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15
Defining Multicast Properties
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To map an IP Multicast group address to an Layer 2 Multicast address:
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For IPv4, this is mapped by taking the 23 low-order bits from the IPv4
address, and adding them to the 01:00:5e prefix. By standard, the upper
nine bits of the IP address are ignored, and any IP addresses that only
differ in the value of these upper bits are mapped to the same Layer 2
address, since the lower 23 bits that are used are identical. For example,
234. 1 29.2.3 is mapped to a MAC Multicast group address
01:00:5e:01:02:03. Up to 32 IP Multicast group addresses can be
mapped to the same Layer 2 address.
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For IPv6, this is mapped by taking the 32 low-order bits of the Multicast
address, and adding the prefix of 33:33. For example, the IPv6 Multicast
address FF00:1122:3344 is mapped to Layer 2 Multicast
33:33:11:22:33:44.
The Properties page enables you to configure the Bridge Multicast filtering status.
By default, all Multicast frames are flooded to all ports of the VLAN. To selectively
forward only to relevant ports and filter (drop) the Multicast on the rest of the ports,
enable Bridge Multicast filtering status in the Properties page.
If filtering is enabled, Multicast frames are forwarded to a subset of the ports in the
relevant VLAN as defined in the Multicast Forwarding Data Base. Multicast filtering
is enforced on all traffic. By default, such traffic is flooded to all relevant ports, but
you can limit forwarding to a smaller subset.
A common way of representing Multicast membership is the (S,G) notation where
S is the (single) source sending a Multicast stream of data, and G is the IPv4 or IPv6
group address. If a Multicast client can receive Multicast traffic from any source of
a specific Multicast group, this is saved as (*,G).
The following are ways of forwarding Multicast frames:
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MAC Group Address—Based on the destination MAC address in the
Ethernet frame.
As mentioned before, one or more IP Multicast group addresses
NOTE
can be mapped to a MAC group address. Forwarding, based on the MAC
group address, can result in an IP Multicast stream being forwarded to
ports that have no receiver for the stream.
Cisco Small Business 200 Series Smart Switch Administration Guide
Multicast
Defining Multicast Properties
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