Networking – Why is it improper network design to have two physical NICs, one machine, same subnet, same netmask, same gateway, two different IP addresses

networkingsubnet

All I read and can see says to put two different IP addresses on one NIC. I thought if I had two NICs, I'd be better off for speed/throughput.

Why is it a wrong topology to have two separate physical NICS with the same subnet on one physical machine, two different IP addresses.

Example:

 NIC1/eth0  192.168.20.50/24 192.168.50.1
 NIC2/eth1  192.168.20.55/24 192.168.50.1

Why do they sell servers that are Quadport, if that is the case? Is it expected they will all be on different subnets and gateways?

Best Answer

When you have multiple interfaces with the same subnets on a given machine you are going to have connectivity to the same network via multiple interfaces. Machine would not load-balance and likely pick a single interface for sending traffic out. e.g. Juniper routers pick the interface with the least IP address:

http://kb.juniper.net/InfoCenter/index?page=content&id=KB24928

Multiple interfaces can give you redundancy but note that you can only have one default-gateway esp. if this is a Windows server. I am not sure if other OS will load-balance dual default-gateway routes.

Some redundant solutions include:

  • NIC-teaming.
  • Enable routing protocols like RIP or BGP on the server that can use all interfaces and learn the default-route dynamically.

Quad-port servers usually use: 2-ports for Public/Production Network. 1-port for private/management purposes, 1-port for data backup. Sometimes, some servers in clusters use 1 or 2 ports for heartbeats/keepalives.

Thanks.

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