Multicast
Multicast Forwarding Overview
264 Cisco 350, 350X and 550X Series Managed Switches, Firmware Release 2.4, ver 0.4
15
The IGMP/MLD Snooping querier resumes sending general query messages if it does hear
another querier for the following interval:
Query passive interval = Robustness * Query Interval + 0.5*Query Response Interval.
NOTE It is recommended to disable IGMP/MLD Querier election mechanism if there is an IPM
Multicast router on the VLAN.
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 that 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 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.
IGMP/MLD Proxy
IGMP/MLD Proxy is a simple IP Multicast protocol.
Using IGMP/MLD Proxy to replicate Multicast traffic on devices, such as the edge boxes, can
greatly simplify the design and implementation of these devices. By not supporting more
complicated Multicast routing protocols, such as Protocol Independent Multicast (PIM) or
Distance Vector Multicast Routing Protocol (DVMRP), it reduces not only the cost of the
devices, but also the operational overhead. Another advantage is that it makes the proxy
devices independent of the Multicast routing protocol used by the core network routers.
Hence, proxy devices can be easily deployed in any Multicast network.
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