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New Member

Network fluctuation

Hello,

we have a wireless network composed by Cisco Wireless Lan Controllers (WLCs) and Access Points (APs) providing access to a large university.

For WLC we're using the 5508 model (four controllers in total) with the latest software version - 7.0.220.0.

For APs we're using the 3500 Series (AIR-CAP3502I-T-K9) and the 1200 Series (AIR-LAP1262N-T-K9), also with the latest version (upgraded automatically by the WLC). We have around 200 APs.

We have established VLANs specific for the dynamic interfaces (providing the WLANs) and a management VLANs (id 254).

Our users are experiencing performance issues with the network, specially when the number of users connected is high (around 1700~2000 users).

Analysing the packets received by a client, we detected a lot of Gratuitous ARP packages originating from the WLCs, for each APs. According to Wireshark:

No.     Time           Source                Destination   Protocol Length Info

     31 31.954314000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.63 (Reply)

     33 32.552240000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.236 (Reply)

     34 32.559302000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.115 (Reply)

     35 32.616899000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.171 (Reply)

     36 33.386340000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.112 (Reply)

     37 33.500793000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.40 (Reply)

     39 33.719575000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.18 (Reply)

     40 33.987327000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.89 (Reply)

     42 58.180785000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.158 (Reply)

     43 58.181200000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.243 (Reply)

     46 58.508351000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.121 (Reply)

     47 58.646692000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.38 (Reply)

     48 58.858960000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.157 (Reply)

     49 59.353822000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.72 (Reply)

     50 59.430118000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.69 (Reply)

     51 59.641935000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.156 (Reply)

     52 60.010860000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.42 (Reply)

     53 60.177897000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.215 (Reply)

     60 83.766096000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.107 (Reply)

     61 83.766538000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.29 (Reply)

     63 84.074924000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.123 (Reply)

     67 86.723782000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.162 (Reply)

     68 86.812189000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.202 (Reply)

     69 86.822574000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.204 (Reply)

     71 87.871292000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.234 (Reply)

     73 87.989734000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.179 (Reply)

     75 91.582985000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.63 (Reply)

     76 91.795397000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.119 (Reply)

     77 92.004989000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.161 (Reply)

     79 92.549156000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.236 (Reply)

     80 92.555894000   Cisco_61:xx:xx        Broadcast      ARP      42     Gratuitous ARP for NNN.NNN.254.115 (Reply)

Cisco_61:xx:xx is the WLC mac. All these IP addresses (NNN.NNN.254.XXX) are APs.

As you can see, the clients are receiving broadcast related to the APs in the mangament VLAN (254). In our understanding this traffic should ocurr only between WLCs and APs. We are already using two separate ports in the WLCs - one for the 254 VLAN (management) and other for the dynamic interfaces (WLANs).

We have left a ping command running in the client, and have noticed that when the clients receives many of these broadcast the connection fluctuates.

Such broadcasts from the management network in the WLANs are expected behaviour?

6 REPLIES

Re: Network fluctuation

Are all your APs in the same subnet ?

How about separating the APs into different subnets that are different than the management subnet ?

New Member

Re: Network fluctuation

Hi Nicolas,

thanks for answering.

Yes, the APs are all in the same subnet/vlan (254).

By separting you mean putting the WLCs in a different subnet (ex: 253) and leaving the APs in the subnet 254, doing L3 to communicate with the WLCs?

Re: Network fluctuation

Rod,

There was a cisco guide and I cant seem to find it now that recommend not to have more than 100 aps in a subnet. Now, this guide was a bit older and Im sure Nick can speak to this....

So it seems like your APs are all in the same managment vlan as your WLC. What about your clients, they are in their own vlan correct ?

__________________________________________________________________________________________ "Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin ___________________________________________________________
Hall of Fame Super Silver

Re: Network fluctuation

George,

You referring to this link:

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a0080810880.shtml#t3

Sent from my iPhone

-Scott
*** Please rate helpful posts ***

Re: Network fluctuation

yes sir!

Network Design

These are the best practices for network design:

  • Limit the number of access points per VLAN. A good number is around  60 to 100 if you use a later code version. This helps to minimize  reassociation problems in case of network failure. Cisco IOS based APs  can be deployed on higher densities subnetworks. Always make sure that  the underlying layer 2 and layer 3 topology is properly configured  (spanning tree, loadbalancing, and so forth).

__________________________________________________________________________________________ "Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin ___________________________________________________________
New Member

Re: Network fluctuation

Thanks for your replies.

George, regarding the clients, yes they are in a specific VLAN.

We're organizing how to promote the following changes in our network, following your advices:

1) We're separting the WLCs into a secondary VLAN (ex: 252), leaving the APs in the current management (to be turned into ap-management only) interface (254).

2) We will divide the APs into more subnets.

As soon as we promote these change, I will post whether they resolved the issue or not.

If you have any other suggetions, please post them as we can try while promoting these two changes.

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