[Discuss] vlans and arp

[Matthew Gillen]

I think I'm convinced it's a hardware problem with the ethernet port on 
serv2 (the NUC box).  Manually forcing the arp entries on both sides 
didn't fix it, so it wasn't just an arp problem.

Reconfiguring all the vlans to use the other physical interface on the 
NUC (eno1) makes everything work.

I had noticed that the (suspected) bad port on the NUC wouldn't 
negotiate a 1Gbps speed with the Netgear, but when I forced it to 
100Mbps the link would show as 'up'.  I guess that failure to auto 
negotiate was probably indicative of a bigger problem.

Ugh.

On 7/1/2022 8:48 AM, Matthew Gillen wrote:
> I'm pulling my hair out.  I have a fedora box (serv1) that is my network 
> management hub.  It has a physical interface with no address, and a 
> series of vlans configured (via NetworkManager).
> 
> This is plugged into a netgear switch that sends vlan tagged packets to 
> serv1.  This works great.  (side note: this is a great way to have one 
> dhcpd serve a lot of VLANs: the client requests come in to the vlan 
> interfaces, and then a single dhcpd instance knows which subnet it 
> should issue an address for)
> 
> A botched upgrade made me finally want to separate the net management 
> from my file server.  So I have a little NUC-like box that I'm trying to 
> set up as a replacement for serv1.
> 
> serv2 is RHEL9.  Tried to set it up the same way using NetworkManager: a 
> base interface with no address, and VLAN interfaces.
> 
> The netgear switch has the same vlan config for the ports serv1 and 
> serv2 are using (ie every vlan is using tagging).
> 
> On serv2, a tcpdump of the base interface (enp1s0) sees VLAN-tagged 
> traffic for several vlans, as expected.  A tcpdump of the VLAN interface 
> shows the untagged traffic for that VLAN, again as expected.
> 
> The problem is serv1 never sees any arp requests or replies from serv2. 
> serv2 sees ARP coming in from serv1, and sends a reply.  tcpdump on 
> serv2 shows the reply being sent (both the untagged version going out 
> the VLAN interface, and the VLAN-tagged version going out the base 
> interface.  But serv1 never sees anything.  It's like the outgoing 
> arp-reply gets dropped on the floor sometime after the tcpdump on serv2 
> sees it.
> 
> What would make arp only work in one direction like that?  My google-fu 
> turned up some stuff about ARP-flux, but that seems like a different 
> problem (my arp things appear to be routed to the correct interface, 
> they're just getting dropped somewhere).
> 
> Any ideas?
> 
> Thanks
> Matt
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