l2 l3 borders

[Balma1969]

a very basic question:

inside a single LAN/VLANI shhould create only one IP subnet.

What are the contraindications in creating more than 1 subnet in a single LAN(VLAN ?

[Seb Rupik]

Hi there,

Technically speaking there is no hard limit on the number of subnets that can be supported in a single VLAN. Layer2 switching has no interest in the upper layers, it is only concerned with MAC addresses.

However a VLAN becomes a Layer3 object when you add a gateway (either SVI or routed interface) . At this point have a 1:1 mapping between VLAN ID and subnet ID becomes administratively advantageous. You can obviously configure secondary IP addresses on a Layer 3 interface, the number of which varies depending on platform.

For the sake of maintenance it is typically a good idea to just have a single subnet per VLAN (or two if you are thinking about IPv6 and dual-stack).

cheers,

Seb.

[Joseph W. Doherty]

The most common reason for having more than one subnet on the same VLAN (excluding mixing IPv4 and IPv6) is while readdressing hosts on the subnet.

Another possible (rare) reason would be if you have a multi-address host on the VLAN, where you need different subnet IPs for some form of load balancing and/or perhaps security.

[Cristian Matei]

Hi,

    While it's just not good design, in the end it's gonna work and for the two subnets to speak in between they still need to go via their default gateway; you'll just have two subnets within same VLAN and the default gateway attached to a primary and a secondary subnet. The main drawback will be a larger broadcast domain, as now all broadcast from both subnets are contained within the VLAN. You may see weird messages on hosts as they get ARP messages from non-connected hosts.

    While it can be done and working, i don't really see a reason for doing it.

Regards,

Cristian Matei.