cloud-flat-eqiad1: either 172.16.8.0/22 (1k addresses) or 172.16.8.0/21 (2k addresses)

Even with the ability to more easily have the vlan stretched across many racks, there are disadvantages to having too large a broadcast domain. i.e. ARPs/ND get flooded to all hosts in the subnet. With 2,000 hosts this can end up being quite a lot, so my instinct here would be to go with the /22.

I guess the other factor weighing into the decision is what the options are if we had more than 1,000 hosts. Is it easy enough to add another similar network with another /22? Or would it be quite complicated to add additional networks if we outgrow the subnet we allocate now?

As of this writing we have a 172.16.0.0/21 for eqiad1, with about ~800 VMs running.

source:

• tofu https://gitlab.wikimedia.org/repos/cloud/cloud-vps/tofu-infra/-/blob/04639876f44342ef196cacb37bee43703d7de966/data/subnets.yaml#L77
• netbox https://netbox.wikimedia.org/ipam/prefixes/84/

All VMs live in the same subnet, so effectively the same broadcast domain as you say.

If we manage to set it up nicely, it should be a problem to grow later, or to have another range. Well, in fact, our target is to have tenant networks, so that means even less neighbors in the /22 subnet.

I agree going with 172.16.8.0/22 (1k addresses) for now for eqiad1.

Created https://netbox.wikimedia.org/ipam/prefixes/1076/

updates:

• cloud-flat-1-ipv4only (eqiad1) 172.16.8.0/21 https://netbox.wikimedia.org/ipam/prefixes/1076/
• cloud-flat-1-dualstack-ipv4 (eqiad1) 172.16.16.0/21 https://netbox.wikimedia.org/ipam/prefixes/1103/
• cloud-flat-codfw1dev-ipv4only 172.16.130.0/24 https://netbox.wikimedia.org/ipam/prefixes/1101/