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ISCOM2600G-HI (A) Series Configuration Guide
Figure 8-3 Mapping between IPv4 multicast address and multicast MAC address
The first 4 bits of IP multicast address are 1110, indicating multicast identification. In the last
28 bits, only 23 bits are mapped to the multicast MAC address, and the missing of 5 bits
makes 32 IP multicast addresses mapped to the same multicast MAC address. Therefore, in
Layer 2, the ISCOM2600G-HI series switch may receive extra data besides IPv4 multicast
group, and these extra multicast data needs to be filtered by the upper layer on the
ISCOM2600G-HI series switch.
Basis of multicast protocol
To implement complete set of multicast services, you need to deploy a variety of multicast
protocols in various positions of network and make them cooperate with each other.
Typically, IP multicast working at network layer is called Layer 3 multicast, so the
corresponding multicast protocol is called Layer 3 multicast protocol, including Internet
Group Management Protocol (IGMP). IP multicast working at data link layer is called Layer 2
multicast, so the corresponding multicast protocol is called Layer 2 multicast protocol,
including Internet Group Management Protocol (IGMP) Snooping.
Figure 8-4 shows operating of IGMP and Layer 2 multicast features.
Figure 8-4 Operating of IGMP and Layer 2 multicast features
IGMP, a protocol in TCP/IP protocol suite, is responsible for managing IPv4 multicast
members. IGMP runs between the multicast router and host, defines the establishment and
maintenance mechanism of multicast group membership between hosts and the multicast
router. IGMP is not involved in transmission and maintenance of group membership between
multicast routers, which is completed by the multicast routing protocol.
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