Hi all, Ihave a question about ARP:
---background----
At one of our remote offices we recently elected to have an alarm system installed. We have an RB2011 with dual wan running there. The alarm, which apparently is one of those Christopher Columbus brought with him back in 1493, is supposed to be internet monitored. Because of scheduling issues I couldn't be present for the installation of the system but prepped all the network cabling beforehand so the alarm tech wouldn't have an excuse to lay his greasy paws on my rackmounted equipment.
Long story short, the guy says there's a problem with our networking gear.
The networking cable is fine (triple checked by both of us) but the alarm constantly renegotiates ethernet speed. When forced to 10 mbit half duplex, it maintains a stable link but it won't connect to their server. I told him that this can't be correct (who in the world still produces 10Base-T half duplex gear in 2019!?!?) so he took the equiment back to their shop. They tested it there and contacted the manufacturer and it alledgedly all checked out. As a final test measure he brought back a cheapo "router" with him, plugged the alarm in, and that into our network. Still no connection.
----current status----
Our routerboard gets bombarded with DHCP requests from the alarm (one every 2-3 minutes). This tells me that his "router" isn't routing; merely acting as a switch. I'm also guessing that since the "router" isn't forced to 10 half duplex it's constantly renegotiating, hence the requests. In any case, the alarm is taking the IP and is pingable. Lease time is set to 72 hours.
Now, I'm running the sniffer on the routerboard, streaming to my PC over the VPN and I'm seeing two things. DHCP requests (no surprise there) and ARP requests for the alarm server at 181.XX.XX.XX (I'm guessing the IP is hardcoded into it) but no reply from the router.
Every other piece of hardware on the network (servers, smartphones, laptops, PCs, weather station, ip cameras, radios, swtiches, access points, etc) work perfectly.
----question----
I've always taken ARP for granted so I'm not very knowledgeable on the inner workings of it. My understanding is that either:
A: Host sends out "who has 181.x.x.x? ARP request". If no reply, host sends it to the gateway.
B: Host sends out "who has 181.x.x.x? ARP request", router replies "I know the way, send it to me".
So why is neither of those happening? Could it be that it's taking the IP from DHCP but not the gateway? Anyone ever seen such a thing?