Tag: Beacons

DFS channels and why to avoid them (even though you say you cannot)

DFS channels and why to avoid them (even though you say you cannot)

 

These days most wireless guys say they cannot stick to only 8 channels. Most people agree that you should use all 26 channels (including all the DFS channels). The truth is you can use an 8-channel plan if you use 20 MHz wide channels exclusively. I know …I know every marketing person, every manager and every user just lost their collective minds. Everyone wants to use 80 and 160 MHz wide channels and most people (even Wi-Fi people) want to use 40 MHz channels everywhere. If you use wide channels then you cannot stick to an 8-channel plan. On the other hand, if you agree that using 20 MHz channels is a more efficient use of the channels and spectrum, then you can very easily get away with using only 8 channels.

 

There are 9 non-DFS channels but when you use channel 165 (the highest channel in UNII-3) you may run into issues since there is not enough separation between Channel 165 and UNII-4 band channels. This has been known to cause issues, especially with voice clients. I always recommend staying away from channel 165 which leaves us with an 8-channel plan (36, 40, 44, 48, 149, 153, 157, and 161).

 

Back in the day when we used 2.4 GHz only networks, you were limited to only 3 channels, and despite this people more clever than, I got it to work with very few issues. Yes, there was always co-channel interference, but the better engineers would work to minimize it. In 5 GHz, suddenly using 8 channels is big a hassle for most people. Have we gotten lazy, or have we bought into the lie that wider channels are better? I will save my arguments for why 40MHz channels are highly inefficient for another blog. If you do not want to wait, you can look at Devin Akin’s blog at https://divdyn.com/wi-fi-throughput/  he does an amazing job diving into this point.

 

DFS Channels

What is a DFS channel? These channels share the spectrum with Weather Radar and Radar systems. For the FCC and IEEE to approve the use of these channels in WIFI, a mechanism had to be in place where these channels could co-exist. A mechanism called DFS (Dynamic Frequency Selection) was created to have the WIFI devices listen for radar events and either stop using the channels or automatically move off these channels. When RRM/ARM is used, and an AP hears a radar event it must pick a new channel and inform its clients to move to this new channel. If RRM/ARM is not used, then AP after hearing a radar event must stop transmitting for 30 minutes.

 

 

 

 

 

Why are DFS channels so bad?

 

There are 16 DFS channels in the UNII-2 and UNII-2e space (52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, and 144). These channels have two major drawbacks, especially for voice clients. These drawbacks are 1.) The length of time it takes for a client to scan DFS channels. 2.) When radar events are heard the APs and device must move off that channel. The rest of this blog I will talk about these two issues.

 

The first issue is how long it takes a client to scan the DFS channels. This changes a bit with different clients but since I know the Vocera devices I will use them as an example. When the Vocera badge scans non-DFS channels, it immediately sends a probe request and gets back a probe response this takes roughly 15 – 20ms per channel. The device must return to its original channel for 50ms to do any TX/RX it may need. The total time to scan each channel is 65ms. When you multiply 65ms by 8 channels you get a total scan time of 520ms. When the badge scans a DFS channel it cannot send a probe right away. It must make sure there is no radar on this channel. It does this by listening for a beacon (100 – 104ms). The device will then send a Probe Request and receive a Probe Response only after it hears a beacon. Since it has stayed off its channel so long it needs to return to its original channel for 150ms for any Tx/Rx. The round trip time for every channel is roughly 250ms. When you multiply 250ms by 16 channels you get a total scan time of 4000ms (4 seconds). Four seconds may seem like a short time but, for a voice client that has a jitter buffer of 120ms, a 4-second delay can seem like a lifetime. When the device is in the middle of a call and needs to find another AP to roam to a delay beyond 120ms will cause choppy audio. It will take even longer if you hide the SSID since the badge must take an extra step of sending a Probe Request with the wildcard in it so all APs will respond.

 

This improves greatly when you have 802.11k enabled. When using 802.11k after the device connects to the AP it will request a Neighbor list. This neighbor list will (normally…if everything is working right) contain a list of APs that are its closest neighbors. This will cut down the number of channels the device has to scan. If the device happens to move away from all the APs in the neighbor list it will need to do a full scan.

 

The second issue with DFS channels is when a DFS event occurs (or even a false positive happens) the AP must send out a channel change announcement which tells the devices that it must move. Most clients will treat this as a new roaming event without the benefit of pre-scanning the channels before the move. They will have to rescan all the channels instead of just the one where the AP is moving to. If you are a laptop surfing the internet or reading email you may never notice the delay but if you are a voice client in the middle of a call you will experience choppy audio during the forced roam.

 

 

In conclusion, stick with 20 MHz channels and use only the 8 Non-DFS channels. By doing this you will avoid all the issues that come along with the DFS channels. If your devices or applications need to move massive amounts of data, either stick a cable in it or switch over to Wi-Fi 6e and you can have all the bonded channels you want. You will not have to worry about the fact you are running highly inefficient bonded channels until you start using 320 MHz bonded channels and Wi-Fi 6e will be down to 4 channels and this will restart the discussion all over bonded channels again.

 

Multicast on a Cisco Wireless Network

This is not meant to be a definitive guide to multicast. There are more detailed documents and white papers from Cisco and others on this subject. This is just an attempt to give a basic understanding and encourage you to learn more about this subject.

This is a two-part blog. The 1st blog named “Settings that can improve Multicast on a Wireless Network” which discussed the settings needed to optimize a Wireless/Wired network for multicast traffic. This blog will discuss more about settings and troubleshooting on a Cisco Controller based wireless network.

I work for a wireless communications vendor that uses multicasts as an integral part of the product. I have been onsite and on conference calls with customers trying to troubleshoot multicast issues. The issue almost always turns out that multicast packets are being blocked somewhere on the network. There seems to be a lot of confusion about what happens to multicast packets and how to prevent them from being blocked on your network. This blog will try to explain the paths the multicast packets take on the wireless and wired network.

The topics I will cover in this blog are multicast packets on a controller-based network, configuring multicast on a Cisco Controller, IGMP snooping on the controller, troubleshooting multicast issues, troubleshooting using Multicast Hammer, Cisco commands and some links to blogs and articles I found helpful.

Background

The Cisco network uses IGMP (Internet Group Messaging Protocol) to manage the multicast traffic. When a multicast session is started a client or server will send an IGMP join message to a certain multicast address. Any clients that want to be part of this group will send a join message to the same multicast address. These join messages will get recorded on your local network’s router as well as the Rendezvous point (RP). If your local router sees traffic for this multicast address it will pass the traffic to your local network. If the router sees multicast for a different address it will dump these packets. When IGMP snooping is enabled on the switch, it will record which ports need multicast traffic.

Terms discussed in this blog

Multicast Group ID (MGID) is created by the controller and passed to each AP. The MGID is the ID number which maps multicast source, the multicast address and the VLAN. This helps the controller and APs to keep track of multicast groups.

Delivery Traffic Indication Map (DTIM) is an informational element in a beacon that will inform a station if there is any multicast traffic the AP has for the station.

Rendezvous Point (RP) is a router in a multicast network domain that acts as a shared multicast tree. There can be multiple RP on any given network.

 

Configuring Multicast on Cisco Controller

There are two modes you can use on your controller; Multicast-Unicast and Multicast-Multicast. If you use Multicast-Unicast, then the controllers will make a copy of every multicast packet and sends these packets out as unicast packets to every AP. This would work OK for a smaller network but in a larger network, this would create too much traffic. In larger networks Multicast-Multicast mode is much more efficient use of bandwidth.

When selecting the Multicast-Multicast option on the controller you will need to choose a multicast address. This address must be different on every controller on your network.

Screenshot courtesy of mrn-cciew

When an AP connects to a controller, the controller will tell the AP to join this multicast address you have chosen. All the APs will send a join message to this multicast address. When the controller receives a multicast packet it sends out one packet and all the APs who have joined the multicast group will receive this packet. Multicast can be best described as one to many, where unicast is described as one to one.

IGMP snooping on the Controller

When IGMP snooping is enabled on the controller, it creates a Multicast Group ID (MGID) table, which keeps track of the clients and what multicast groups they have joined. The controller sends this MGID table to all the APs that are connected to it. When multicast packets are sent to the APs, the APs will check the MGID table to see if they have any clients that have joined this multicast session. If they have a client, the AP will send the traffic. If the AP doesn’t have a client, the AP will discard the traffic.

If IGMP snooping isn’t enabled on your controller all APs that have the same VLAN where the multicast traffic had been requested (even by a client on another AP), will receive these multicast packets and will send them out. This means every AP will be sending out multicast packets even if they do not have a client for this traffic. IGMP snooping will limit the multicast packets that are sent by the APs.

 

Multicast Packets on a Controller based network

When the wireless client sends an IGMP join message to the AP, the AP sends this join message through the CAPWAP tunnel to the controller. The controller absorbs this join message and sends a new join message to the local router on the VLAN of the client using its own IP address (not the IP address of the client). The router then adds this multicast group to the interface creating a (*,G) entry.

When multicast packets are sent from the wired side of the network, the router which is acting as Rendezvous Point (RP) will send these packets to the controller. The controller receives these packets and encapsulates them in its multicast address. The controller then sends these multicast packets to all the APs on that controller. The AP strips the controller’s multicast address and sees the original multicast address. The AP checks its MGID table to see if it has a client for this multicast address. If the AP has no clients that have subscribed to the multicast, then the AP drops the packets. If the AP has clients that have subscribed to the multicast address, then the AP sets the AID of that client in the DTIM section of the beacon. The AP then sends the packets down to the client immediately after the beacon. The AP cannot hold multicast packets longer than the DTIM value, the AP sends these packets even if the client is sleeping. This is different from unicast packets where the AP will hold onto them until the client requests the packets.

When multicast packets are sent from a wireless client, the wireless client sends the packets to the AP as unicast packets to the multicast address. The AP then sends these unicast packets through the CAPWAP tunnel to the controller. The controller then makes two copies. One copy is sent out to the locally connected LAN where the router will forward these packets to the RP. The second copy the controller encapsulates in its multicast address and sends these multicast packets to all the APs on that controller. The AP strips the controller multicast address and sees the original multicast address. The AP checks its MGID table to see if it has a client for this multicast address If the AP has no clients that have subscribed to the multicast then the AP drops the packets. If the AP has clients that have subscribed to the Multicast address, then the AP sets the AID of that client in the DTIM section of the beacon. The AP then sends the packets down to the client immediately after the beacon.

Troubleshooting Multicast issues

When troubleshooting multicast issues there are many settings you should look for in the Wireless controller and on your switches and routers. These settings are discussed in greater detail in my blog “Settings that can improve Multicast on Wireless Networks” which can be found here.

Most multicast issues are caused by not having PIM set up correctly on your network. Two ways you can troubleshoot this are; 1) check all the VLANs where the multicast traffic will traverse or 2.) check to see if the join messages are being recorded on your router and switches (if you have snooping enabled).

Protocol-Independent Multicast (PIM) needs to be enabled or set on all your VLANs where multicast traffic will traverse. The VLANs you need to enable it on are the management VLAN, AP management VLAN (if different from the management VLAN), AP VLAN, the VLAN of the sending device and the VLAN of the receiving device. The management VLANs are very important since the controller sends multicast packets to the APs using either the management VLAN, AP management VLAN or the AP VLAN. These VLANs are often overlooked. Checking all the interfaces and VLANs where traffic will traverse always seems to cause Network Admin the most trouble. They usually ask me to list out all the interfaces where PIM needs to be enabled. This is difficult for them but impossible for an outsider with no direct knowledge or access to their network. We always recommend opening a case with Cisco (or your wireless vendor) to help find all the VLANs and interfaces where the multicast traffic may traverse.

If you need to verify that join messages are making it through your network, you will need to SSH to all routers and switches in the path and make sure you see the join messages from the clients. If you get to a router or switch where you no longer see join messages, you probably have found the issue.

Another common issue is multicast packets getting stuck at the core. If the multicast packets are flowing over the core, make sure the VLAN the EtherChannel/Port Channel have PIM enabled on them. I have seen issues where one EtherChannel had PIM enabled the other EtherChannel did not. This caused one multicast session to work and the next multicast session to fail.

After checking the VLANS for PIM and checking to see if the messages are traversing your network you should check other settings including DTIM, TKIP, RP, IGMP snooping, multicast buffering and roaming.

DTIM should be set to 1 especially if you are using multicast for voice. You can check this either on the GUI of the controller, in the controller config file or a wireless capture. The DTIM will be in the beacons of the wireless capture under the Traffic Indication Map. If DTIM count is set to 0 then you are seeing a DTIM beacon if the DTIM period is set to 1 then the next beacon will also be a DTIM beacon (see below).

 

 

We all know TKIP has been deprecated but you may still have it lingering on your network, if you have TKIP and AES on the same SSID this will cause issues with multicast. You can check this either on the GUI of the controller, the controller config file or you can look at the beacons in a wireless capture. The capture below shows AES in the Pairwise Cipher suite list. If TKIP was enabled, you would see it in the same section.

 

Multiple Rendezvous Points (RP) can cause issues with multicast if you have devices on multiple VLANs and each VLAN is directing you to different RPs. This will cause your devices on one VLAN to get the multicast traffic and the other devices on a different VLAN will never get the multicast traffic. If you have two RPs, they must communicate and update each other or better yet have one RP for the entire network.

If you are having issue with delivery of multicast packets you should verify you have IGMP snooping set on your switches. To verify if IGMP snooping is enabled on the switch run the command show ip igmp snooping

If it is not enabled on the switch you should set it globally, then you can set it per VLAN. Enabling it globally, run the command ip igmp snooping enable to enable it on a VLAN 2 you would use the command ip igmp snooping vlan 2 enable

If you have higher DTIM values (which causes the AP to hold on to multicast traffic for longer periods of time) and your network experiences a high volume of multicast traffic, you may experience issues where some multicast packets are being dropped. This may be caused by your multicast buffers being over-run. Each radio can handle 50 Multicast packets at any given time. These are shared equally across all SSIDs. You can verify the Multicast buffers by running the command show controller dot11radio0 | begin –\ In-Prog . You can enable a WLAN to use more than 50 using the command config wlan multicast buffer enable (30-60) <wlan-id> Increasing the Multicast buffer size will allow the AP to hold on to more multicast packets.

Roaming can cause issues with multicast especially if your client doesn’t send a new join message after each roam. This issue is more common with Autonomous APs or even cloud based AP. Cisco controllers will prompt the client to send a join request or they will send a new join message on behalf of the client after a roam. In Autonomous APs or even cloud based APs if a new join message is not sent and you have igmp snooping enabled on your switches the multicast packets might get delivered to the wrong AP.

.

Troubleshooting using Multicast Hammer

Multicast Hammer is a free tool that can help you troubleshoot multicast issues. You can download Multicast Hammer installer and setup instructions from the internet. It is a simple program to use and install that was created by Nortel (maybe the only thing left of Nortel in use these days). Multicast Hammer simulates multicast traffic on your network. It is a valuable tool that will remove questions about certain applications and show you raw multicast traffic (or lack of it) on your network. You should run MC Hammer on two machines that are on the same WLAN. Set one up as a server and one as a client. Run the test to see if data is flowing between machines. Once you see data you should move one of the machines to another network and retest. This will help you find out where multicast traffic is being blocked.

I won’t go through the install of Multicast Hammer (or as Vocera has dubbed it MC Hammer….can’t touch this!!!!) but I wanted to show you two screen shots one set up as server and one and client.

Server

The set-up is rather simple. You want to make sure you have the server radio button selected, choose your network interface, and then choose an available Multicast address (not the MC address on your controller). After you hit start on the server and client you will see data flowing. If you only see the data on the server then you know multicast traffic is being blocked somewhere on your network.

Client Side

The set-up is rather simple. You want to make sure you have the Client radio button selected, choose your network interface and then choose the same Multicast address as you chose in the server side of the application. After you hit start on the server and client you will see data flowing. If you only see the data on the server then you know multicast traffic is being blocked somewhere on your network.

 

Cisco Multicast Troubleshooting commands

show ip pim interface

show ip mroute

show ip igmp

Debug bcast * enable

Debug ip igmp

show ip igmp groups

On the controller verify multicast mode

show network

On the controller check L2 and L3 MGIDs

show network multicast mgid summary

show network multicast mgid detail <mgid>

 

Links to Multicast articles, videos and blogs

https://www.youtube.com/watch?v=Gjt2L9jAYNA From Kevin Wallace Cisco Multicast Routing for CCNA, CCNP, and CCIE candidates

https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/7-4/vocera_config_guide/vocera_config_guide/vocera_config_guide_chapter_01011.pdf Cisco guide on how to configure multicast for Vocera

https://supportforums.cisco.com/document/56511/multicast-and-wireless-lan-controller-wlc This is from Stephen Rodriquez 7 years ago but still good info in there.

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus1000/sw/4_0/troubleshooting/configuration/guide/n1000v_troubleshooting/trouble_14mcast.html

https://community.cisco.com/t5/wireless-mobility-documents/understanding-multicast-in-unified-wireless-networks/ta-p/3125021 This a power point Understanding Multicast in Unified Wireless Networks by Jeff Keown Cisco Wireless TAC

http://www.labminutes.com/wl0025_wlc_multicast_videostream_1 excellent video on setting up your controller for multicast.

https://mrncciew.com/2012/11/17/configuring-multicast-on-wlc/ everything on mrc-cciew site is awesome and I learn a ton every time I visit

Thank you for reading this blog. I hope reading this blog gives you more insight into multicast on a Cisco network. Please leave comments and continue this discussion on Twitter and Slack. If you haven’t followed me on Twitter please use this link to follow me @wifi_nc

 

 

 

Settings that can improve Multicast on a Wireless Network

This is a two-part blog; this blog will go over settings that will improve multicast on your network. The next blog “Multicast on a Cisco Wireless Network” will go over more troubleshooting on a Cisco Network.

I work for a wireless communications vendor that uses multicast as an integral part of the product. There are certain settings on the wireless/wired network that when implemented will improve multicast.

The issues and settings we will discuss in the blog are; enabling multicast on the router and VLANs, Which VLANs do you enable PIM on, the purpose of DTIM and Beacon settings, basic and mandatory data rates, issues using TKIP and AES on the same SSID, Issues with Rendezvous Points (RP), IGMP snooping on switches, roaming issues with multicast and lastly issues with multicast buffers.

 

Enabling Multicast on the Router and VLANs

To enable multicast routing on your routers and VLANs you will need two commands. The first command you need to issue on your router (or layer 3 switches) is ip multicast-routing. The second command is the pim command (Protocol Independent Multicast), which enables multicast routing on your VLANs. You can set this command as dense-mode, sparse-mode or sparse-dense-mode. Dense mode is good to use if you have a small network, but the PIM sparse-dense-mode will allow the router to use both sparse-mode and dense-mode. The differences between sparse-mode and dense-mode center around Rendezvous Points and how multicast traffic and multicast routes are updated in the network. Most networks I work with use sparse-dense-mode. The command pim sparse-dense mode needs to be issued on all the VLANs where you want multicast traffic to flow.

 

Which VLANs do you enable PIM on?

This is the million-dollar question that can cause a lot of confusion. The VLANs you need to enable multicast on using the pim sparse-dense mode are the management VLAN, AP management VLAN (if different from the management VLAN), AP VLAN (if different from the management VLAN) and all the VLANs of the sending and receiving devices. The management VLANs are very important since the controller sends multicast packets to the APs using either the management VLAN or the AP Management VLAN.

If the multicast packets are flowing over the core, make sure the VLAN of the EtherChannel/Port Channel have PIM enabled on them if they are different from the management VLAN. I have seen issues where one EtherChannel had PIM enabled the other EtherChannel did not. This caused one multicast session to work and the next multicast session to fail.

 

 

 

The purpose of DTIM and Beacon settings

If you are using multicast to deliver voice packets you must set the DTIM to 1 and the beacons to 100ms. These settings tell the AP how often to set either a Traffic Indication Map (TIM) information element or a Delivery Traffic Indication Map (DTIM) information element inside the beacon. There are no TIM or DTIM beacons per se. There are only Information Elements inside the beacon (but for ease of use I will use terms TIM beacons and DTIM beacons). The TIM beacon will tell the client if the AP has unicast packets buffered for that client. The DTIM beacons will tell the clients they have multicast packets about to be delivered (as well as unicast packets buffered). If you set the DTIM to a higher value to either 2 or 3 then the AP will only deliver the multicast packets to the clients every 200ms or 300ms. Most VOIP clients will have between a 90ms and 150ms buffer. If the client gets multicast packets every 200 or 300ms then, the user will hear choppy audio.

 

Some device manufacturers want you to set the DTIM to the higher value, giving the devices more time to sleep. If the devices know the DTIM will only come every 200, 300 msec or more then the client device can sleep that much longer. This saves battery life and is somewhat of a valid concern but when Voice is being delivered over multicast packets the DTIM needs to be set to 1 or the user will hear choppy audio. I said this is somewhat of a valid concern but the fact of the matter it is not mandatory that the client wakes up every DTIM. Before adjusting your DTIM check with your device manufacturer to see if the device wakes up for every DTIM, you might be presently surprised to find the devices don’t wake up every beacon. Some devices will stay asleep longer than the DTIM. You can test this by pinging the device. While the device is idle ping it. You may find the device only responds every 500msec or so (or maybe longer). You can then ping the device while on an active call and see how often it responds and then compare the two values.

 

 

Basic and Mandatory Data rates

Cisco recommends using 2 basic data rates 12Mbps and 24Mbps. When you have two basic data rates set, management traffic will go out at the lower data rate, but Cisco will send multicast traffic at the higher data rate. This can cause issues for clients that have rate shifted down to 12Mps or lower. If the AP is sending multicast traffic out at 24 Mbps and the client is only able to receive at 12 Mbps your client may miss multicast packets. This will be very difficult to troubleshoot since some devices will get the multicast packets and others will not receive them. Trying to replicate the issue would prove difficult. I would always recommend setting only one Basic Data rate to help offset this issue.

 

 

Issues with using TKIP and AES on the same SSID

I have seen issues with multicast where TKIP and AES are enabled on the same SSID. When you have both enabled on the same SSID the AP must send multicast packets out using TKIP. If your clients are using AES they will have issues decrypting the multicast packets. Hopefully, everyone is using AES instead of TKIP (especially since TKIP has been deprecated) but if you need TKIP then it is better to have only one encryption per SSID. Of course, I strongly recommend only using AES.

 

 

Issues with Rendezvous point (RP)

There are two ways you can use Rendezvous points. You can assign a router as a Rendezvous point or you can let the network assign one for you. You can program multiple RPs on your network, but this may cause issues. When a client sends a join message routers in the path will create a (*,G) entry on the interface so the router knows what interface has clients that have subscribed to this multicast address. These join messages will eventually make it to the RP. When the RP gets this join message, it will build a path back to each client/network segment.

If you have multiple RPs you may find an issue where one client may get the multicast traffic and one client will not. This happens if two devices are on different VLANs and each device sends the join message to a different RP. In this case, one device or network segment will get the multicast traffic and one will not. Having multiple RPs might be by design but if you have multiple RPs you should ensure that the RPs share information of all related VLANs.

 

 

IGMP snooping on switches

When IGMP snooping is enabled on a switch, the switch can send the multicast packets out the right interfaces. When a switch sees a normal packet, it will look up the MAC address in its CAM table, if the switch has the MAC address of the client it can forward these packets the right interface. If the MAC address is not in the CAM table, it will flood the packet out all interfaces. The same is true of the multicast packets if the switch has IGMP snooping enabled. The switch will keep track of all the join messages sent by clients/APs. The switch then records what interfaces need the multicast packets. When a multicast packet is sent to the switch it looks in its table and sends the traffic only to those interfaces that need the packets. This cuts down on multicast packets flooding the network. If the switch does not have information on a multicast address, then the switch will send the packets out on all ports.

 

Roaming issues with Multicast

When the clients roam from AP to AP the controller will request the device to send a new join message, so the controller, AP, router, and the network knows the client has moved APs. If your clients fail to send a new join message, the controller will not update its MGID table and the new AP that your client is connected to may drop multicast traffic because the AP will not know there are any clients who have subscribed to these multicast groups.

This can be a real issue in an Autonomous network or even cloud based where the client doesn’t send a join message on a roam and there is no controller to request a new join message. If your client doesn’t send a new join message you will lose Multicast session as you roam. The fix this I would contact the device manufacture to see if there is a firmware update that will fix this issue.

 

 

Issues with Multicast Buffer

The multicast buffer is shared across all BSSIDs on the AP. If there are a high number of SSIDs on your network, you may experience issues where the buffers fill up and the AP starts dumping multicasts packets. This may cause choppy audio on your multicast sessions. If your SSIDs have a higher DTIM value, the APs/Controllers will need to store packets for a longer period.  When you are experiencing issues with multicast traffic you may need to increase your multicast buffer size and then limit which WLANs can use this buffer. Multicast traffic is often crucial to voice clients and other clients/WLANs may never use multicast packets. If you limit which WLANs can use the multicast buffer there will be available space for applications that have a critical need for multicast.

 

It is important to note that the AP can only buffer multicast packets for the length of the DTIM value. When this value has expired the AP will inform the clients and immediately send the multicast packets whether the client is listening or not. This, of course, is different from the way unicast frames are delivered. If the AP has unicast frames for the client, the AP will set the clients AID in the Partial Virtual Bitmap. The AP will buffer these frames until the client wakes up and requests these frames.

 

 

Links to Multicast articles, videos, and blogs

 

https://www.youtube.com/watch?v=Gjt2L9jAYNA From Kevin Wallace   Cisco Multicast Routing for CCNA, CCNP, and CCIE candidates

 

https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/7-4/vocera_config_guide/vocera_config_guide/vocera_config_guide_chapter_01011.pdf Cisco guide on how to configure multicast for Vocera

 

https://supportforums.cisco.com/document/56511/multicast-and-wireless-lan-controller-wlc This is from Stephen Rodriquez 7 years ago but still good info in there.

 

 

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus1000/sw/4_0/troubleshooting/configuration/guide/n1000v_troubleshooting/trouble_14mcast.html

 

https://community.cisco.com/t5/wireless-mobility-documents/understanding-multicast-in-unified-wireless-networks/ta-p/3125021  This a power point Understanding Multicast in Unified Wireless Networks by Jeff Keown Cisco Wireless TAC

 

http://www.labminutes.com/wl0025_wlc_multicast_videostream_1   excellent video on setting up your controller for multicast.

 

 

Thank you for reading this blog. I hope reading this blog gives you more insight into multicast and the settings needed on your network. Please leave comments and continue this discussion on Twitter and Slack. If you haven’t followed me on Twitter I am at @wifi_nc. Stay tuned for my next blog in this series called “Multicast on a Cisco Wireless Network”. That blog will go into more details and troubleshooting multicast on Cisco Networks.