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Help diagnosing strange networking issue.

We have two switches connected via fibre. Both ends appear to be up and the config is correct. Port is tagged on the correct VLANs both sides.

Looking at the counters on the interface I can see broadcast traffic but no unicast traffic. Also doing a "show mac-address" doesn't show an mac-addresses on the port. Switches have been working fine for 6 months and no config change from what I can tell.

Assuming possible SFP fault or fibre patch is damaged. Any other troubleshooting steps to diagnose?

14 comments
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Start at layer 1 and work your way up... Reseat cables, change out SFPs, power cycle, then move on to more complex things.

Worth trying different SFP ports as well.

Any crc errors?

Original Poster1 point · 1 month ago

The stats on the port don't show any errors logged in the last 24 hours.

Unicast traffic, are you able to test through two machines pinging each other? If there's no MAC address shown on the MAC-TABLE you are probably experiencing port-security or filters dropping frames.

What type of switches? The command looks like Procurve maybe? Look in the logs "show logging" and see if there were any errors when you plugged them in. "show int transceiver" might show you if you possibly have the wrong SFP's in. Do the ports have link lights?

doesn't touch anything < 1Gbps
1 point · 1 month ago

Spanning Tree blocking the port?

Original Poster1 point · 1 month ago

How would we diagnose that?

Show commands should tell you what is root / forwarding

doesn't touch anything < 1Gbps
1 point · 1 month ago

Hard to say without knowing make and model of the switches. On Juniper switches you'd run "show spanning-tree interface" and look for ports in BLK state.

Original Poster1 point · 1 month ago

Ah ah. The port in question is indeed being blocked! Now how to find out why!

doesn't touch anything < 1Gbps
1 point · 1 month ago

There's another link between those two switches that's forwarding and so Spanning Tree has blocked this other link to avoid a loop. LLDP (or CDP) should give you a good starting point to working out which port goes where.

Certs? I don't need no stinking certs
1 point · 1 month ago

Assuming some "security" team hasn't disabled it...

there's a special circle of hell reserved for people who block ping and ICMP unreachable, along with those who shut off lldp/cdp on switches.

-1 points · 1 month ago

Any crc errors ?

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