Table of Contents

Chapter 1 Multicast Overview.. 1-1

1.1 Multicast Overview. 1-1

1.1.1 Information Transmission in the Unicast Mode. 1-1

1.1.2 Information Transmission in the Broadcast Mode. 1-2

1.1.3 Information Transmission in the Multicast Mode. 1-3

1.1.4 Roles in Multicast 1-4

1.1.5 Advantages and Applications of Multicast 1-5

1.2 Multicast Models. 1-6

1.3 Multicast Architecture. 1-7

1.3.1 Multicast Protocols. 1-10

1.4 Multicast Packet Forwarding Mechanism.. 1-12

1.4.1 Implementation of the RPF Mechanism.. 1-13

1.4.2 RPF Check. 1-13

Chapter 2 Common Multicast Configuration. 2-1

2.1 Common Multicast Configuration. 2-1

2.1.1 Configuring Suppression on the Multicast Source Port 2-1

2.1.2 Configuring a Multicast MAC Address Entry. 2-2

2.1.3 Configuring Dropping Unknown Multicast Packets. 2-3

2.2 Displaying Common Multicast Configuration. 2-4

Chapter 3 IGMP Snooping Configuration. 3-1

3.1 IGMP Snooping Overview. 3-1

3.1.1 Principle of IGMP Snooping. 3-1

3.1.2 Basic Concepts in IGMP Snooping. 3-2

3.1.3 Work Mechanism of IGMP Snooping. 3-3

3.2 IGMP Snooping Configuration Task List 3-5

3.2.1 Enabling IGMP Snooping. 3-6

3.2.2 Configuring the Version of IGMP Snooping. 3-6

3.2.3 Configuring Timers. 3-7

3.2.4 Configuring Fast Leave Processing. 3-8

3.2.5 Configuring a Multicast Group Filter 3-9

3.2.6 Configuring the Maximum Number of Multicast Groups on a Port 3-10

3.2.7 Configuring IGMP Snooping Querier 3-11

3.2.8 Suppressing Flooding of Unknown Multicast Traffic in a VLAN. 3-12

3.2.9 Configuring Static Member Port for a Multicast Group. 3-13

3.2.10 Configuring a Static Router Port 3-14

3.2.11 Configuring a Port as a Simulated Group Member 3-14

3.2.12 Configuring a VLAN Tag for Query Messages. 3-16

3.2.13 Configuring Multicast VLAN. 3-16

3.3 Displaying and Maintaining IGMP Snooping. 3-18

3.4 IGMP Snooping Configuration Examples. 3-19

3.4.1 Configuring IGMP Snooping. 3-19

3.4.2 Configuring Multicast VLAN. 3-21

3.5 Troubleshooting IGMP Snooping. 3-24

 

Chapter 1  Multicast Overview

 

 

1.1  Multicast Overview

With the development of 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.

1.1.1  Information Transmission in the Unicast Mode

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.

1.1.3  Information Transmission in the Multicast Mode

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.

1.1.4  Roles in Multicast

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.

 

 

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.

1.2  Multicast Models

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.

1.3  Multicast Architecture

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.

Multicast Address

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

 

 

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.

1.3.1  Multicast Protocols

 

 

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.

1.4.1  Implementation of the RPF Mechanism

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.

1.4.2  RPF Check

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.

 

Chapter 2  Common Multicast Configuration

 

 

2.1  Common Multicast Configuration

Table 2-1 Complete the following tasks to perform common multicast configurations:

Task	Remarks
Configuring Suppression on the Multicast Source Port	Optional
Configuring a Multicast MAC Address Entry	Optional
Configuring Dropping Unknown Multicast Packets	Optional

 

2.1.1  Configuring Suppression on the Multicast Source Port

Some users may deploy unauthorized multicast servers on the network. This affects the use of network bandwidth and transmission of multicast data of authorized users by taking network resources. You can configure multicast source port suppression on certain ports to prevent unauthorized multicast servers attached to these ports from sending multicast traffic to the network.

I. Configuring multicast source port suppression in system view

Follow these steps to configure multicast source port suppression in system view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure multicast source port suppression	multicast-source-deny [ interface interface-list ]	Optional Multicast source port suppression is disabled by default.

 

II. Configuring multicast source port suppression in Ethernet port view

Follow these steps to configure multicast source port suppression in Ethernet port view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Configure multicast source port suppression	multicast-source-deny	Optional Multicast source port suppression is disabled by default.

 

2.1.2  Configuring a Multicast MAC Address Entry

In Layer 2 multicast, the system can add multicast forwarding entries dynamically through a Layer 2 multicast protocol. Alternatively, you can statically bind a port to a multicast MAC address entry by configuring a multicast MAC address entry manually.

Generally, when receiving a multicast packet for a multicast group not yet registered on the switch, the switch will flood the packet within the VLAN to which the port belongs. You can configure a static multicast MAC address entry to avoid this.

Follow these steps to configure a multicast MAC address entry in system view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Create a multicast MAC address entry	mac-address multicast mac-address interface interface-list vlan vlan-id	Required The mac-address argument must be a multicast MAC address.

 

Follow these steps to configure a multicast MAC address entry in Ethernet port view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Create a multicast MAC address entry.	mac-address multicast mac-address vlan vlan-id	Required The mac-address argument must be a multicast MAC address.

 

 

2.1.3  Configuring Dropping Unknown Multicast Packets

Generally, if the multicast address of the multicast packet received on the switch is not registered on the local switch, the packet will be flooded in the VLAN. When the function of dropping unknown multicast packets is enabled, the switch will drop any multicast packets whose multicast address is not registered. Thus, the bandwidth is saved and the processing efficiency of the system is improved.

Follow these steps to configure dropping unknown multicast packet:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure dropping unknown multicast packets	unknown-multicast drop enable	Required By default, the function of dropping unknown multicast packets is disabled.

 

2.2  Displaying Common Multicast Configuration

Follow these commands to display common multicast configuration:

To do...	Use the command...	Remarks
Display the statistics information about multicast source port suppression	display multicast-source-deny [ interface interface-type [ interface-number ] ]	Available in any view
Display the created multicast MAC table entries	display mac-address multicast [ static { { { mac-address vlan vlan-id | vlan vlan-id } [ count ] } | count } ]	Available in any view

 

Chapter 3  IGMP Snooping Configuration

When configuring IGMP snooping, go to these sections for information you are interested in:

l           IGMP Snooping Overview

l           IGMP Snooping Configuration Task List

l           Displaying and Maintaining IGMP Snooping

l           IGMP Snooping Configuration Examples

l           Troubleshooting IGMP Snooping

 

 

3.1  IGMP Snooping Overview

Internet Group Management Protocol Snooping (IGMP Snooping) is a multicast constraining mechanism that runs on Layer 2 devices to manage and control multicast groups.

3.1.1  Principle of IGMP Snooping

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 3-1, 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 3-1 Before and after IGMP Snooping is enabled on Layer 2 device

3.1.2  Basic Concepts in IGMP Snooping

I. IGMP Snooping related ports

As shown in Figure 3-2, 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 3-2 IGMP Snooping related ports

Ports involved in IGMP Snooping, as shown in Figure 3-2, 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 3-1 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

 

3.1.3  Work Mechanism of IGMP Snooping

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.

 

 

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.

 

 

3.2  IGMP Snooping Configuration Task List

Complete the following tasks to configure IGMP Snooping:

Task	Remarks
Enabling IGMP Snooping	Required
Configuring the Version of IGMP Snooping	Optional
Configuring Timers	Optional
Configuring Fast Leave	Optional
Configuring a Multicast Group Filter	Optional
Configuring the Maximum Number of Multicast Groups on a Port	Optional
Configuring IGMP Snooping Querier	Optional
Suppressing Flooding	Optional
Configuring Static Member Port for a Multicast Group	Optional
Configuring a Static Router Port	Optional
Configuring a Port as a Simulated Group Member	Optional
Configuring a VLAN Tag for Query Message	Optional
Configuring Multicast VLAN	Optional

 

3.2.1  Enabling IGMP Snooping

Follow these steps to enable IGMP Snooping:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable IGMP Snooping globally	igmp-snooping enable	Required By default, IGMP Snooping is disabled globally.
Enter VLAN view	vlan vlan-id	—
Enable IGMP Snooping on the VLAN	igmp-snooping enable	Required By default, IGMP Snooping is disabled on all the VLANs.

 

 

3.2.2  Configuring the Version of IGMP Snooping

With the development of multicast technologies, IGMPv3 has found increasingly wide application. In IGMPv3, a host can not only join a specific multicast group but also explicitly specify to receive or reject the information from a specific multicast source. Working with PIM-SSM, IGMPv3 enables hosts to join specific multicast sources and groups directly, greatly simplifying multicast routing protocols and optimizing the network topology.

Follow these steps to configure the version of IGMP Snooping:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter VLAN view	vlan vlan-id	—
Configure the version of IGMP Snooping	igmp-snooping version version-number	Optional The default IGMP Snooping version is version 2.

 

 

3.2.3  Configuring Timers

This section describes how to configure the aging timer of the router port, the aging timer of the multicast member ports, and the query response timer.

Follow these steps to configure timers:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure the aging time of the router port	igmp-snooping router-aging-time seconds	Optional By default, the aging time of the router port is 105 seconds.
Configure the general query response timer	igmp-snooping max-response-time seconds	Optional By default, the general query response timeout time is 10 seconds.
Configure the aging time of the multicast member port	igmp-snooping host-aging-time seconds	Optional By default, the aging time of multicast member ports is 260 seconds

 

3.2.4  Configuring Fast Leave Processing

With fast leave processing enabled, when the switch receives an IGMP leave message on a port, the switch directly removes that port from the forwarding table entry for the specific group. If only one host is attached to the port, enable fast leave processing to improve bandwidth management.

I. Enabling fast leave processing in system view

Follow these steps to enable fast leave processing in system view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable fast leave processing	igmp-snooping fast-leave [ vlan vlan-list ]	Required By default, the fast leave processing feature is disabled.

 

II. Enabling fast leave processing in Ethernet port view

Follow these steps to enable fast leave processing in Ethernet view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Enable fast leave processing for specific VLANs	igmp-snooping fast-leave [ vlan vlan-list ]	Required By default, the fast leave processing feature is disabled.

 

 

3.2.5  Configuring a Multicast Group Filter

On an IGMP Snooping-enabled switch, the configuration of a multicast group allows the service provider to define restrictions on multicast programs available to different users.

In an actual application, when a user requests a multicast program, the user’s host initiates an IGMP report. Upon receiving this report message, the switch checks the report against the ACL rule configured on the receiving port. If the receiving port can join this multicast group, the switch adds this port to the IGMP Snooping multicast group list; otherwise the switch drops this report message. Any multicast data that has failed the ACL check will not be sent to this port. In this way, the service provider can control the VOD programs provided for multicast users.

Make sure that an ACL rule has been configured before configuring this feature.

I. Configuring a multicast group filter in system view

Follow these steps to configure a multicast group filter in system view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure a multicast group filter	igmp-snooping group-policy acl-number [ vlan vlan-list ]	Required No group filter is configured by default, namely hosts can join any multicast group.

 

II. Configuring a multicast group filter in Ethernet port view

Follow these steps to configure a multicast group filter in Ethernet port view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Configure a multicast group filter	igmp-snooping group-policy acl-number [ vlan vlan-list ]	Optional No group filter is configured by default, namely hosts can join any multicast group.

 

 

3.2.6  Configuring the Maximum Number of Multicast Groups on a Port

By configuring the maximum number of multicast groups that can be joined on a port, you can limit the number of multicast programs on-demand available to users, thus to regulate traffic on the port.

Follow these steps to configure the maximum number of multicast groups on a port:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Limit the number of multicast groups on a port	igmp-snooping group-limit limit [ vlan vlan-list [ overflow-replace ] ]	Required The maximum number of multicast groups on a port is 256 by default.

 

 

3.2.7  Configuring IGMP Snooping Querier

In an IP multicast network running IGMP, a multicast router is responsible for sending IGMP general queries, so that all Layer 3 multicast devices can establish and maintain multicast forwarding entries, thus to forward multicast traffic correctly at the network layer. This router or Layer 3 switch is called IGMP querier.

However, a Layer 2 multicast switch does not support IGMP, and therefore cannot send general queries by default. By enabling IGMP Snooping querier on a Layer 2 switch in a VLAN where multicast traffic needs to be Layer-2 switched only and no multicast routers are present, the Layer 2 switch will act as a querier to send IGMP general queries, thus allowing multicast forwarding entries to be established and maintained at the data link layer.

You can also configure the source address and interval of general queries to be sent from the IGMP Snooping querier.

Follow these steps to configure IGMP Snooping querier:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable IGMP Snooping	igmp-snooping enable	Required By default, IGMP Snooping is disabled.
Enter VLAN view	vlan vlan-id	—
Enable IGMP Snooping	igmp-snooping enable	Required
Enable IGMP Snooping querier	igmp-snooping querier	Required By  default, IGMP Snooping querier is disabled.
Configure the interval between IGMP general queries	igmp-snooping query-interval seconds	Optional By default, the interval between IGMP general queries is 60 seconds.
Configure the source IP address of IGMP general queries	igmp-snooping general-query source-ip { current-interface | ip-address }	Optional By default, the source IP address of IGMP general queries is 0.0.0.0.

 

3.2.8  Suppressing Flooding of Unknown Multicast Traffic in a VLAN

With IGMP Snooping enabled in a VLAN, multicast traffic for unknown multicast groups is flooded within the VLAN by default. This wastes network bandwidth and affects multicast forwarding efficiency.

With the unknown multicast flooding suppression function enabled, when receiving a multicast packet for an unknown multicast group, an IGMP Snooping switch creates a nonflooding entry and relays the packet to router ports only, instead of flooding the packet within the VLAN. If the switch has no router ports, it drops the multicast packet.

The difference between the IGMP Snooping non-flooding function and the function of dropping unknown multicast packets is in that the former passes unknown multicast packets to the router ports while the latter directly discards unknown multicast packets.

Follow these steps to suppress flooding of unknown multicast traffic in the VLAN:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable unknown multicast flooding suppression	igmp-snooping nonflooding-enable	Required By default, unknown multicast flooding suppression

 

 

3.2.9  Configuring Static Member Port for a Multicast Group

If the host connected to a port is interested in the multicast data for a specific group, you can configure that port as a static member port for that multicast group.

I. In Ethernet port view

Follow these steps to configure a static multicast group member port in Ethernet port view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Configure the current port as a static member port for a multicast group in a VLAN	multicast static-group group-address vlan vlan-id	Required By default, no port is configured as a static multicast group member port.

 

II. In VLAN interface view

Follow these steps to configure a static multicast group member port in VLAN interface view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter VLAN interface view	interface vlan-interface interface-number	—
Configure specified port(s) as static member port(s) of a multicast group in the VLAN	multicast static-group group-address interface interface-list	Required By default, no port is configured as a static multicast group member port.

 

 

3.2.10  Configuring a Static Router Port

In a network where the topology is unlikely to change, you can configure a port on the switch as a static router port, so that the switch has a static connection to a multicast router and receives IGMP messages from that router.

I. In Ethernet port view

Follow these steps to configure a static router port in Ethernet port view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Configure the current port as a static router port	multicast static-router-port vlan vlan-id	Required By default, no static router port is configured.

 

II. In VLAN view

Follow these steps to configure a static router port in VLAN view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter VLAN view	vlan vlan-id	—
Configure a specified port as a static router port	multicast static-router-port interface-type interface-number	Required By default, no static router port is configured.

 

3.2.11  Configuring a Port as a Simulated Group Member

Generally, hosts running IGMP respond to the IGMP query messages of the multicast switch. If hosts fail to respond for some reason, the multicast switch may consider that there is no member of the multicast group on the local subnet and remove the corresponding path.

To avoid this from happening, you can configure a port of the VLAN of the switch as a multicast group member. When the port receives IGMP query messages, the multicast switch will respond. As a result, the port of the VLAN can continue to receive multicast traffic.

Through this configuration, the following functions can be implemented:

l           When an Ethernet port is configured as a simulated member host, the switch sends an IGMP report through this port. Meanwhile, the switch sends the same IGMP report to itself and establishes a corresponding IGMP entry based on this report. 

l           When receiving an IGMP general query, the simulated host responds with an IGMP report. Meanwhile, the switch sends the same IGMP report to itself to ensure that the IGMP entry does not age out.

l           When the simulated joining function is disabled on an Ethernet port, the simulated host sends an IGMP leave message.

Therefore, to ensure that IGMP entries will not age out, the port must receive IGMP general queries periodically.

Follow these steps to configure a port as a simulated group member:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Configure the current port as a simulated multicast group member	igmp host-join group-address [source-ip source-address ] vlan vlan-id	Required Simulated joining is disabled by default.

 

 

3.2.12  Configuring a VLAN Tag for Query Messages

By configuring the VLAN tag carried in IGMP general and group-specific queries forwarded and sent by IGMP Snooping switches, you can enable multicast packet forwarding between different VLANs In a Layer-2 multicast network environment.

Follow these steps to configure VLAN tag for query message:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure a VLAN tag for query messages	igmp-snooping vlan-mapping vlan vlan-id	Required By default, the VLAN tag in IGMP general and group-specific query messages is not changed.

 

 

3.2.13  Configuring Multicast VLAN

In traditional multicast implementations, when users in different VLANs listen to the same multicast group, the multicast data is copied on the multicast router for each VLAN that contains receivers. This is a big waste of network bandwidth.

In an IGMP Snooping environment, by configuring a multicast VLAN and adding ports to the multicast VLAN, you can allow users in different VLANs to share the same multicast VLAN. This saves bandwidth because multicast streams are transmitted only within the multicast VLAN. In addition, because the multicast VLAN is isolated from user VLANs, this method also enhances the information security.

Multicast VLAN is mainly used in Layer 2 switching, but you must make the corresponding configurations on the Layer 3 switch.

Follow these steps to configure multicast VLAN on the Layer 3 switch:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Create a multicast VLAN and enter VLAN view	vlan vlan-id	—
Return to system view	quit	—
Enter VLAN interface view	interface Vlan-interface vlan-id	—
Enable IGMP	igmp enable	Required By default, the IGMP feature is disabled.
Return to system view	quit	—
Enter Ethernet port view for the Layer 2 switch to be configured	interface interface-type interface-number	—
Define the port as a trunk or hybrid port	port link-type { trunk | hybrid }	Required
Specify the VLANs to be allowed to pass the Ethernet port	port hybrid vlan vlan-id-list { tagged | untagged }	Required The multicast VLAN defined on the Layer 2 switch must be included, and the port must be configured to forward tagged packets for the multicast VLAN if the port type is hybrid.
	port trunk permit vlan vlan-list

 

Follow these steps to configure multicast VLAN on the Layer 2 switch:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable IGMP Snooping	igmp-snooping enable	—
Enter VLAN view	vlan vlan-id	—
Enable IGMP Snooping	igmp-snooping enable	Required
Enable multicast VLAN	service-type multicast	Required
Return to system view	quit	—
Enter Ethernet port view for the Layer 3 switch	interface interface-type interface-number	—
Define the port as a trunk or hybrid port	port link-type { trunk | hybrid }	Required
Specify the VLANs to be allowed to pass the Ethernet port	port hybrid vlan vlan-list { tagged | untagged }	Required The multicast VLAN must be included, and the port must be configured to forward tagged packets for the multicast VLAN if the port type is hybrid.
	port trunk permit vlan vlan-list
Enter Ethernet port view for a user device	interface interface-type interface-number	—
Define the port as a hybrid port	port link-type hybrid	Required
Specify the VLANs to be allowed to pass the port	port hybrid vlan vlan-id-list { tagged | untagged }	Required The multicast VLAN must be included, and the port must be configured to forward tagged packets for the multicast VLAN.

 

 

3.3  Displaying and Maintaining IGMP Snooping

To do...	Use the command…	Remarks
Display the current IGMP Snooping configuration	display igmp-snooping configuration	Available in any view
Display IGMP Snooping message statistics	display igmp-snooping statistics	Available in any view
Display the information about IP and MAC multicast groups in one or all VLANs	display igmp-snooping group [ vlan vlan-id ]	Available in any view
Clear IGMP Snooping statistics	reset igmp-snooping statistics	Available in user view

 

3.4  IGMP Snooping Configuration Examples

3.4.1  Configuring IGMP Snooping

I. Network requirements

To prevent multicast traffic from being flooded at Layer 2, enable IGMP snooping on Layer 2 switches.

l           As shown in Figure 3-3, Router A connects to a multicast source (Source) through Ethernet 1/0/2, and to Switch A through Ethernet 1/0/1.

l           Run PIM-DM and IGMP on Router A. Run IGMP snooping on Switch A. Router A acts as the IGMP querier.

l           The multicast source sends multicast data to the multicast group 224.1.1.1. Host A and Host B are receivers of the multicast group 224.1.1.1.

II. Network diagram

Figure 3-3 Network diagram for IGMP Snooping configuration

III. Configuration procedure

1)         Configure the IP address of each interface

Configure an IP address and subnet mask for each interface as per Figure 3-3. The detailed configuration steps are omitted.

2)         Configure Router A

# Enable IP multicast routing, enable PIM-DM on each interface, and enable IGMP on Ethernet 1/0/1.

<RouterA> system-view

[RouterA] multicast routing-enable

[RouterA] interface Ethernet 1/0/1

[RouterA-Ethernet1/0/1] igmp enable

[RouterA-Ethernet1/0/1] pim dm

[RouterA-Ethernet1/0/1] quit

[RouterA] interface Ethernet 1/0/2

[RouterA-Ethernet1/0/2] pim dm

[RouterA-Ethernet1/0/2] quit

3)         Configure Switch A

# Enable IGMP Snooping globally.

<SwitchA> system-view

[SwitchA] igmp-snooping enable

  Enable IGMP-Snooping ok.

# Create VLAN 100, assign Ethernet 1/0/1 through Ethernet 1/0/4 to this VLAN, and enable IGMP Snooping in the VLAN.

[SwitchA] vlan 100

[SwitchA-vlan100] port Ethernet 1/0/1 to Ethernet 1/0/4

[SwitchA-vlan100] igmp-snooping enable

[SwitchA-vlan100] quit

4)         Verify the configuration

# View the detailed information of the multicast group in VLAN 100 on Switch A.

<SwitchA> display igmp-snooping group vlan100

  Total 1 IP Group(s).

  Total 1 MAC Group(s).

 

  Vlan(id):100.

    Total 1 IP Group(s).

    Total 1 MAC Group(s).

    Static Router port(s):

    Dynamic Router port(s):

                     Ethernet1/0/1

    IP group(s):the following ip group(s) match to one mac group.

        IP group address: 224.1.1.1

        Static host port(s):

        Dynamic host port(s):

                     Ethernet1/0/3          Ethernet1/0/4

    MAC group(s):

        MAC group address: 0100-5e01-0101

        Host port(s): Ethernet1/0/3          Ethernet1/0/4

As shown above, the multicast group 224.1.1.1 has been registered on Switch A, with the dynamic router port Ethernet 1/0/1 and dynamic member ports Ethernet 1/0/3 and Ethernet 1/0/4. This means that Host A and Host B have joined the multicast group 224.1.1.1.

3.4.2  Configuring Multicast VLAN

I. Network requirements

As shown in Figure 3-4, Workstation is a multicast source. Switch A forwards multicast data from the multicast source. A Layer 2 switch, Switch B forwards the multicast data to the end users Host A and Host B.

Table 3-2 describes the network devices involved in this example and the configurations you should make on them.

Table 3-2 Network devices and their configurations

Device	Device description	Networking description
Switch A	Layer 3 switch	The interface IP address of VLAN 20 is 168.10.1.1. Ethernet 1/0/1 is connected to the workstation and belongs to VLAN 20. The interface IP address of VLAN 10 is 168.10.2.1. Ethernet 1/0/10 belongs to VLAN 10. Ethernet 1/0/10 is connected to Switch B.
Switch B	Layer 2 switch	l      VLAN 2 contains Ethernet 1/0/1 and VLAN 3 contains Ethernet 1/0/2. l      The default VLANs of Ethernet 1/0/1 and Ethernet 1/0/2 are VLAN 2 and VLAN 3 respectively. l      VLAN 10 contains Ethernet 1/0/10, Ethernet 1/0/1, and Ethernet 1/0/2. Ethernet 1/0/10 is connected to Switch A. l      VLAN 10 is a multicast VLAN. l      Ethernet 1/0/1 sends untagged packets for VLAN 2 and VLAN 10. l      Ethernet 1/0/2 sends untagged packets for VLAN 3 and VLAN 10.
Host A	User 1	Host A is connected to Ethernet 1/0/1 on Switch B.
Host B	User 2	Host B is connected to Ethernet 1/0/2 on Switch B.

 

In this configuration example, you need to configure the ports that connect Switch A and Switch B to each other as hybrid ports. The following text describes the configuration details. You can also configure these ports as trunk ports. The configuration procedure is omitted here. For details, see Configuring Multicast VLAN.

Configure a multicast VLAN, so that users in VLAN 2 and VLAN 3 can receive multicast streams through the multicast VLAN.

II. Network diagram

Figure 3-4 Network diagram for multicast VLAN configuration

III. Configuration procedure

The following configuration is based on the prerequisite that the devices are properly connected and all the required IP addresses are already configured.

1)         Configure Switch A:

# Set the interface IP address of VLAN 20 to 168.10.1.1 and enable PIM DM on the VLAN interface.

<SwitchA> system-view

[SwitchA] multicast routing-enable

[SwitchA] vlan 20

[SwitchA–vlan20]port Ethernet 1/0/1

[SwitchA-vlan20] quit

[SwitchA] interface Vlan-interface 20

[SwitchA-Vlan-interface20] ip address 168.10.1.1 255.255.255.0

[SwitchA-Vlan-interface20] pim dm

[SwitchA-Vlan-interface20] quit

# Configure VLAN 10.

[SwitchA] vlan 10

[SwitchA-vlan10] quit

# Define Ethernet 1/0/10 as a hybrid port, add the port to VLAN 10, and configure the port to forward tagged packets for VLAN 10.

[SwitchA] interface Ethernet 1/0/10

[SwitchA-Ethernet1/0/10] port link-type hybrid

[SwitchA-Ethernet1/0/10] port hybrid vlan 10 tagged

[SwitchA-Ethernet1/0/10] quit

# Configure the interface IP address of VLAN 10 as 168.10.2.1, and enable PIM-DM and IGMP.

[SwitchA] interface Vlan-interface 10

[SwitchA-Vlan-interface10] ip address 168.10.2.1 255.255.255.0

[SwitchA-Vlan-interface10] igmp enable

[SwitchA-Vlan-interface10] pim dm

2)         Configure Switch B:

# Enable the IGMP Snooping feature on Switch B.

<SwitchB> system-view

[SwitchB] igmp-snooping enable

# Create VLAN 2, VLAN 3 and VLAN 10, configure VLAN 10 as the multicast VLAN, and then enable IGMP Snooping on it.

[SwitchB] vlan 2 to 3

Please wait.... Done.

[SwitchB] vlan 10

[SwitchB-vlan10] service-type multicast

[SwitchB-vlan10] igmp-snooping enable

[SwitchB-vlan10] quit

# Define Ethernet 1/0/10 as a hybrid port, add the port to VLAN 2, VLAN 3, and VLAN 10, and configure the port to forward tagged packets for VLAN 2, VLAN 3, and VLAN 10.

[SwitchB] interface Ethernet 1/0/10

[SwitchB-Ethernet1/0/10] port link-type hybrid

[SwitchB-Ethernet1/0/10] port hybrid vlan 2 3 10 tagged

[SwitchB-Ethernet1/0/10] quit

# Define Ethernet 1/0/1 as a hybrid port, add the port to VLAN 2 and VLAN 10, configure the port to forward untagged packets for VLAN 2 and VLAN 10, and set VLAN 2 as the default VLAN of the port.

[SwitchB] interface Ethernet 1/0/1

[SwitchB-Ethernet1/0/1] port link-type hybrid

[SwitchB-Ethernet1/0/1] port hybrid vlan 2 10 untagged

[SwitchB-Ethernet1/0/1] port hybrid pvid vlan 2

[SwitchB-Ethernet1/0/1] quit

# Define Ethernet 1/0/2 as a hybrid port, add the port to VLAN 3 and VLAN 10, configure the port to forward untagged packets for VLAN 3 and VLAN 10, and set VLAN 3 as the default VLAN of the port.

[SwitchB] interface Ethernet 1/0/2

[SwitchB-Ethernet1/0/2] port link-type hybrid

[SwitchB-Ethernet1/0/2] port hybrid vlan 3 10 untagged

[SwitchB-Ethernet1/0/2] port hybrid pvid vlan 3

[SwitchB-Ethernet1/0/2] quit

3.5  Troubleshooting IGMP Snooping

Symptom: Multicast function does not work on the switch.

Solution:

Possible reasons are:

1)         IGMP Snooping is not enabled.

l           Use the display current-configuration command to check the status of IGMP Snooping.

l           If IGMP Snooping is disabled, check whether it is disabled globally or in the specific VLAN. If it is disabled globally, use the igmp-snooping enable command in both system view and VLAN view to enable it both globally and on the corresponding VLAN at the same time. If it is only disabled on the corresponding VLAN, use the igmp-snooping enable command in VLAN view only to enable it on the corresponding VLAN.

2)         Multicast forwarding table set up by IGMP Snooping is wrong.

l           Use the display igmp-snooping group command to check if the multicast groups are expected ones.

l           If the multicast group set up by IGMP Snooping is not correct, contact your technical support personnel.