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Cisco AVVID Network Infrastructure IP Multicast Design - page 49 / 98





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Chapter 3

IP Multicast in a Wireless LAN

Other Considerations

Step 5

Enable the debug for the high-rate stream.

ROUTER#debug ip igmp

Step 6

On the PC, open the IP/TV viewer and request the program associated with the group.

The following ACL console message should appear on ROUTER showing that the traffic was denied (number of packets may vary):

1w1d: %SEC-6-IPACCESSLOGS: list IPMC-BRIDGE denied 1 packet

The following debug entry should appear for the discarded IGMP join attempt:

1w1d: IGMP(0): Discard report at boundary (FastEthernet0/1) for

There should be no multicast state active for the

Other Considerations

The following additional considerations apply to deploying IP multicast in a WLAN environment:

The WLAN LAN extension via EAP and WLAN static WEP solutions can support multicast traffic on the WLAN; the WLAN LAN extension via IPSec solution cannot.

The WLAN has an 11 Mbps available bit rate that must be shared by all clients of an AP. If the AP is configured to operate at multiple bit-rates, multicasts and broadcasts are sent at the lowest rate to ensure that all clients receive them. This reduces the available throughput of the network because traffic must queue behind traffic that is being clocked out at a slower rate.

WLAN clients can roam from one AP to another seamlessly within the same subnet. If roaming multicast is to be supported, Cisco Group Management Protocol (CGMP) and/or Internet Group Management Protocol (IGMP) snooping must be turned off for the port to which the AP is connected because a multicast user roaming from one AP to another is roaming from one switch port to another. The new switch port might not have this stream setup and it has no reliable way of determining the required multicast stream.

Multicast and broadcast from the AP are sent without requiring link-layer acknowledgement. Every unicast packet is acknowledged and retransmitted if unacknowledged. The purpose of the acknowledgement is to overcome the inherent unreliable nature of wireless links. Broadcasts and multicasts are unacknowledged due to the difficulty in managing and scaling the acknowledgements. This means that a network that is seen as operating well for unicast applications, can experience degraded performance in multicast applications.

Enterprise customers who are using WLAN in laptops would normally use (Constant Awake Mode) CAM as the Power-Save Mode. If delay-sensitive multicast traffic is being sent over the WLAN, customers should ensure that only the CAM configuration is used on their WLAN clients. Based on the 802.11 standard, if the client is in power-save mode, then the AP will buffer broadcast and multicast traffic until the next beacon period that contains a delivery traffic information map (DTIM) transmission. The default period is 200ms. Enterprises that use WLAN on small handheld devices will most likely need to use the WLAN power-save features (Max or Fast) and should not attempt to run delay-sensitive multicast traffic over the same WLAN.

Cisco AVVID Network Infrastructure IP Multicast Design



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