VLAN – How to Map Several Subnets with VLAN

subnetvlan

I've read the post Multiple Subnets in a VLAN and am wondering about the possible conflicts when using several subnets in a VLAN.

The recommended design is of course put a IP subnet in a VLAN.

If I configure several IP subnets in a VLAN, e.g. some hosts in 10.1.1.0/24, some hosts in 10.2.2.0/24 and both under a VLAN, will these two networks interrupt each other? If there's actually no impact then why is such a design not preferred?

Best Answer

This is possible however:

  • you still need a router to have the hosts from one subnet to communicate with the hosts in the other subnet
  • in a modern switched environment there's no collision, so this is not a issue, but you still have broadcast. All hosts will see the broadcasts from both domains which take some part of the bandwidth.
  • Access Control Lists are bounded to interfaces. So to use ACL to limit traffic between the subnets you are limited to a single interface to place ingress / outgress rules; this is stil doable but more limited
  • any host can potentially hear some traffic pertaining to the other subnet. That's a security issue.
  • when troubleshooting a network issue, you may have to sniff traffic (with utility like tcpdump or wireshark), having several IP networks mixed render this or other troubleshooting more difficult
  • overall, this is more complex to maintain and less scalable than having one subnet per vlan.

Basically there's rarely a benefit in doing so, even if you can encounter some corner case where it is the simplest solution (or a temporary dirty fix, but in IT, a temporary setup often last years....).

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