Multicast on LWAP AP

Unanswered Question
Mar 10th, 2009

any one experiencing on Multicast problem. In my network, Multicast is perfectly working on wired to wired network, but once using wireless to Wired (multicast server on wired).

Currently I am using WLC (LWAP AP, any advice to solve my problem?

Many thanks in advance,



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Overall Rating: 5 (1 ratings)
m_zabetian Fri, 03/20/2009 - 09:46

change your Ethernet Multicast Mode configuration from Disable to unicast (it is located under Controller genera)

ronaldo.melo Thu, 03/26/2009 - 09:56

Hi Sam,I'm having the same issue at a customer the quality of image at the clients in wireless is worst.I had configured in both modes (unicast and multicast) but still having problems, in unicast a little better. The code at my WLC is WLC4400-K9-4-2-176. Your problem was resolved?



gnijs Wed, 04/29/2009 - 12:38

I tried also doing video multicast over Wifi. Stream was low (200 kbs) bandwidht, but i discovered that there were much to many dropped or missed packets (around 11%). PCs didn't notice this, but multicast video was unacceptable. I tried changing the QOS settings on the controllers to gold, but didn't help, neither did enabling WMM extensions. Checked also interference via WLC and WISM, but that was ok according to the controller. So know we disabled multicast on wifi because the quality just wasn't good enough

ronaldo.melo Wed, 04/29/2009 - 12:55

When you change the configuration at controllers to Gold the multicast packets must come market for you have any result. So you need configure a Cos to multicast traffic. I'm having the same problem and I will do QoS for multicast packets soon. I will post the results.

j-mccarthy Wed, 05/20/2009 - 02:23

I have a TAC case open about wireless multicast packet loss at the moment. I have done extensive testing with both WiSM and 4404 and I am unable to prevent packet loss when multicasting from wired to wireless network. Unicasting between the same two devices works perfectly so I think there's a multicast issue in the controller software. I've been testing with on both WiSM and 4404. Packet loss occurs on both platforms. Today I tried the new 4.2.205 code on the 4404, still has the problem. I've tried both unicast multicast and multicast multicast delivery methods. I get fairly constant 3-5% loss on a 1mbps stream. I'm verifying the packetloss using RTP Stream analysis on Wireshark on the client machine and also with Iperf.

Has anyone got wireless multicast streams working properly using WLC & lightweight APs with no packet loss?

ronaldo.melo Wed, 05/20/2009 - 06:16

Hi folks,

Anyone tried with a AP in autonomous mode? At the WLC if you put the traffic “multicast” to unicast mode the WLC will have problems with CPU and performance for normal traffic, I am assuming using more than 6 clients connected at the same time at AP. I have a TAC opened and they explain the quality will never be the same of wired and you need mark the multicast traffic for the WLC and put the SSID to Gold. I didn't have a time yet to do the tests because the wired network of my client is not Cisco.

j-mccarthy Wed, 05/20/2009 - 06:40

I've tested unicast multicast mode in my lab with one access point and one wireless client and traffic is being dropped. There is an issue there that is not related to scale.

I've also set the SSID to Gold, had no impact on the problem. If there is no congestion present there should be no need for QoS anyway?

ronaldo.melo Wed, 05/20/2009 - 06:51

According TAC even you have only one client at AP it is necessary the QoS??? I agree it is not a problem of QoS but you know the TAC… you must following the procedures.



dancampb Wed, 05/20/2009 - 09:06

QoS isn't required for multicast, but definitely recommended especially if the multicast stream is voice or video.

The most common cause of multicast not working is that PIM is not enabled on all of the VLAN's needed. You should have PIM enabled on the VLAN of the controller's management interface, the VLAN of the dynamic interface mapped to the WLAN, any VLAN AP's are on, and any other wired VLAN needed.

Also you should make sure IGMP snooping is enabled on the controller. This of course isn't available until the 4.2 train.

j-mccarthy Wed, 05/20/2009 - 16:32

I have multicast working fine. The problem I'm seeing is packet loss of the multicast stream to the client. Consistent 3-4% packet loss makes a 1 mbit video stream pretty unwatchable. There should be no problem receiving a stream of that size.

Unicasting the same stream from the same source (wired PC) to the same destination (wireless laptop) shows 0% packet loss.

mpatalberta1 Thu, 05/21/2009 - 19:40

The wireless will queue the unicast packet during a roam.

It does NOT do this for multicast.

mpatalberta1 Thu, 05/21/2009 - 18:35

Set dtim = 2

set beacon = 100

The multicast packet will come out at the beacon.

Unlike unicast the multicast packet is NOT ack knowledged by the client

j-mccarthy Thu, 05/21/2009 - 19:08

On my network DTIM=1 Beacon=100

"The DTIM Period is the number of beacon intervals that elapse between transmission of Beacon frames containing a TIM element whose DTIM Count field is 0. Default 1 interval."

That explanation doesnt really help me :)

Regarding the acking of packets - the receipt of UDP traffic unicast is reliable on wireless wheras the receipt of UDP multicast is unreliable?

Why does the client ack a unicast UDP packet but not a multicast UDP packet?

mpatalberta1 Thu, 05/21/2009 - 19:39

The multicast packet is sent by the infrastructure on all ap. it does not know who is going to receive them. For example dhcp does this. Thus you cannot rely on who will ack the multicast.

I am using the multicast for voice with wpa2 personal. In general except during a roam we get no loss. However we do have 1 redundant data packet in the multicast packet.

The roam with an open infrastructure we do not loose any noticable packets.

The roam on wpa2/psk as result of a soft handoff we have no loss. We only have data loss during a distressed roam.

Turn off the aironet ie.

mpatalberta1 Thu, 05/21/2009 - 19:42

I should mention you could use the cisco igmp method. This option is suppose to improve the multicast performance.

j-mccarthy Thu, 05/21/2009 - 19:47

I am using Cisco IGMP. There is no roaming involved. One client, one access point, one WLC & one multicast server.

Voice (MOH I assume)is probably ok because the bandwidth of the stream is so low (64kbps).

With a 1mbps video stream the packet loss is really noticeable. Looks like delivering video over wireless multicast is a non runner.

Matthew Fowler Thu, 05/21/2009 - 21:58

Probably the biggest issue is, as mentioned, the fact that 802.11 multicast is not ACKed while 802.11 unicast is. The test would be to get a wireless sniff of the unicast video and see if the retransmit percentage is about the same as the multicast drop percentage.

What power saving is the client doing? Have you tried setting it to being constantly awake?

j-mccarthy Thu, 05/21/2009 - 22:02

hi mat, i haven't mucked around with power settings on the client to be honest. thats interesting idea because i am seeing different levels of packet loss with different clients. ibm t60 with an intel 3945 seems a lot worse than a dell d600 for example.

Matthew Fowler Thu, 05/21/2009 - 22:24

I ran a test after I posted. I have a 4965, set the Power Management bar to lowest performance and a 1Mbps iperf test resulted in 2-9% loss. I then set it to highest performance and I only got 0.18% loss.

So, definitely worth checking/trying.

JASON BOYERS Mon, 05/25/2009 - 19:39

It looks like the 4965 is using Power Save Polling mode, with the default being to conserve as much power as possible. However, that means that the client is not listening all the time for traffic. And, thus when the AP sends the multicast traffic, the client doesn't hear it. From what I'm seeing, PSP really doesn't give you much more battery life with current devices. I'd change the Power Management to highest performance, like the last post indicated.

dziminski Wed, 08/12/2009 - 11:28

I am just starting to test multicast video myself and I am running into the same issue. Even a well designed wireless LAN with QOS is still going to have collisions. With unicast video, the packets can be resent at the 802.11 layer. With multicast, you are going to get some packet loss since there are no ACKs and therefore no retries. Aruba has a feature that converts multicast to unicast from the AP to the client to get around this. I don't think the cisco controllers have this (at least I haven't seen it).

lomonaco Tue, 07/07/2009 - 11:19


What about your data rates. Take a look this information (Mobility Design 4.1)

It might seen logical to choose the default configuration of APs and clients, thereby allowing all data rates.

However, there are three key reasons for limiting the data rate to the highest rate at which full coverage is obtained:

Broadcast and multicast (if enabled) are sent at the lowest associated data rate (to ensure that all clients can receive the packets). this reduces the throughput of the WLAN because traffic must wait until frames are processed at the slower rate. Clients that are farther away, and therefore acessing the network at a lower data rate, decrease the overall throughput by causing delays while the lower bit rates are being serviced. It might me better to force the clients to roam to a closer AP so as not to impact the performance of the rest of the network.

Rate Modes

If more than one data rate is set to mandatory, multicast and broadcast frames are sent at the highest common mandatory transmission rate of all associated clients (the lower mandatory receive rate of all of the clients) This allows all clients to receive broadcast packets. The lowest mandatory rate is normally set at 1 Mbs/s

My Best Regards

Andre Lomonaco

dziminski Wed, 08/12/2009 - 11:35

Good point about the data rates. That could have a huge impact as well if your multicast traffic is sent at 1 MBPS.

Aruba has a feature that will bump up the data rate for broadcast and multicast to the highest supported rate that all associated clients on an AP support. Hopefully Cisco will develop a similar feature.

Edit: I did some testing with data rates. I set the 18 mbps rate as the only mandatory rate and I was able to get the multicast to transmit at that rate. However, I didn't see a big improvement on performance. Still losing about 3-5% of video packets. The video is very jumpy. I am testing with the 1142 AP and a Lenovo T400 with an Intel 5300 WLAN card.


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