I really recommend using a big block of private IP addresses and NAT them (I am 
on a 10.x.x.x /17 right now) - this allows you to have really big subnets where 
needed, with reasonable DHCP lease times.  DHCP goes through to our BlueCat 
servers. One can then arrange to have enough public IP addresses tied in your 
NAT service to support the numbers of clients...

If your router can't cope with the ARP traffic (which for a /17 is roughly 5 
packets per second assuming a default 4 hour ARP timeout) then it's going to be 
easy to take down with a single misbehaving client...

Richard Letts

Director, Networking and Telecommunications
ITaP Infrastructure Services
Purdue University
rle...@purdue.edu<mailto:rle...@purdue.edu>
O: 765-496-1663
C: 206-790-5837

From: The EDUCAUSE Wireless Issues Community Group Listserv 
<WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU> On Behalf Of Glinsky, Eric
Sent: Wednesday, August 28, 2019 3:36 PM
To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU
Subject: [WIRELESS-LAN] WLC interface groups?

This question is for large universities with WLCs that tunnel traffic through a 
controller. Do you use a single interface (VLAN) for, say, 30k clients, or do 
you use two or more interfaces in an interface group, and why? Do you use DHCP 
proxy? Is there any documentation or generally-accepted rules of thumb on this?

Historically, on all three Cisco 8540 pairs, we had a core interface and an 
interface for res halls, and depending on the AP's location (6k APs) our 
branded SSID would map clients to one interface or the other.

All our wireless clients have public IPs, and we've faced issues running out. 
Throughout the day, we'd see the majority of clients move from the res hall 
network to the core network, and vice versa at night. At one point, we merged 
both the interfaces in an interface group to utilize all IPs at all times. 
However, the way it's currently set up, there are more IPs available in the 
core interface than in the res hall interface.

We are considering these options on how to move forward with or without the 
interface group:


1.      Consolidating down to one interface. More efficient use of IP space, 
clients wouldn't change IPs as often. Could probably increase lease time to 1 
hour, but what about broadcast and ARP traffic for all 30k addresses in the 
VLAN at the router - understanding that client device broadcast traffic doesn't 
leave the controller except DHCP (we do not use DHCP proxy in the controllers).

2.      Staying with the group of two interfaces and balancing the IP space 
between them. Avoids wasted IPs, depending how intelligent the 8540s are at 
distributing clients between all interfaces in the group.

3.      Splitting out to more interfaces. We'd cut down on broadcast traffic 
but we'd be liable to have one client taking up three or more addresses between 
all the interfaces for up to the 30-minute lease time we have, and a client 
would change IPs more throughout the day as it re-associates and gets put in a 
different interface.

Interestingly, a consultant we're working with hasn't seen a single customer 
besides us use interface groups.

Eric Glinsky
Network Technician
University of Connecticut
ITS - Network Operations
Temporary Administration Building
25 Gampel Service Drive | Storrs, CT 06269-1138
(860) 486-9199
e...@uconn.edu<mailto:e...@uconn.edu>


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