-1

We have a data feed coming into a single Linux server (CentOS 7). We need this server to forward all the data to two different VLANs with the same subnet. We currently have software running on the server that can mirror data to two different subnets but it is not VLAN aware (as the VLANs are implemented by a Cisco proprietary protocol), so we need to configure the server at the OS level to mirror the data out of two separate interfaces. Is there a way to configure Linux to do this? (All connections are IP over TCP)

One solution is to change the subnet for one of the VLANs, however this is likely to cause downtime which we would prefer not to have.

dippynark
  • 233
  • 3
  • 13
  • Please clarify "Data feed" and what protocols it uses. Please also show network configuration. – Tero Kilkanen Aug 24 '17 at 09:02
  • Good point, everything is IP over TCP, we don't actually have the server yet so I don't know the details of any higher level protocols being used, it was more of a theoretical question in preparation for when it does arrive – dippynark Aug 24 '17 at 09:47
  • The question still cannot be answered without a network diagram with information about IP networks. – Tero Kilkanen Aug 24 '17 at 09:50
  • @TeroKilkanen what sort of network configuration details would you need? – dippynark Aug 24 '17 at 09:51
  • As in how low level would I need to go? I can provide the IP addresses and subnets of the various interfaces involved? – dippynark Aug 24 '17 at 09:53
  • Yes, subnets, IP addresses of the interfaces on the server, and more information about the clients connecting to the server. – Tero Kilkanen Aug 24 '17 at 09:54
  • I'm not quite sure how to answer that because I'm asking about how I can configure the network, so we don't have a working configuration at the moment. The VLANs can be associated with any subnet you choose and it won't affect the question (I don't think) as long as it is the same subnet for both VLANs and the interfaces on the server can be configured to be on any other subnet. I don't think the details of the clients will affect the answer, just suppose the server is sending arbitrary information to something on these two VLANs, I probably shouldn't have mentioned the data feed at all. – dippynark Aug 24 '17 at 10:13

1 Answers1

1

"two different VLANs with the same subnet" - as in "IP subnet"? This creates a routing problem that is not trivial to solve.

If you're using a router to forward into the different VLANs, the router needs to split the subnet into smaller subnets, ie. to address 192.168.1.3 and 192.168.1.5 you split to 192.168.1.0/30 and 192.168.1.4/30.

If you directly attach the VLANs to the server you need to do the same with NIC IP bindings.

Zac67
  • 10,320
  • 2
  • 12
  • 32
  • Yeah IP subnet, but we need to send the same data to the same IP address on both subnets so I don't think splitting the subnet into two pieces with a different IP range for each piece helps in this case – dippynark Aug 24 '17 at 10:18
  • No, with identical IP addresses that won't work. Do consider renumbering. Using identical IP addresses in different networks involves destination NATing and often can't be done with a single router. In order to solve this on the host level, the software needs to be able to use different source interfaces (instead of relying on OS routing). – Zac67 Aug 24 '17 at 10:31
  • Yeah renumbering looks like the only sensible solution so far, thanks – dippynark Aug 24 '17 at 10:37