this post was submitted on 19 Jun 2023
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Managed switch works for Linux but not for u-boot? (beehaw.org)
submitted 2 years ago by [email protected] to c/[email protected]
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I had the weirdest of a problem. Two computers communicating with each other over ping and TFTP works. When I boot one of them into U-boot (a bootloader that supports TFTP boot) it can’t ping not load tftp of the other machine complaining on ARP timeouts.

I swapped with a dumb switch - all works. Everything else (machines, cables) are the same. The managed switch is a Cisco switch and I have a serial console to it, but I’m not familiar with managing those switches - what feature is potentially blocking u-boot's arp packets?

I’ve double checked with tcpdump - the other machine never seer u-boot's arp packets, but does when the same board is booted into Linux. I’ve also checked Cisco's monitor event-trace arp continuous and it didn’t print any packets but it did say link status went from up to down to back up when I rebooted.

Is there some sort of Mac filter on Cisco switches?

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[–] [email protected] 1 points 2 years ago* (last edited 2 years ago) (1 children)

Reason I'm asking for configs is that it might be an access port vs trunk port config, we just don't know without more detail. Happy to look over everything and help you out though.

@[email protected] you good? People trying to help and you never replied.

[–] [email protected] 2 points 2 years ago (1 children)

I think lemmy is messing up. I replied a week ago.

But we did figure it out. Fastport was disabled and uboot resets the electrical lines which caused the managed switch to rebuild its tree which was still happening by the time uboot started sending arp requests

[–] [email protected] 1 points 2 years ago* (last edited 2 years ago)

Wow. I totally don’t see that comment on my end at all. I am so sorry about that.

As for your issue, that is a strange one. Can’t say I’ve ever seen it before. Is this a legacy switch and software version?

Edit: Oh wait, you meant PortFast. So this was a Spanning-Tree issue. Makes sense. Spanning Tree is the devil btw.