Cisco Small Business 200 Series Administration Manual page 212

Multicast
Multicast Forwarding
• 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.
IGMP/MLD Proxy Tree
IGMP/MLD Proxy works in a simple tree topology in which it is not necessary to run a robust Multicast
routing protocol (for example, PIM). It is sufficient to use a simple IPM Routing protocol based on learning
group membership information and proxy group membership information and forward Multicast packets
based upon that information.
The tree must be manually configured by designating upstream and downstream interfaces on each proxy
device. In addition, the IP addressing scheme applied to the proxying tree topology should be configured to
ensure that a proxy device can win the IGMP/MLD Querier election to be able to forward Multicast traffic.
There should be no other Multicast routers except the proxy devices within the tree, and the root of the tree
is expected to be connected to a wider Multicast infrastructure.
A proxy device performing IGMP/MLD-based forwarding has a single upstream interface and one or more
downstream interfaces. These designations are explicitly configured; there is no protocol to determine
what type each interface is. A proxy device performs the router portion of IGMP/MLD on its downstream
interfaces, and the host portion of IGMP/MLD on its upstream interface.
Only one tree is supported.
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