D-Link DES-7100 User Manual download pdf (Page 48)

DES-7000/DES-7100 Layer 2 Switch User’s Guide

Switch Management

• The TCP/IP stack of the operating systems at both the sending and receiving end must be

capable of sending and receiving multicast traffic. Most modern operating systems for servers

and workstations support multicasting.

• Intervening devices (Layer 3 and Layer 2) must be capable of supporting multicast discovery

and routing protocols.

Multicast Addressing

Multicast addresses do not identify individual hosts like Class A, B or C IP networks and multicast

address cannot appear as a source address. Multicasting uses group membership, employing a

address/port combination to define the members of a multicast group. A host may be a member of one

or several multicast groups, but each host must request and be granted membership in the group

before it will be allowed to receive multicast data.

The Class D IP address range is assigned to network devices that comprise a multicast group. The

four most significant four bits of a Class D address are set to "1110". The following 28 bits are referred

to as the 'multicast group ID'. Some Class D address groups are registered with the Internet Assigned

Numbers Authority (IANA) for special purposes. For example, the block of multicast addresses ranging

from 224.0.0.1 to 224.0.0.225 is reserved for use by routing protocols and some other low-level

topology discovery and maintenance protocols. A full listing of multicast addresses and other useful

information can be found at http://www.iana.org/assignments/multicast-addresses.

Multicast Groups

There are three types of IP v4 addresses: unicast, broadcast, and multicast. Unicast addresses are

used to transmit messages from a single network device to another, single network device. Broadcast

packets are sent to all devices on the subnet. Multicast defines a group of network devices or hosts

that will receive the multicast packets. The members of this group are not necessarily on the same

subnet or VLAN. Specially designated multicast addresses are used to send multicast packets to the

group members. The Ethernet multicast destination address is a function of a portion of the multicast

IP address (within the reserved range) and the MAC address of the recipient.

Multicast groups can be administered manually using the Static Multicast Forwarding table (see page

66).

IGMP

Internet Group Management Protocol (IGMP) is a Layer 3 protocol used by multicast recipient hosts to

communicate multicast group membership information to local (or nearest) routers. Receiving hosts

use IGMP to indicate to their desire to join a multicast group with a membership report (one per group),

the routers in turn periodically send a membership query to establish whether any hosts are still

interested in receiving an active group. If a router receives no reply after three consecutive

membership queries, the router can stop the transmission of the group to the LAN, and prune itself

from a multicast routing tree.

A later version of IGMP (IGMPv2) allows hosts to indicate a desire to leave a multicast group by

sending a leave group message. A leave message instructs the multicasting agent and (and the

Switch if IGMP Snooping is enabled) to discontinue transmission of multicast data, without having to

wait for a timeout. This can save resources, particularly if hosts are involved in frequent group

changing (e.g. an application that “changes channels”, selecting from various video/audio streams).

IGMP Snooping

Even though IGMP is a Layer 3 function, the Switch is capable of inspecting IGMP packets that pass

through it. The ability to examine IGMP information from multicast source and destination systems (i.e.

group membership reports, leave group messages and group membership queries) enables much

more efficient management of multicast data. Without IGMP snooping it would be necessary to

broadcast all multicast traffic to all ports and all VLANs. This would cause sever bandwidth problems

on networks that are simultaneously running just a few multicast applications. When IGMP snooping is

enabled (IGMP snooping is disabled by default on the DES-7000 and DES-7100), the Switch makes

delivery decisions on multicast traffic, matching multicast groups to Switch ports and eliminating the

need to broadcast. In order to use IGMP Snooping it must first be enabled for the entire Switch using

the Advanced Settings (see page 52) and then enabled for individual VLANs (see IGMP Snooping

Settings on page 60).

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D-Link DES-7100 User Manual Page 48

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