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I have a network with a large class B broadcast domain (172.16.0.0/18 - 172.16.192.0/18) with a single DHCP server.
I have lots of L2 switches in this network but there is no VLAN configured.
My problem is When I connect an access point (Linksys) to network the connection is very very poor and slow while Ethernet connections are OK

I think CSMA/CA with a large broadcast domain causes the problem.
I started a wireshark and captured packets for 2 minutes; I got 90K pcks with 50K of ARP packets

Can this large broadcast domain be the reason poor quality of my wireless ?
is there any way to fix this?

  • Network classes are dead, killed in 1993 by RFCs 1517, 1518, and 1519, which defined CIDR (Classless Inter-Domain Routing). Modern networking doesn't use network classes, Please let them rest in peace. Also, consumer-grade devices (your Linksys) are explicitly off-topic here. – Ron Maupin Nov 28 '17 at 14:54
  • Also, a broadcast domain is not a layer 3 concept determined by IP addresses, it's a layer 2 concept that isn't controlled by addressing. Several different subnets could all share one broadcast domain, or a subnet could be used in more than one broadcast domain (although that could be a strange and tricky configuration). – Todd Wilcox Nov 28 '17 at 15:03
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Feb 19 '18 at 20:13
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First, please forget about classfull networks, they are dead for decades now.

Having a /18 network is not recommended at all. As you pointed out, it is a very large broadcast domain that will cripple performance.

Apart broadcast, another problem is the number of MAC addresses a device can handle. This lead to two distinct issues:

Limit of switch MAC table size

Newest enterprise class device can handle 16 000 Mac address, but some are limited to 4096 MAC addresses. A consumer grade device like your linksys has generally a lower limit (a quick google search show 2000 but I didn't verified this).

So if you have too many devices, once the MAC table of a switch is full, it will flood the frames destined for the overflowing hosts, increasing the broadcast issue.

Limit of hosts ARP cache size
The ARP cache maintained but hosts can be pretty limited in size. I found a value of 265 for some edition of Windows. When the cache is full, it is no more effective and ARP requests will dramatically increases.

I got 90K pcks with 50K of ARP packets

seems to confirm that this is actually your issue.

Additionally, I suppose (due to the size of the networks) that you have many Wireless clients, and SOHO devices are not designed to handle many clients. That may also explain the poor performance, but it's hard to say with the info provided.

is there any way to fix this?

You need to split your network in smaller, isolated (by VLAN) subnets, and use enterprise class access points (preferably WAVE2) to handle a great number of Wifi clients.

  • Along with splitting the network into VLANs, the asker will probably want to create smaller IP subnets and configure some kind of routing for traffic to go from VLAN to VLAN. – Todd Wilcox Nov 28 '17 at 15:09
  • @JFL If you have more MAC addresses in a network than a switch can handle, the switch will drop active MACs from its overflowing SAT. Subsequently, the switch will broadcast frames destined for these MACs to all other ports due to an unknown port association (aka "hub mode"). Enterprise-class switches usually have 16,000 SAT entries today. – Zac67 Nov 29 '17 at 11:56
  • @Zac67 you're right I'll edit, thanks. – JFL Nov 29 '17 at 12:13
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In a word: YES.

Broadcast (and thus, multicast) is a serious problem for wireless. The frame will only be sent once, so it has to be transmit such that every station can hear it. This is the basic-rate. Basic rates are typically very low. The AP may be able to speak to you at 300Mbps, but a more distant, less capable station may be reachable at only 24Mbps. For both of you to hear it, it has to come over the air at 24Mbps. (actually, lower because there's no opportunity for retransmission.)

This isn't a problem for wired links as they're running 100Mbps, or faster, on switches where there are no collisions and traffic can be buffered.

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