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Question regarding the routing table (lemmy.world)

submitted 7 months ago by RalphFurley@lemmy.world to c/networking@sh.itjust.works

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Let's say I have a Linux VM. Default route is the gateway to the top of rack switch for public internet and a public IP is bound on one virtual nic.

2nd interface is on a private network so the VM can be reached anywhere on the VPN. This is a management network where the gateway is on the other side of the data center.

A lot of stuff sits on the 10.0.0.0/8 that needs to reach this vm so a static route for the second interface points that /8 to that gateway on say 10.100.100.1

Now inside the same cabinet are devices sitting on 10.20.20.0/24.

If I didn't do anything, would hitting something on say 10.20.20.2 route traffic through gateway outside of the cab and back? I would think so as it sees the routing table and has no way of knowing.

If I want to optimize traffic so nothing is routed and traffic stays local to the cab, could I just add a third nic and give it an IP of say 10.20.20.3 and hitting .2 would arp / hit it directly through the switch in the cab?

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[–] Markaos@lemmy.one 1 points 7 months ago* (last edited 7 months ago)

Like others here, I have trouble following your post, but one thing is instantly sticking out to me: you mentioned 10.0.0.0/8 and 10.20.20.0/24 networks being present.

Unless your 10.20.20.x are actually in a 10.0.0.0/8 network (so configured with addresses like 10.20.20.x/8), this is definitely gonna cause a lot of issues. Like most things in networking, it can probably be made to somewhat work for some specific setup with some voodoo magic, but you really need to understand what you're doing on a very deep level to do that.

The first issue you will probably encounter if you want to communicate between the two networks is that ARP cannot be forwarded, so your /8 network will never discover the /24 network (unless you just bridge them together - that will let the /8 network discover the the /24 devices, but not the other way around).

If you want to make your network routed, you will need to split it into non-overlapping networks. Otherwise give everything a 10.x.y.z/8 IP address and make sure that the whole network is connected via switches (hardware) / bridges (software).