Cisco Cisco Catalyst 6000 Multilayer Switch Feature Card MSFC2 Bulletins
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Therefore, depending on the platform, the controller would need to generate up to 300 copies of each multicast packet. This mechanism is
inefficient, placing a large processing burden on the controller and flooding the network with a large number of packets.
Release 3.2 introduces a more efficient way of delivering multicast traffic from the controller to the access points. Instead of unicasting each
multicast packet over the LWAPP tunnel to each access point, an LWAPP multicast group is used to deliver the multicast packet to each access
point. This works as follows:
1.
When an access point boots up, it issues an Internet Group Management Protocol (IGMP) join request to join the LWAPP multicast group
2.
When the controller receives a multicast packet, it transmits the packet to the LWAPP multicast group via the management interface
3.
The network delivers the multicast packet to each of the access points that have joined the LWAPP multicast group, replicating the packet along
the way as needed (Figure 6)
the way as needed (Figure 6)
Figure 6. Enhanced Multicast Forwarding Mechanism
In order for this to work, the network between the controller and the access points must be multicast-enabled. To accommodate networks that do not
support multicast, the controller will continue to support the original multicast forwarding mechanism. The system administrator, through a
configuration option, will be able to choose between the two mechanisms.
Notes on Multicast Performance
•
To use the multicast performance enhancements, the wireless LAN controller must operate in Layer 3 LWAPP Mode.
•
If the network between the controller and access points consists of multiple Layer 3 subnets, a multicast routing protocol such as Protocol-
Independent Multicast (PIM) must be enabled.
•
The multicast address used for the LWAPP multicast group is configurable on the system. The controller downloads the LWAPP multicast group
address to the LWAPP access points during the join process. If the administrator changes the multicast group, the access points are informed of
the change, and will leave the old group address and join the new group address.
the change, and will leave the old group address and join the new group address.
•
All controllers in the same cluster (mobility group) must use the same multicast address for the LWAPP multicast group.
•
All multicast packets are sent out at the lowest QoS level.
•
Multicast traffic on a WLAN is transmitted by every access point, even if that access point has no clients associated with it that have requested the
multicast traffic (i.e. no IGMP snooping).
•
The access points use IGMPv1 to join the LWAPP multicast group.
•
In a mobility event (a client moves from an access point on their anchor controller to an access point on a foreign controller), the client can only
receive multicast packets if one of the following happens: