Laptop with 2 NICs for 2 different subnets but always uses the NIC with prioriy 1

Question

Laptop with 2 NICs for 2 different subnets but always uses the NIC with prioriy 1

Salam ELIAS 302

Hi, I have a laptop and desktop both have 2 NICs, 1 is 1Gbs used for local subnet, the 2nd NIC is 10Gbs I need it for accessing the internet

1 NIC is in a 192.168.1.0/24 attached tothe router which serves as a DHCP and I have a local DNS to server the local subnet as mydns.mynetwork.net.

2nd NIC is in the 192.168.0.0/24 attached to the internet provider box directly (in order to be able to have more than 1Gbs when I browse internet). Here is a snapshot of the network

User's image

DNS for both machines on NIC 1 is my internal DNS in 192.168.1.0/24. With both NICs active, I cant reach any machine in the local subnet 192.168.1.0/24 but when I disable the 2nd NIC everyting goes alright and can ping any machine in local network.

I understand that it is not possible that both NICs have a gateway so NIC 1 is witout gateway but I did a rout add to 192.168.1.0/24 using the router internal IP 192.168.1.1 as the gateway.

when I do a route on the laptop I get

IPv4 Route Table

===========================================================================

Active Routes: Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 192.168.1.1 192.168.1.6 30 0.0.0.0 0.0.0.0 192.168.0.254 192.168.0.2 25 127.0.0.0 255.0.0.0 On-link 127.0.0.1 331 127.0.0.1 255.255.255.255 On-link 127.0.0.1 331 127.255.255.255 255.255.255.255 On-link 127.0.0.1 331 192.168.0.0 255.255.255.0 On-link 192.168.0.2 281 192.168.0.2 255.255.255.255 On-link 192.168.0.2 281 192.168.0.255 255.255.255.255 On-link 192.168.0.2 281 192.168.1.0 255.255.255.0 On-link 192.168.1.6 281 192.168.1.6 255.255.255.255 On-link 192.168.1.6 281 192.168.1.255 255.255.255.255 On-link 192.168.1.6 281 224.0.0.0 240.0.0.0 On-link 127.0.0.1 331 224.0.0.0 240.0.0.0 On-link 192.168.1.6 281

On the desktop

IPv4 Route Table

Active Routes: Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 192.168.0.254 192.168.0.182 2 127.0.0.0 255.0.0.0 On-link 127.0.0.1 331 127.0.0.1 255.255.255.255 On-link 127.0.0.1 331 127.255.255.255 255.255.255.255 On-link 127.0.0.1 331 192.168.1.0 255.255.255.0 192.168.1.1 192.168.1.3 10 192.168.1.3 255.255.255.255 On-link 192.168.1.3 263 192.168.2.0 255.255.255.0 192.168.1.1 192.168.1.3 8

When I ping desktop (whic has 192.168.1.3 IP )from laptop which has 192.168.1.6 IP , desktop respond with the other subnet IP

User's image

whereas I ping laptop from desktop it also responds on the other subnet NIC but with different behavior

User's image

Am I missing something in NICs configurations?

Thanks for your help

0 comments

1 answer

Your answer

Answer 1

The issue is the routing design, not just NIC priority.

Windows decides whether traffic is local or remote by comparing the destination IP with the local IP and subnet mask. For hosts on the same subnet, traffic is sent directly on-link and does not need a gateway. For remote networks, traffic is sent to a default gateway.

In this setup, 192.168.1.x/24 traffic should stay on the 192.168.1.x NIC and should not be routed through 192.168.1.1 as a gateway. The route on the desktop:

192.168.1.0    255.255.255.0    192.168.1.1    192.168.1.3

is wrong for a directly connected subnet. That subnet should be on-link through the 192.168.1.x interface, not via the router.

Also, having two default routes:

0.0.0.0 -> 192.168.1.1
0.0.0.0 -> 192.168.0.254

causes Windows to prefer the lower-metric default route for non-local traffic. That affects which NIC is chosen for replies and can make the behavior look inconsistent.

Use this configuration instead:

On the 192.168.0.x NIC only, keep the default gateway 192.168.0.254 for Internet access.
On the 192.168.1.x NIC, remove the default gateway.
Do not add a route for 192.168.1.0/24 through 192.168.1.1. Since that subnet is directly attached, Windows should use the on-link route automatically.
Verify both machines have a direct route like this for the local NIC:
```
   192.168.1.0    255.255.255.0    On-link    192.168.1.x
```
If a wrong persistent route was added, delete it and recheck with route print.

Expected result:

Access to local devices on 192.168.1.0/24 uses the 1 Gb NIC directly.
Internet traffic uses the 10 Gb NIC through 192.168.0.254.
Replies to 192.168.1.x peers should come from the 192.168.1.x address, not the 192.168.0.x address.

One more important point: Windows behavior with multiple adapters can be unexpected, especially when routing is nonstandard. Microsoft also notes that multiple adapters do not provide load balancing by themselves, and routing table order determines which adapter is used.

References: