Dell FCX624-I Laptop User Manual


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728 PowerConnect B-Series FCX Configuration Guide
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Overview of IP multicasting
25
IPv4 multicast group addresses
In IPv4 Multicast, host groups are identified by Class D addresses, i.e., those with “1110” as their
higher-order four bits. In Internet standard "dotted decimal" notation, these group addresses range
from 224.0.0.0 to 239.255.255.255. However, the IANA IPv4 Multicast Address Registry
(referencing RFC 3171) stipulates that the range 224.0.0.0 through 224.0.0.255 should not be
used for regular multicasting applications.
“The range of addresses between 224.0.0.0 and 224.0.0.255, inclusive, is reserved for the use of
routing protocols and other low-level topology discovery or maintenance protocols, such as gateway
discovery and group membership reporting. Multicast routers should not forward any multicast
datagram with destination addresses in this range, regardless of its TTL.”
Mapping of IPv4 Multicast group addresses to
Ethernet MAC addresses
The IANA owns a block of Ethernet MAC addresses for Multicast usage that are in the range
0100.5e00.0000 through 0100.5e7F.FFFF. For a given IPv4 Multicast group, there is a simple way
of obtaining the appropriate Ethernet Destination MAC address that must be used in Layer 2
encapsulation. This is defined in RFC 1112, as follows:
“An IP host group address is mapped to an Ethernet multicast address by placing the low-order
23-bits of the IP address into the low-order 23 bits of the Ethernet multicast address
01-00-5E-00-00-00 (hex). Because there are 28 significant bits in an IP host group address, more
than one host group address may map to the same Ethernet multicast address.”
NOTE
Since there are 5 bits in the IPv4 Group address that are not used in the mapping, there is a
possibility for up to 32 IPv4 Multicast Groups to use the same Ethernet Destination MAC address.
Taking this into account along with the reserved IPv4 Group address range, it is discouraged for
applications to use IPv4 Multicast Group Addresses that may conflict with the reserved addresses
at the Layer 2 level. This is because some devices may use just the Ethernet Destination MAC
address to take actions on the packet.
Supported Layer 3 multicast routing protocols
Layer 3 Switches support t multicast routing protocol- Protocol-Independent Multicast (PIM)
protocol along with the Internet Group Membership Protocol (IGMP).
PIM is broadcast and pruning multicast protocols that deliver IP multicast datagrams. The
protocols employ reverse path lookup check and pruning to allow source-specific multicast delivery
trees to reach all group members. PIM build a different multicast tree for each source and
destination host group.
NOTE
Both DVMRP and PIM can concurrently operate on different ports of a Layer 3 Switch.