With development of networks on the
Internet, more and more interaction services such as data, voice, and video
services are running on the networks. In addition, highly bandwidth- and
time-critical services, such as e-commerce, Web conference, online auction,
video on demand (VoD), and tele-education have come into being. These services
have higher requirements for information security, legal use of paid services,
and network bandwidth.
In the network,
packets are sent in three modes: unicast, broadcast and multicast. The
following sections describe and compare data interaction processes in unicast,
broadcast, and multicast.
In unicast, the system establishes a
separate data transmission channel for each user requiring this information, and
sends a separate copy of the information to the user, as shown in Figure 1-1:
Figure 1-1 Information transmission in
the unicast mode
Assume that Hosts B, D and E need this
information. The source server establishes transmission channels for the
devices of these users respectively. As the transmitted traffic over the
network is in direct proportion to the number of users that receive this
information, when a large number of users need this information, the server
must send many pieces of information with the same content to the users.
Therefore, the limited bandwidth becomes the bottleneck in information transmission.
This shows that unicast is not good for the transmission of a great deal of
information.
1.1.2 Information
Transmission in the Broadcast Mode
When you adopt
broadcast, the system transmits information to all users on a network. Any user
on the network can receive the information, no matter the information is needed
or not. Figure 1-2 shows information transmission in broadcast mode.
Figure 1-2 Information transmission in
the broadcast mode
Assume that Hosts B, D, and E need the
information. The source server broadcasts this information through routers, and
Hosts A and C on the network also receive this information.
As we can see from the information transmission
process, the security and legal use of paid service cannot be guaranteed. In
addition, when only a small number of users on the same network need the
information, the utilization ratio of the network resources is very low and the
bandwidth resources are greatly wasted.
Therefore,
broadcast is disadvantageous in transmitting data to specific users; moreover,
broadcast occupies large bandwidth.
As described in the previous sections,
unicast is suitable for networks with sparsely distributed users, whereas
broadcast is suitable for networks with densely distributed users. When the
number of users requiring information is not certain, unicast and broadcast
deliver a low efficiency.
Multicast solves this problem. When some users on a network require
specified information, the multicast information sender (namely, the multicast
source) sends the information only once. With multicast distribution trees
established for multicast data packets through multicast routing protocols, the
packets are duplicated and distributed at the nearest nodes, as shown in Figure 1-3:
Figure 1-3 Information transmission in
the multicast mode
Assume that Hosts B, D and E need the
information. To transmit the information to the right users, it is necessary to
group Hosts B, D and E into a receiver set. The routers on the network
duplicate and distribute the information based on the distribution of the
receivers in this set. Finally, the information is correctly delivered to Hosts
B, D, and E.
The advantages of multicast over unicast
are as follows:
l
No matter how many receivers exist, there is
only one copy of the same multicast data flow on each link.
l
With the multicast mode used to transmit information,
an increase of the number of users does not add to the network burden
remarkably.
The advantages of multicast over broadcast
are as follows:
l
A multicast data flow can be sent only to the
receiver that requires the data.
l
Multicast brings no waste of network resources
and makes proper use of bandwidth.
The following roles are involved in
multicast transmission:
l
An information sender is referred to as a multicast
source (“Source” in Figure 1-3).
l
Each receiver is a multicast group member (“Receiver”
in Figure 1-3).
l
All receivers interested in the same information
form a multicast group. Multicast groups are not subject to geographic
restrictions.
l
A router that supports Layer 3 multicast is
called multicast router or Layer 3 multicast device. In addition to providing
multicast routing, a multicast router can also manage multicast group members.
For a better understanding of the multicast
concept, you can assimilate multicast transmission to the transmission of TV
programs, as shown in Table
1-1.
Table 1-1 An
analogy between TV transmission and multicast transmission
|
Step
|
TV transmission
|
Multicast transmission
|
|
1
|
A TV station transmits a TV program
through a television channel.
|
A multicast source sends multicast data
to a multicast group.
|
|
2
|
A user tunes the TV set to the channel.
|
A receiver joins the multicast group.
|
|
3
|
The user starts to watch the TV program
transmitted by the TV station via the channel.
|
The receiver starts to receive the
multicast data that the source sends to the multicast group.
|
|
4
|
The user turns off the TV set.
|
The receiver leaves the multicast group.
|
l
A multicast source does not necessarily belong
to a multicast group. Namely, a multicast source is not necessarily a multicast
data receiver.
l
A multicast source can send data to multiple
multicast groups at the same time, and multiple multicast sources can send data
to the same multicast group at the same time.
1.1.5 Advantages and Applications of
Multicast
I. Advantages of multicast
Advantages of multicast include:
l
Enhanced efficiency: Multicast decreases network
traffic and reduces server load and CPU load.
l
Optimal performance: Multicast reduces redundant
traffic.
l
Distributive application: Multicast makes
multiple-point application possible.
II. Application of multicast
The multicast technology effectively
addresses the issue of point-to-multipoint data transmission. By enabling
high-efficiency point-to-multipoint data transmission, over an IP network,
multicast greatly saves network bandwidth and reduces network load.
Multicast provides the following
applications:
l
Applications of multimedia and flow media, such
as Web TV, Web radio, and real-time video/audio conferencing.
l
Communication for training and cooperative
operations, such as remote education.
l
Database and financial applications (stock), and
so on.
l
Any point-to-multiple-point data application.
Based on the multicast source processing
modes, there are three multicast models:
l
Any-Source Multicast (ASM)
l
Source-Filtered Multicast (SFM)
l
Source-Specific Multicast (SSM)
I. ASM model
In the ASM model, any sender can become a
multicast source and send information to a multicast group; numbers of
receivers can join a multicast group identified by a group address and obtain
multicast information addressed to that multicast group. In this model,
receivers are not aware of the position of a multicast source in advance.
However, they can join or leave the multicast group at any time.
II. SFM model
The SFM model is derived from the ASM model.
From the view of a sender, the two models have the same multicast group
membership architecture.
Functionally, the SFM model is an extension
of the ASM model. In the SFM model, the upper layer software checks the source
address of received multicast packets so as to permit or deny multicast traffic
from specific sources. Therefore, receivers can receive the multicast data from
only part of the multicast sources. From the view of a receiver, multicast
sources are not all valid: they are filtered.
III. SSM model
In the practical life, users may be
interested in the multicast data from only certain multicast sources. The SSM
model provides a transmission service that allows users to specify the
multicast sources they are interested in at the client side.
The radical difference between the SSM
model and the ASM model is that in the SSM model, receivers already know the
locations of the multicast sources by some means. In addition, the SSM model
uses a multicast address range that is different from that of the ASM model,
and dedicated multicast forwarding paths are established between receivers and
the specified multicast sources.
The purpose of IP multicast is to transmit information
from a multicast source to receivers in the multicast mode and to satisfy
information requirements of receivers. You should be concerned about:
l
Host registration: What receivers reside on the
network?
l
Technologies of discovering a multicast source:
Which multicast source should the receivers receive information from?
l
Multicast addressing mechanism: Where should the
multicast source transports information?
l
Multicast routing: How is information
transported?
IP multicast is a kind of peer-to-peer
service. Based on the protocol layer sequence from bottom to top, the multicast
mechanism contains addressing mechanism, host registration, multicast routing,
and multicast application:
l
Addressing mechanism: Information is sent from a
multicast source to a group of receivers through multicast addresses.
l
Host registration: A receiving host joins and
leaves a multicast group dynamically using the membership registration
mechanism.
l
Multicast routing: A router or switch transports
packets from a multicast source to receivers by building a multicast distribution
tree with multicast routes.
l
Multicast application: A multicast source must
support multicast applications, such as video conferencing. The TCP/IP protocol
suite must support the function of sending and receiving multicast information.
As receivers are multiple hosts in a
multicast group, you should be concerned about the following questions:
l
What destination should the information source
send the information to in the multicast mode?
l
How to select the destination address?
These questions are about multicast
addressing. To enable the communication between the information source and
members of a multicast group (a group of information receivers), network-layer
multicast addresses, namely, IP multicast addresses must be provided. In
addition, a technology must be available to map IP multicast addresses to
link-layer MAC multicast addresses. The following sections describe these two
types of multicast addresses:
I. IP multicast address
Internet Assigned Numbers Authority (IANA)
categorizes IP addresses into five classes: A, B, C, D, and E. Unicast packets
use IP addresses of Class A, B, and C based on network scales. Class D IP
addresses are used as destination addresses of multicast packets. Class D
address must not appear in the IP address field of a source IP address of IP
packets. Class E IP addresses are reserved for future use.
In unicast data transport, a data packet is
transported hop by hop from the source address to the destination address. In
an IP multicast environment, there are a group of destination addresses (called
group address), rather than one address. All the receivers join a group. Once
they join the group, the data sent to this group of addresses starts to be
transported to the receivers. All the members in this group can receive the
data packets. This group is a multicast group.
A multicast group has the following
characteristics:
l
The membership of a group is dynamic. A host can
join and leave a multicast group at any time.
l
A multicast group can be either permanent or
temporary.
l
A multicast group whose addresses are assigned
by IANA is a permanent multicast group. It is also called reserved multicast
group.
Note that:
l
The IP addresses of a permanent multicast group
keep unchanged, while the members of the group can be changed.
l
There can be any number of, or even zero,
members in a permanent multicast group.
l
Those IP multicast addresses not assigned to
permanent multicast groups can be used by temporary multicast groups.
Class D IP addresses range from 224.0.0.0
to 239.255.255.255. For details, see Table 1-2.
Table 1-2 Range
and description of Class D IP addresses
|
Class D address range
|
Description
|
|
224.0.0.0 to 224.0.0.255
|
Reserved multicast addresses (IP
addresses for permanent multicast groups). The IP address 224.0.0.0 is
reserved. Other IP addresses can be used by routing protocols.
|
|
224.0.1.0 to 231.255.255.255
233.0.0.0 to 238.255.255.255
|
Available any-source multicast (ASM)
multicast addresses (IP addresses for temporary groups). They are valid for
the entire network.
|
|
232.0.0.0 to 232.255.255.255
|
Available source-specific multicast (SSM)
multicast group addresses.
|
|
239.0.0.0 to 239.255.255.255
|
Administratively scoped multicast
addresses, which are for specific local use only.
|
As specified by
IANA, the IP addresses ranging from 224.0.0.0 to 224.0.0.255 are reserved for
network protocols on local networks. The following table lists commonly used
reserved IP multicast addresses:
Table 1-3
Reserved IP multicast addresses
|
Class D address range
|
Description
|
|
224.0.0.1
|
Address of all hosts
|
|
224.0.0.2
|
Address of all multicast routers
|
|
224.0.0.3
|
Unassigned
|
|
224.0.0.4
|
Distance vector multicast routing
protocol (DVMRP) routers
|
|
224.0.0.5
|
Open shortest path first (OSPF) routers
|
|
224.0.0.6
|
Open shortest path first designated
routers (OSPF DR)
|
|
224.0.0.7
|
Shared tree routers
|
|
224.0.0.8
|
Shared tree hosts
|
|
224.0.0.9
|
RIP-2 routers
|
|
224.0.0.11
|
Mobile agents
|
|
224.0.0.12
|
DHCP server/relay agent
|
|
224.0.0.13
|
All protocol independent multicast (PIM) routers
|
|
224.0.0.14
|
Resource reservation protocol (RSVP)
encapsulation
|
|
224.0.0.15
|
All core-based tree (CBT) routers
|
|
224.0.0.16
|
The specified subnetwork bandwidth
management (SBM)
|
|
224.0.0.17
|
All SBMS
|
|
224.0.0.18
|
Virtual router redundancy protocol (VRRP)
|
|
224.0.0.19 to 224.0.0.255
|
Other protocols
|
Like having reserved the private network segment 10.0.0.0/8 for
unicast, IANA has also reserved the network segment 239.0.0.0/8 for multicast.
These are administratively scoped addresses. With the administratively scoped
addresses, you can define the range of multicast domains flexibly to isolate IP
addresses between different multicast domains, so that the same multicast
address can be used in different multicast domains without causing collisions.
II. Ethernet multicast MAC address
When a unicast IP packet is transported in
an Ethernet network, the destination MAC address is the MAC address of the
receiver. When a multicast packet is transported in an Ethernet network, a
multicast MAC address is used as the destination address because the
destination is a group with an uncertain number of members.
As stipulated by IANA, the high-order 24
bits of a multicast MAC address are 0x01005e, while the low-order 23 bits of a
MAC address are the low-order 23 bits of the multicast IP address. Figure 1-4 describes
the mapping relationship:
Figure 1-4 Multicast address mapping
The high-order four bits of the IP multicast address are 1110,
representing the multicast ID. Only 23 bits of the remaining 28 bits are mapped
to a MAC address. Thus, five bits of the multicast IP address are lost. As a
result, 32 IP multicast addresses are mapped to the same MAC address.
l
Generally, we refer to IP multicast working at
the network layer as Layer 3 multicast and the corresponding multicast
protocols as Layer 3 multicast protocols, which include IGMP, PIM, and MSDP; we
refer to IP multicast working at the data link layer as Layer 2 multicast and
the corresponding multicast protocols as Layer 2 multicast protocols, which
include IGMP Snooping.
l
This section provides only general descriptions
about applications and functions of the Layer 2 and Layer 3 multicast protocols
in a network. For details about these protocols, refer to the related chapters
of this manual.
I. Layer 3 multicast protocols
Layer 3 multicast protocols include
multicast group management protocols and multicast routing protocols. Figure 1-5 describes
where these multicast protocols are in a network.
Figure 1-5 Positions of Layer 3
multicast protocols
1)
Multicast management protocols
Typically, the Internet Group Management
Protocol (IGMP) is used between hosts and Layer 3 multicast devices directly
connected with the hosts. These protocols define the mechanism of establishing
and maintaining group memberships between hosts and Layer 3 multicast devices.
2)
Multicast routing protocols
A multicast
routing protocol runs on Layer 3 multicast devices to establish and maintain
multicast routes and forward multicast packets correctly and efficiently. Multicast
routes constitute a loop-free data transmission path from a data source to
multiple receivers, namely a multicast distribution tree.
In the ASM model, multicast routes come in
intra-domain routes and inter-domain routes.
l
An intra-domain multicast routing protocol is
used to discover multicast sources and build multicast distribution trees within
an autonomous system (AS) so as to deliver multicast data to receivers. Among a
variety of mature intra-domain multicast routing protocols, protocol
independent multicast (PIM) is a popular one. Based on the forwarding
mechanism, PIM comes in two modes – dense mode (often referred to as PIM-DM) and sparse mode (often referred
to as PIM-SM).
l
An inter-domain multicast routing protocol is
used for delivery of multicast information between two ASs. So far, mature
solutions include multicast source discovery protocol (MSDP).
For the SSM model, multicast routes are not
divided into inter-domain routes and intra-domain routes. Since receivers know
the position of the multicast source, channels established through PIM-SM are
sufficient for multicast information transport.
II. Layer 2 multicast protocols
Layer 2 multicast protocols include IGMP
Snooping and multicast VLAN. Figure
1-6 shows where these protocols are in the network.
Figure 1-6 Positions of Layer 2
multicast protocols
Running on Layer 2 devices, Internet Group
Management Protocol Snooping (IGMP Snooping) are multicast constraining
mechanisms that manage and control multicast groups by listening to and
analyzing IGMP messages exchanged between the hosts and Layer 3 multicast
devices, thus effectively controlling the flooding of multicast data in a Layer
2 network.
1.4 Multicast Packet Forwarding Mechanism
In a multicast model, a multicast source
sends information to the host group identified by the multicast group address
in the destination address field of the IP packets. Therefore, to deliver
multicast packets to receivers located in different parts of the network,
multicast routers on the forwarding path usually need to forward multicast
packets received on one incoming interface to multiple outgoing interfaces.
Compared with a unicast model, a multicast model is more complex in the
following aspects.
l
In the network, multicast packet transmission is
based on the guidance of the multicast forwarding table derived from the
unicast routing table or the multicast routing table specially provided for
multicast.
l
To process the same multicast information from
different peers received on different interfaces of the same device, every
multicast packet is subject to a reverse path forwarding (RPF) check on the
incoming interface. The result of the RPF check determines whether the packet
will be forwarded or discarded. The RPF check mechanism is the basis for most
multicast routing protocols to implement multicast forwarding.
The RPF mechanism enables multicast devices
to forward multicast packets correctly based on the multicast route
configuration. In addition, the RPF mechanism also helps avoid data loops
caused by various reasons.
Upon receiving a multicast packet that a
multicast source S sends to a multicast group G, the multicast device first
searches its multicast forwarding table:
1)
If the corresponding (S, G) entry exists, and
the interface on which the packet actually arrived is the incoming interface in
the multicast forwarding table, the router forwards the packet to all the
outgoing interfaces.
2)
If the corresponding (S, G) entry exists, but
the interface on which the packet actually arrived is not the incoming
interface in the multicast forwarding table, the multicast packet is subject to
an RPF check.
l
If the result of the RPF check shows that the
RPF interface is the incoming interface of the existing (S, G) entry, this
means that the (S, G) entry is correct but the packet arrived from a wrong path
and is to be discarded.
l
If the result of the RPF check shows that the
RPF interface is not the incoming interface of the existing (S, G) entry, this
means that the (S, G) entry is no longer valid. The router replaces the
incoming interface of the (S, G) entry with the interface on which the packet
actually arrived and forwards the packet to all the outgoing interfaces.
3)
If no corresponding (S, G) entry exists in the
multicast forwarding table, the packet is also subject to an RPF check. The
router creates an (S, G) entry based on the relevant routing information and
using the RPF interface as the incoming interface, and installs the entry into
the multicast forwarding table.
l
If the interface on which the packet actually
arrived is the RPF interface, the RPF check is successful and the router
forwards the packet to all the outgoing interfaces.
l
If the interface on which the packet actually
arrived is not the RPF interface, the RPF check fails and the router discards
the packet.
The basis for an RPF check is a unicast
route. A unicast routing table contains the shortest path to each destination
subnet. A multicast routing protocol does not independently maintain any type
of unicast route; instead, it relies on the existing unicast routing
information in creating multicast routing entries.
When performing an RPF check, a router
searches its unicast routing table. The specific process is as follows: The
router automatically chooses an optimal unicast route by searching its unicast
routing table, using the IP address of the “packet source” as the
destination address. The outgoing interface in the corresponding routing entry
is the RPF interface and the next hop is the RPF neighbor. The router considers
the path along which the packet from the RPF neighbor arrived on the RPF
interface to be the shortest path that leads back to the source.
Assume that unicast routes exist in the
network, as shown in Figure
1-7. Multicast packets travel along the SPT from the multicast
source to the receivers.
Figure 1-7 RPF check process
l
A multicast packet from Source arrives to VLAN-interface
1 of Switch C, and the corresponding forwarding entry does not exist in the
multicast forwarding table of Switch C. Switch C performs an RPF check, and
finds in its unicast routing table that the outgoing interface to
192.168.0.0/24 is VLAN-interface 2. This means that the interface on which the
packet actually arrived is not the RPF interface. The RPF check fails and the
packet is discarded.
l
A multicast packet from Source arrives to VLAN-interface
2 of Switch C, and the corresponding forwarding entry does not exist in the
multicast forwarding table of Switch C. The router performs an RPF check, and
finds in its unicast routing table that the outgoing interface to
192.168.0.0/24 is the interface on which the packet actually arrived. The RPF
check succeeds and the packet is forwarded.
Internet Group Management Protocol Snooping
(IGMP Snooping) is a multicast constraining mechanism that runs on Layer 2
devices to manage and control multicast groups.
By analyzing received IGMP messages, a
Layer 2 device running IGMP Snooping establishes mappings between ports and
multicast MAC addresses and forwards multicast data based on these mappings.
As shown in Figure 2-1, when IGMP Snooping is not
running on the switch, multicast packets are broadcast to all devices at Layer
2. When IGMP Snooping is running on the switch, multicast packets for known
multicast groups are multicast to the receivers, rather than broadcast to all
hosts, at Layer 2. However, multicast packets for unknown multicast groups are still
broadcast at Layer 2.
Figure 2-1 Before and after IGMP
Snooping is enabled on Layer 2 device
I. IGMP Snooping related ports
As shown in Figure 2-2, Router A connects to the
multicast source, IGMP Snooping runs on Switch A and Switch B, Host A and Host
C are receiver hosts (namely, multicast group members).
Figure 2-2 IGMP Snooping related ports
Ports involved in IGMP Snooping, as shown
in Figure 2-2,
are described as follows:
l
Router port: A router port is a port on the
Layer 3 multicast device (DR or IGMP querier) side of the Ethernet switch. In
the figure, Ethernet 1/0/1 of Switch A and Ethernet 1/0/1 of Switch B are
router ports. A switch registers all its local router ports in its router port
list.
l
Member port: A member port is a port on the
multicast group member side of the Ethernet switch. In the figure, Ethernet
1/0/2 and Ethernet 1/0/3 of Switch A and Ethernet 1/0/2 of Switch B are member
ports. The switch records all member ports on the local device in the IGMP
Snooping forwarding table.
II. Port aging timers in IGMP
Snooping and related messages and actions
Table 2-1
Port aging timers in IGMP Snooping and related
messages and actions
|
Timer
|
Description
|
Message before expiry
|
Action after expiry
|
|
Router port aging timer
|
For each router port, the switch sets a
timer initialized to the aging time of the route port
|
IGMP general query or PIM hello
|
The switch removes this port from its
router port list
|
|
Member port aging timer
|
When a port joins a multicast group, the
switch sets a timer for the port, which is initialized to the member port
aging time
|
IGMP membership report
|
The switch removes this port from the
multicast group forwarding table
|
A switch running IGMP Snooping performs
different actions when it receives different IGMP messages, as follows:
I. When receiving a general query
The IGMP querier periodically sends IGMP
general queries to all hosts and routers on the local subnet to find out
whether active multicast group members exist on the subnet.
Upon receiving an IGMP general query, the
switch forwards it through all ports in the VLAN except the receiving port and
performs the following to the receiving port:
l
If the receiving port is a router port existing
in its router port list, the switch resets the aging timer of this router port.
l
If the receiving port is not a router port
existing in its router port list, the switch adds it into its router port list
and sets an aging timer for this router port.
II. When receiving a membership
report
A host sends an IGMP report to the
multicast router in the following circumstances:
l
Upon receiving an IGMP query, a multicast group
member host responds with an IGMP report.
l
When intended to join a multicast group, a host
sends an IGMP report to the multicast router to announce that it is interested
in the multicast information addressed to that group.
Upon receiving an IGMP report, the switch
forwards it through all the router ports in the VLAN, resolves the address of
the multicast group the host is interested in, and performs the following to
the receiving port:
l
If the port is already in the forwarding table,
the switch resets the member port aging timer of the port.
l
If the port is not in the forwarding table, the
switch installs an entry for this port in the forwarding table and starts the
member port aging timer of this port.
A switch will not forward an IGMP report through a non-router port
for the following reason: Due to the IGMP report suppression mechanism, if
member hosts of that multicast group still exist under non-router ports, the
hosts will stop sending reports when they receive the message, and this
prevents the switch from knowing if members of that multicast group are still
attached to these ports.
III. When receiving a leave
message
When an IGMPv1 host leaves a multicast
group, the host does not send an IGMP leave message, so the switch cannot know
immediately that the host has left the multicast group. However, as the host
stops sending IGMP reports as soon as it leaves a multicast group, the switch
deletes the forwarding entry for the member port corresponding to the host from
the forwarding table when its aging timer expires.
When an IGMPv2 or IGMPv3 host leaves a
multicast group, the host sends an IGMP leave message to the multicast router
to announce that it has leaf the multicast group.
Upon receiving an IGMP leave message on the
last member port, a switch forwards it out all router ports in the VLAN.
Because the switch does not know whether any other member hosts of that
multicast group still exists under the port to which the IGMP leave message
arrived, the switch does not immediately delete the forwarding entry
corresponding to that port from the forwarding table; instead, it resets the
aging timer of the member port.
Upon receiving the IGMP leave message from
a host, the IGMP querier resolves from the message the address of the multicast
group that the host just left and sends an IGMP group-specific query to that
multicast group through the port that received the leave message. Upon
receiving the IGMP group-specific query, a switch forwards it through all the
router ports in the VLAN and all member ports of that multicast group, and
performs the following to the receiving port:
l
If any IGMP report in response to the
group-specific query arrives to the member port before its aging timer expires,
this means that some other members of that multicast group still exist under
that port: the switch resets the aging timer of the member port.
l
If no IGMP report in response to the group-specific
query arrives to the member port before its aging timer expires as a response
to the IGMP group-specific query, this means that no members of that multicast
group still exist under the port: the switch deletes the forwarding entry
corresponding to the port from the forwarding table when the aging timer
expires.
Caution:
After an Ethernet switch enables IGMP Snooping, when it receives the
IGMP leave message sent by a host in a multicast group, it judges whether the
multicast group exists automatically. If the multicast group does not exist,
the switch drops this IGMP leave message.
The following table lists all the IGMP
Snooping configuration tasks:
Table 2-2
IGMP Snooping configuration tasks
