SFP28
Negotiates at 10G, not 25G Cable is 10G SFP+ or a forced port speed → set speed forced 25gfull on the switch or align auto-neg
Transceiver not detected / rejection logs Third-party block policy → check show logging | include transceiver, verify allowed model/policy
Link flaps (intermittent drops) One side auto, the other fixed → fix both the same. Consider firmware update
Throughput stuck at ~10G PCIe x4 downgrade (check LnkSta), single-stream limit → use -P multi-stream, tune IRQ/RSS
FEC corrected surges + uncorrected appears Cable quality/length limit → replace cable, apply stronger FEC (RS)

[17] Summary Checklist

Step Where Check
STEP 04 Switch show interfaces Et1 status → connected, 25G
STEP 04 Switch show running-config interfaces Et1 → port configured for 25G (auto / forced 25gfull)
STEP 05 Switch show interfaces Et1 transceiver → transceiver detected
STEP 06 Switch + server FEC mode identical on both (RS recommended)
STEP 07 Server ethtool ens1f0 → 25000Mb/s, Link: yes
STEP 08 Server ethtool -i → latest driver/firmware
STEP 09 Server lspci → PCIe x8 / Gen3+
STEP 11 Both MTU 9000 matched, ping -M do -s 8972 succeeds
STEP 12 Server iperf3 -P 8 → ≥ 23 Gbps both directions
STEP 13 Both End-to-end: PC A send ≈ PC B receive ≈ 23 Gbps+ (incl. --bidir full-duplex)
STEP 15 Both CRC/FEC uncorrected delta 0

When all items pass, consider the compatibility and performance between the new 25G NIC and the Arista switch verified. In particular, when STEP 13 (end-to-end) shows PC B receiving the 25G that PC A sent, it proves in one shot that the switch and both PCs’ NICs all satisfy 25G.

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