3Com Router User Manual

							VII
MULTICAST
Chapter 33IP Multicast
Chapter 34Configuring IGMP
Chapter 35Configuring PIM-DM
Chapter 36Configuring PIM-SM 
						

							498 
						

							33
IP MULTICAST
This chapter covers the following topics:
■IP Multicast Overview
■IP Multicast Addresses
■IP Multicast Features
■IP Multicast Routing Protocols
■IP Multicast Packet Forwarding
■IP Multicast Application
IP Multicast OverviewWhen the destination addresses carrying information (data, voice, and video) 
transmit with only a few subscribers in the network, multiple transmission 
methods such as unicast and broadcast can be employed. Unicast transmission 
means establishing a separate data transmission channel for each subscriber, while 
broadcast transmissionmeans sending the message to all the subscribers in the 
network no matter whether they need it or not. If 200 subscribers in network 
require receiving the same message, traditionally there are two solutions for this. 
One is to send such message 200 times to ensure that all the subscribers are able 
to get it. The other one is to transmit the data within the whole network to enable 
subscribers to get the necessary data directly from the network by adopting the 
broadcast method.
Using the unicast method to transmit to 200 subscribers results in wasted 
bandwidth. Using the broadcast method risks information security and 
confidentiality. IP multicast technology solves both of these problems. The 
multicast source sends the information only once. The transmitted information is 
duplicated and distributed continuously at key network nodes. In this way, the 
information can be sent accurately and efficiently to each subscriber who requires 
it. 
In simple terms, IP multicast is a bandwidth-saving technology. It sends a single 
information flow to several receivers simultaneously to reduce network traffic. In 
case a router does not support multicast in the network, the router can employ 
the tunnel method to encapsulate the multicast packets in the unicast packets, 
and send them to the adjacent multicast router. Adjacent multicast routers drop 
the unicast IP header, and then continue the multicast transmission to avoid 
causing a change to the network structure. 
IP Multicast AddressesIP multicasting uses Class D addressing. Each multicast address stands for a 
multicast group, not for a host. Because the maximum four-digit number of a  
						

							500CHAPTER 33: IP MULTICAST
Class D address is 1110, the range of the multicast addresses is from 224.0.0.0 to 
239.255.255.255. 
The multicast group can be either permanent or temporary. The permanent group 
has a constant group address assigned by IANA, while the number of members in 
the group can be random, even zero. Temporary multicast groups can use that 
group address, which is not reserved, but the number of members in the 
temporary multicast group cannot be zero. 
The range and meaning of Class D address are as follows:
Ta b l e 574   Range and Meaning of Class D Addresses
The reserved multicast addresses, which are frequently used, are as follows: 
Ta b l e 575   List for Reserved Multicast Addresses
The multicast protocol changes the Class D address into the hardware/media 
address. For example, in an Ethernet MAC address, the range of the reserved 
corresponding Ethernet addresses that IANA obtains the IEEE-802 MAC is from 
01-00-5e-00-00-00 to 01-00-5E-ff-ff-ff.
IP Multicast FeaturesIn simple TCP/IP routing, the path of a data packet transmission is from the source 
address to the destination address following the principle of hop-by-hop. But in 
Class D address rangeMeaning
224.0.0.0 to 224.0.0.255Reserved multicast address (Permanent group 
address)
224.0.1.0 to 238.255.255.255Subscriber available multicast address (Temporary 
group address)
239.0.0.0 to 239.255.255.255The multicast address not managed or at specific 
locations
Class D address range Meaning
224.0.0.0Reference addresses (reserved)
224.0.0.1All systems on this subnet
224.0.0.2All routers on this subnet
224.0.0.3Not for distribution
224.0.0.4DVMRP routers
224.0.0.5OSPF routers
224.0.0.6OSPF DR
224.0.0.7ST routers
224.0.0.8ST hosts
224.0.0.9RIP-2 routers
224.0.0.11Active agents
224.0.0.12DHCP Server/trunk agent
224.0.0.13All the PIM routers
224.0.0.14RSVP encapsulation
224.0.0.15All the CBT routers
224.0.0.16Assigned SBM
224.0.0.17All the SBMS
224.0.0.18VRRP
………… 
						

							IP Multicast Routing Protocols501
the IP multicast environment, the destination address of a data packet is not one 
address but a group, forming a group address. All the information receivers are 
added to a group, and once they access the group, data flowing to the destination 
address begin to transmit to the receivers of that particular group. All the group 
members can receive the data packet. Therefore, to get the data packet, they have 
to become group members first. The data packet transmitter is not required to be 
a group member. In the multicast environment, data will be sent to all the group 
members, and the subscribers who are not group members will not receive the 
data packets.
Generally, IP multicast has the following features: 
■The membership of the host group is dynamic .There is no restriction on the 
location or the number of members in the host group. Independent hosts 
access or leave the multicast group at any time. These members can be 
anywhere on the Internet. One host can be a member of several multicast 
groups simultaneously. 
■One host can send data packets to a multicast group even though it is not a 
group member. When sending the message to all the IP hosts in a multicast 
group, it is necessary to send a message to the group address only, just like 
unicast.
■There is no need for the router to save the membership for all the hosts. It is 
only necessary to know whether there is any host belonging to a certain 
multicast group on the network segment. The physical interface is located on 
the network segment. The host can only save the multicast groups it has 
joined.
IP Multicast Routing 
ProtocolsThe multicast protocol includes two parts. One part is the Internet Group 
Management Protocol (IGMP) acting as the IP multicast basic signaling protocol. 
The other part includes the multicast routing protocols such as DVMRP, PIM-SM, 
PIM-DM, which implement IP multicast flow routing. 
Internet Group 
Management Protocol
(IGMP)IGMP is a simple protocol for the support of multicast transmission. IGMP is a 
simple leave/join protocol that allows end-user nodes and their multicast-enabled 
routers to exchange messages that describe the wishes of hosts to participate in 
multicast groups. It defines the multicast membership establishment and 
maintenance mechanism between hosts and routers, and it is the foundation of 
the entire IP multicast. 
IGMP informs routers about the group members, and enables routers to know the 
information about other members within the group through the hosts directly 
connected to them. Application programs can learn that information coming from 
one data source goes to a specific group. If a LAN subscriber announces that it has 
joined a certain multicast group via IGMP, the multicast routers in the LAN 
propagate this information by the multicast routing protocol, and finally add this 
LAN as a branch to the multicast tree. When the host, as a member of a certain 
group, begins to receive information, the routers periodically carry out queries on 
this group, and check whether the group members are still participating. As long 
as there is a host still participating, routers can continue to receive data. Only after 
all the subscribers in the LAN exit this multicast group, are the related branches 
deleted from the multicast tree.  
						

							502CHAPTER 33: IP MULTICAST
Multicast Routing 
ProtocolThe group address in the multicast protocol is a virtual address. Therefore, unlike 
unicast, data packets cannot be routed directly from the data source to the 
specific destination address. The multicast application program sends the data 
packet to a group of receivers instead of a single receiver . 
Multicast routing establishes a cyclic data transmission path from one data source 
end to multiple receiving ends. The task of the multicast routing protocol is to 
establish a distribution tree structure. The multicast routers can adopt many 
methods to establish a data transmission path distribution tree. Protocol 
Independent Multicast (PIM) is the protocol that allows multicast routers to 
identify other multicast routers that will receive the packets. Depending on actual 
network conditions, the multicast routing protocol can be divided into two kinds - 
dense mode and sparse mode. 
Protocol Independent 
Multicast--Dense Mode 
(PIM-DM))The dense mode of the multicast routing protocol is suitable for small networks 
with abundant bandwidth. Suppose that each subnet in the network has at least a 
pair of receiving sites interested in multicast. Therefore, multicast data packet are 
distributed to all the sites in the network. Together with this process there is 
consumption of the related resources (bandwidth and the CPU of the router). To 
decrease the consumption of these precious network resources, the dense mode 
of the multicast routing protocol “prunes” the branches that do not have 
multicast data forwarding, and retains only the branches that contain the receiving 
sites. 
To enable the receiving sites with the multicast forwarding demand in the pruned 
branches to receive multicast data flow, the pruned branches can return to 
forwarding state periodically. To reduce the time delay for the pruned branch to 
recover to the forwarding state, the dense mode of the multicast routing protocol 
adopts a grafting mechanism to actively add to the multicast distribution tree. This 
cyclic diffusion and pruning phenomenon is the feature of the dense mode of the 
multicast routing protocol. Generally, the data packet forwarding path in the 
dense mode is an “active tree” with the source being its root and the group 
members being its leaves. 
The typical routing protocol in the dense mode includes Protocol-Independent 
Multicast-Dense Mode (PIM-DM) and Distance Vector Multicast Routing Protocol 
(DVMRP). 
Protocol Independent 
Multicast-Sparse Mode 
(PIM-SM)Dense mode uses the flood-prune technology, which is not applicable for a WAN. 
In a WAN, multicast receivers are sparse and the sparse mode is used. In sparse 
mode, all hosts do not need to receive multicast packets unless there is an explicit 
request for the packets by default. A multicast router must send a join message to 
the rendezvous point (RP), which is created in the network as the virtual place for 
data exchange. The RP corresponds to the group that receives the multicast data 
traffic from the specified group. The join message passes routers and finally 
reaches the root, the RP. The path that the join message used becomes a branch of 
the shared tree. In PIM sparse mode, multicast packets are sent to the RP first and 
then are forwarded along the shared tree rooted at the RP and with members as 
the branches. To prevent the branches of the shared tree from being deleted 
because they are not updated, PIM sparse mode sends join messages to branches 
periodically to maintain the multicast distribution tree. 
						

							IP Multicast Packet Forwarding503
The transmitting end is first registered at the RP if it needs to send data to a 
specific address, and then sends the data to the RP. Once data reaches the RP, 
multicast data packets are duplicated and sent to receivers who are interested in 
getting them along the distribution tree path. The duplication only occurs at the 
crotch of the distribution tree. This process can automatically repeat until the data 
packets finally arrive at the destination point. 
IP Multicast Packet 
ForwardingIn the multicast model, the source host sends information to any host group 
represented by the multicast group addresses in the destination address segment 
of the IP information packet. In contrast to the unicast model, the multicast model 
cannot base forwarding decisions on the destination addresses contained in the 
information packet. Instead, it must forward the multicast information packet to 
multiple external interfaces to send it to all the receiving sites. Therefore, the 
multicast forwarding process is more complicated than the unicast forwarding 
process.
To guarantee that all the multicast information reaches routers by the shortest 
route, the multicast model must use the unicast routing table or the independent 
multicast routing table and check the multicast information packet receiving 
interfaces. This checking mechanism is the basis for most multicast routing 
protocols to carry out the multicast forwarding reverse path forwarding (RPF) 
check. The multicast module checks the source address in the received multicast 
data packet. If the active tree is adopted, this source address is that of the host 
sending the multicast data packet. If the shared tree is adopted, this source 
address is the root address of the shared tree. Thus, the multicast module can 
determine whether the input interface of the arrived data packet is on the shortest 
path from the receiving site to the source address. When the multicast data arrives 
at the router, if the examination has passed, the information packet is forwarded 
according to the multicast forwarding items. Otherwise, the information is 
discarded. 
IP Multicast 
ApplicationIP multicast allows the internal data of the company to be distributed to a large 
number of subscribers. For example, for a company with many chain stores, 
multicast can be used to send its price information to the cash register in each 
chain store. The real-time information can be sent to multicast subscribers by 
media over the Internet, such as the current remote employee management and 
education. 
The traditional data broadcast is based on the broadcast transmission form, which 
requires much Internet bandwidth. Using multicast technology, TV and wireless 
sites can not only multicast data to Internet subscribers who really need them, but 
can also reduce the cost of network maintenance to a large extent.  
						

							504CHAPTER 33: IP MULTICAST 
						

							34
CONFIGURING IGMP 
This chapter covers the following topics:
■IGMP Overview
■Configuring IGMP
■Displaying and Debugging IGMP
■IGMP Configuration Example
IGMP OverviewThe Internet Group Management Protocol (IGMP) is a protocol that is responsible 
for the IP multicast member management among the TCP/IP protocol family. It is 
the basis for IP multicast, and it is used to establish and maintain multicast 
membership between the IP hosts and the multicast routers directly adjacent to 
the hosts. IGMP does not include the propagation and maintenance of the 
membership relationship information between multicast routers, which is 
accomplished by each multicast routing protocol. IGMP operates on a physical 
network, such as a single Ethernet segment.
At present, IGMP Version 1 and IGMP Version 2 are extensively used. IGMP Version 
2 specifies the following three kinds of messages: 
■Membership Query Message: According to different group addresses, it can 
be classified into a general query message or a group-specific query message, 
used to learn if a particular group has any members attached on a network. For 
a group-specific query message, the router is used to check whether there is 
any subscriber in a connecting network who wants to make the query message 
valid, and the target group address must be zero or a valid multicast group 
address. IGMP Version 2 allows routers to send group-specific query messages.
■Membership Report Message: When the host receives a general query or a 
group-specific membership query message, it first identifies the combination 
with the interface sending the query message and sets a host group delay timer 
for each member group. If the remaining time of this timer is larger than the 
maximum response time set in the query message, it is changed to the 
maximum response time value. The host broadcasts the membership report to 
this router before the time runs out. Once the router receives the membership 
report, it adds the group to the membership list of the network it belongs to, 
and starts the group membership interval timer. If the router does not receive 
any membership report with the maximum query response timeout, it becomes 
clear that there is no local group member, and it does not transmit the received 
multicast message to the network it connects to.
■Leave Group Message: IGMP Version 2 allows a host to send a leave group 
message to all routers when it leaves a multicast group (the target group 
address is 224.0.0.2). 
						

							506CHAPTER 34: CONFIGURING IGMP
IGMP is asymmetric between hosts and routers. The host responds to the IGMP 
query message of the multicast router, and makes a response in the membership 
report message. The router periodically sends a general query message. Then it 
determines, based on the response message received, whether a specific group 
has a host access on its own subnet. Meanwhile, when a router exits from a 
group, it sends a message to the multicast router when it exits. When it receives 
the message, the multicast router sends a packet to inquire about the group to 
ensure that the member has already gone.
Configuring IGMPTo configure the IGMP protocol, the multicast routing function is first enabled, and 
then each feature of the IGMP protocol can be configured.
IGMP configuration includes tasks that are covered in the following sections: 
■Enabling Multicast Routing
■Configuring Router Interfaces as Group Members
■Configuring the Version Number of IGMP at the Router Interface
■Configuring the Time Interval of IGMP Host Sending Query Messages 
■Configuring IGMP Maximum Query Response Time 
■Configuring Subnet Querier Survival Time
Enabling Multicast 
RoutingStart the IGMP protocol on all interfaces to enable routers to send multicast 
messages. Only after enabling multicast routing can all the other configurations 
related to the multicast be valid. 
Make the following configuration in system view. 
Ta b l e 576   Enable/disable Multicast Routing
By default, the system disables multicast routing. 
Configuring Router 
Interfaces as Group 
MembersConfiguring router interfaces as group members can not only enable routers to 
access the multicast group by simulating host behaviors, but also enables the static 
multicast group to access the multicast group.
Make the following configuration in the interface view.
Ta b l e 577   Configure Router Interfaces to be Group Members
By default, the router interface has no group member.
OperationCommand
Enable multicast routingmulticast routing-enable
Disable multicast routingundo multicast routing-enable
OperationCommand
Configure router interface to be group 
membersigmp host-join groups-address
Delete router interface from group 
membersundo igmp host-join groups-address