We had previously discussed here about Multicast PIM-Sparse Mode . That article has covered almost everything about Multicast in its more usual form. But, this intend to be a CCNP Wireless subject and as such, where is the Wireless part of it ? Well, I decided to divide this subject in two parts and here we will take a close look in the Wireless part of Multicast.
Actually, the Wireless part is not so large as Multicast in general, but, it has some very interesting theory.
Sadly we are not going to have any practical experience. I am still having problems in getting a LAB running for Wireless. Although a Wireless Lan Controller is relatively simple to acquire due Virtual WLC, it is not possible to virtualize an Access Point and I am not willing to afford a Cisco AP for now.
I have the possibility to use a LAB inside the company I work for, but, I don´t really believe it is too much important for now. Even though I like to much to see packets going back and forth, it is perfectly possible get a very good grasp through the theory.
Going to the point, Multicast over wireless is not something very common. I hope to find an opportunity to handle such thing in a project out there, but, so far, it is just theory for me.
But, as Wireless technology becomes more and more faster, it is more and more common to see applications like voice and video over wireless. Consequently, multicast will very important in those scenarios.
Starting up with the way multicast behaves over Wireless, lets say that Multicast is not enable by default in a Cisco Wireless Lan Controller. It is right to affirm that packets coming from wired network will be dropped if they are multicast packet. Although multicast packet coming from the Wireless network will be perfectly forwarded to the wired network. This dont make any sense but it is the way Cisco organized things in a default Wireless deployment. The reason for that is beyond this article´s scope.
By clicking on "Enable Global Multicast Mode" this WLC will be Multicast ready and will forward multicast traffic back and forth. Once multicast is enable, you need to choose between two mode. It can be Unicast or Multicast modes. By choosing Unicast mode you are saying to the WLC to transform each multicast packet coming from the Wired network in a unicast packet , encapsulates it into a CAPWAP protocol and send them to each AP. APs then take each packet, decapsulate it and send to the client. As you can see, this process consumes a lot of bandwidth and CPU process. It is even not supported for some WLC models. But, Cisco keeps this mode available in the event of the network between WLC and APs does not support multicast at all. I mean, if the switches connecting WLC and APs only support unicast packet, this mode can allow for multicast between WLC and APs.
In the same window, by choosing Multicast mode, it will be requested to inform an IP Multicast Address and Cisco recommends from the range 239.129.0.0 to 239.255.255.255.
This IP will be used between WLC and APs. Now, when an AP joins on the WLC, it will be joining to this Multicast group. For now on, the behavior of the WLC related to multicast will change drasticaly. Every time it receives a multicast packet, it will encapsulate the multicast packet into a CAPWAP packet and send it once. All APs will receive the multicast as a CAPWAP packet, decapsulate it and then, forward to the clients. This new approach saves a lot of time and resource.
Take care to dont mess it up. We are talking about two multicast group here right ? One multicast group is formed between WLC and APs, as explained earlier. Here represented by the IP 239.129.1.5. But, you may notice that inside the packet encapsulated by the WLC, there is another multicast packet, here represented by the IP 239.130.0.1. After all, the whole process is about client inside a wired or wireless network trying to exchange multicast packet. The reasons are beyond the scope here.
Something happens here. According to the multicast group in which the client belongs to, WLC will create Multicast Group Identifier (MGID). MGID associates client VLAN, client MAC address in a common database for a specific group.
We can have as many MGID as we have clients in differents multicast group. Just to make it clear, if we had 50% of clients belonging to a multicast group and the other 50% to a second multicast group, WLC will create two MGID.
Another point that needs to be discussed is about IGMP Snooping. IGMP Snooping is disable by default and it causes a specific behavior. By keeping it disabled, clients from Wireless side is able to be seeing by uplink routers on the wired side.It is unecessary explain the problems related to this scenario.
By enabling IGMP Snooping, WLC takes care about everything. Flow coming from wired and/or wireless side, will be stoped at the WLC. WLC starts to send its own multicast packets to uplink router using its dynamic interface as a source. WLC become the client from the uplink router point of view.
It is very easy to realize that this scenario is far more organized and secure than the previous one.
This kind of scenario is reason for many many question. It is very important to understand and memorize this flow.
Well, as stated before, MGID is related to the VLAN. It turns out that some WLAN is configured with interface group. Each interface is part of different VLANs. In a normal behavior, if clients belonging to different VLANs, WLC should send the same packet as many times as the number of VLANs. To avoid this behavior, you must enable "Multicast VLAN" feature.
There is a situation related to QoS and Multicast. QoS will subject of another article. I promess go deeper on this because it is very important.
For now, lets discuss some specific situation where QoS may interfere with Multicast. It turns out that Multicast packet is sent at the highest mandatory data rate. Lets consider that 24 Mbps is a mandatory datarate for a specific cell. Therefore, client may be on the edge of the cell and as such is not able to demodulate packet at 24 Mbps. That´s why is it important set up mandatory data rates according to the lower speed of the cell. In voice environment, it causes an issue called "one way audio".
The contrary may happen as well. A client positioned close to the AP may sand multicast data at 100 Mbps to AP but will receive at the mandatory datarate. As video quality degrade at lower speed, we can have problem as well. Cisco allows overcome this issue by enabling VideoStream.
The last situation we are going to discuss here is about roam and multicast. There are two possible scenario. WLC in the same VLAN and in different VLAN.
In the first scenario, everything occurs seamless. When a client moves to another WLC, all the information related to that client is moved to the other WLC.
In the second scenario, things happens just like assynchronous traffic flow. The WLC that generate the MGID keeps it as yours and forwards the traffic to the "anchor' WLC.
One picture is worth a thousand words.
That´s all for now. Multicast over wireless, as mentioned above, is not too much used. But in case of find some deployment out there, we have a good start here.
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