L3 decisions in LANs

[mizoufa187]

Suppose I have 2 users with IP config. U1: 192.168.10.2 /29 and U2: 192.168.10.12 /29, obviuosly in different subnets. If a frame is intended to be sent from one of these users to the other, it will be blocked. This is a L3-based decision, while the switch is a L2-based device. So, what exactly in the network makes this decision?

[network.king]

Hi,

A user with ip 192.168.10.2 with a mask of /29 wants to connect to 192.168.10.12 will send the traffic to his Default gateway first ( a router or L3 switch ) and router would decide to route to 192.168.10.12 .

Here the switch doesnt do any routing , pc with ip 192.168.10.2 knows that 192.168.10.12 is not in its network , so it need to send the traffic to Def Gw configured and packet would be with SA : 192.168.10.2 DA:192.168.10.12 and its frame with Source mac as its own mac and Destination Mac as Routers mac . L2 switch does only switching of packets and router takes care of routing .

Hope this helps.

regards

vanesh k

[mizoufa187]

So is this an operating system or an NIC decision?

[sourabhagarwal]

this is decided by TCP/IP protocol stack installed in your NIC adaper.

hope to help ... rate if it does ...

[cratejockey]

As an interesting aside to this conversation a college of mine just reminded me that you should also be able to set the default Gateway of your host to the hosts IP address. Doing this to both host if they exist on the same L2 segment ie a flat switch with no VLANs they should ARP and find each other. In a VLAN environment or across L3 links or potentially even larger L2 networks you would need an L3 gateway to process the communications.

While this is possible its not a good idea and should never be tried on a network that is in production and that you do not control.