Configuring Multicast Forwarding
Multicast Forwarding
Cisco Small Business 300 Series Managed Switch Administration Guide 149
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An IGMP Querier is required to facilitate the IGMP protocol on a given subnet. In
general, a multicast router is also a IGMP Querier. When there are multiple IGMP
Queriers in a subnet, the queriers elect a single querier as the primary querier.
The Sx300 can be configured to be an IGMP Querier as a backup querier, or in
situation where a regular IGMP Querier does not exist. The Sx300 is not a full
capability IGMP Querier.
If the switch is enabled as a IGMP Querier, it starts after 60 seconds have passed
with no IGMP traffic (queries) detected from a Multicast router. In the presence of
other IGMP Queriers, the switch might (or might not) stop sending queries, based
on the results of the standard querier selection process.
Multicast Address Properties
Multicast addresses have the following properties:
• Each IPv4 Multicast address is in the address range 224.0.0.0 to
239.255.255.255.
• The IPv6 Multicast address is FF00:/8.
• To map an IP multicast group address to an Layer 2 Multicast address:
- 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 which are used are identical. For
example, 234.129.2.3 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.
- For IPv6, this is mapped by taking the 32 low order bits of the Multicast
address, and adding them with 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.
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