Score:-2

2 computers on the same subnet of an ISP can't talk

ve flag

I have networks connected to the same ISP. Both routers are configured with a static IP like:

Router 1:
Static IP address: 123.123.123.201
Subnet Mask: 255.255.255.0
Gateway: 123.123.123.1

Router 2:
Static IP address: 123.123.123.203
Subnet Mask: 255.255.255.0
Gateway: 123.123.123.1

So both routers are on the same subnet as each other and I think that's the problem. When I try to ping one from the other, I get a "Destination Host Unreachable" message. However, I can ping both from an outside ISP which has a different subnet.

Router 1 "ip route" contains:
123.123.123.0/24 dev eth8 proto kernel scope link src 123.123.123.201 

Router 2 "ip route" contains:
default via 123.123.123.1 dev eth2  proto zebra 
123.123.123.0/24 dev eth2  proto kernel  scope link  src 123.123.123.203 

How do I make it so the 2 routers can talk to each other? Static route?

Peter Zhabin avatar
cn flag
You're likely hitting some sort of a `port security` configuration on an ISP side - that is client ports can only talk to uplink and not to each other. You should investigate this with `arp -an` and simultaneous `tcpdump` sessions on both routers to see what's going on with ARP resolution. If ARP's are not heard by the other side you should create a ticket with your ISP..
Score:0
ve flag

After further investigation, I found that I could not see any ARP packets on the interface. I talked to my ISP and they added an ARP Proxy on their side which fixed the problem.

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