IP routing involves metrics. This is the cost of each route. If there are multiple routes to a destination then the route with lowest metric/ cost is chosen.
In the context of Windows OS there are two metrics that come into play.
One is the metric of the interface/ NIC itself (that’s the “Automatic metric” checkbox above). By default its set to automatic, and this determines the cost of using that interface itself. For example if both your wireless and wired connection can access the Internet, which one should the machine choose? The interface metric is used to make this decision. You can assign a value to this metric if you want to force a decision.
Each interface can have multiple gateways to various networks it knows of. Could be that it has more than one gateway to the same network – say, your wired connection can connect to the Internet from two different routers on your network, which one should it choose? Here’s where the gateway metric comes into play (circled in the screenshot above). By default when you add a gateway its metric is set to automatic, but here too you can assign a value.
So far so good. Now how does all this come into play together?
The first thing to know is that gateway metrics have a value of 256 by default (when set to “Automatic metric”). So if you have more than one gateway to a particular destination, and the metric is set to automatic, then by default both gateways have a metric value of 256 and hence equal preference. Remember that.
The next thing to know is that interface metrics have a value ranging from 5 to 50 (when set to “Automatic metric”) based on the speed of the interface. Lower numbers are better than higher numbers. See this KB article for the numbers, here’s a screenshot from that article.
So if you have two wired connections for instance, one of speed 1 GB and other of speed 10 GB, then the 1 GB interface has a metric of 10 and the 10 GB interface has a metric of 5 – thus making the latter preferred.
To view the interface & gateway metrics assigned to your interfaces use the netsh interface ip show address command:
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C:\WINDOWS\system32>netsh interface ip show address Configuration for interface "Local Area Connection* 2" DHCP enabled: Yes InterfaceMetric: 5 Configuration for interface "Ethernet" DHCP enabled: Yes IP Address: 10.136.20.16 Subnet Prefix: 10.136.20.0/24 (mask 255.255.255.0) InterfaceMetric: 10 Configuration for interface "Ethernet 2" DHCP enabled: Yes IP Address: 10.171.1.6 Subnet Prefix: 10.171.1.4/30 (mask 255.255.255.252) Gateway Metric: 5 InterfaceMetric: 20 |