11-10-2020 09:35 AM
HI all,
I've noticed, and read that a /22 subnet in an not to large network should be fine.
But it got me thinking about what are the best practices when you take the amount of switches in a broadcast domain in account.
For example (in this case an real life example): we have a 3 tiered network setup, every vlan with is stretched over the whole tier (where needed of course). In total 1 Core 5 distribution switches and a total 12 access siwtches (all are logical switches).
The following will be hypothetical:
All Vlan's have an /24 subnet, but now we create a new /22 subnet and stretch it over all the 3 tiers, almost 70% off the available address will be used, but mostly the same devices.
This should be fine, if you've asked me.
But what if you doubled all the access switches (or quadrable it) what should be the limit or ratio regarding the amount of devices, Tx switches and subnets?
Is there some sort of baseline?
11-10-2020 10:04 AM
Hello,
not really sure what you are asking. A /22 subnet gives you 1022 hosts. You should also look at potential future growth and keep that in mind when you plan the subnet size.
11-10-2020 01:35 PM
The danger of large address space networks, isn't so much to the switches, themselves, but to the hosts. This because broadcasts need to be examined by every host to determine if it's something the receiving host needs to be concerned about. This is also a little bit true with some multicast too, although to a much lower degree, especially if IGMP snooping is active.
Another concern, with large address space networks, you're often more likely to "spread them", across multiple interfaces, that host the SVI to access layer devices. This creates the potential for other issues, especially if using dual links to the downstream access layer from different distribution devices. (Suggest you research unicast flooding, if you're unfamiliar with the topic.)
That all noted, yes in an IP environment, with hosts that aren't too broadcast chatty, you can often "safely" go larger than a /24. (BTW, there are IP environments where even a /24 is too large.) However, the classic /24 (or olde class C) address block serves fairly well in most cases. So, you should very carefully review the need to have larger address host networks. (BTW, one current exception is wireless host networks, which depending on the technology might "safely" go even larger. [We've used /20s for some of those, w/o issues.])
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