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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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Principles

Principles

This section describes the definition, multicast model, multicast address, and multicast protocol of IP multicast.

Multicast Concepts

Multicast transmits data from one source to multiple receivers. Figure 8-3 shows the multicast transmission model. HostA and HostC are interested in information sent from Source and request for reception of the information. The data sent from Source is received only by HostA and HostC.

Figure 8-3 Multicast transmission

  • Multicast group: a group of receivers identified by an IP multicast address. User hosts (or other receiver devices) that have joined a multicast group become members of the group and can identify and receive the IP packets destined for the multicast group address.
  • Multicast source: a sender of multicast data. Source in Figure 8-3 is a multicast source. A multicast source can simultaneously send data to multiple multicast groups. Multiple multicast sources can simultaneously send data to a multicast group. A multicast source does not need to join any multicast groups.
  • Multicast group member: a host that has joined a multicast group. HostA and HostC in Figure 8-3 are multicast group members. Memberships in a multicast group change dynamically. Hosts can join or leave a multicast group anytime. Members of a multicast group are located anywhere on a network.
  • Multicast router: a router or Layer 3 switch that supports IP multicast. The routers in Figure 8-3 are multicast routers. In addition to multicast routing functions, multicast routers connected to user network segments provide multicast member management functions.

Table 8-1 describes concepts involved in IP multicast by using TV channels and programs.

Table 8-1 Analogy between TV watching and multicast transmission

Sequence

TV Broadcasting

Multicast Transmission

1

A television station sends data to its channel.

A multicast source sends data to a multicast group.

2

Some audience turn on their TV sets and select this channel.

Receivers join the multicast group.

3

TV sets play this channel.

Member hosts receive data sent to the multicast group.

4

Audience switch to other channels or turn on/off their TV sets.

Member hosts dynamically join or leave multicast groups.

Multicast Service Models

Multicast service models differ for receiver hosts and do not affect multicast sources. A multicast source sends multicast packets by using its own IP address as the source IP address and a group address as the destination IP address. Depending on whether receiver hosts can select multicast sources, two multicast models are defined: the Any-Source Multicast (ASM) model and Source-Specific Multicast (SSM) model. The two models use multicast group addresses in different ranges.

ASM Model

The ASM model distributes multicast data based on group addresses. A group address identifies a collection of network service, and multicast packets sent from any source to this address obtain the same service. After joining a group, a host can receive multicast data sent from any source with this group address as the destination address.

To improve security, multicast source filter policies can be configured on routers to permit or deny packets from specified multicast sources. This filters data sent to receiver hosts.

In the ASM model, each group address must be unique on the entire multicast network. An ASM group can only be used by a single application at a time. If two applications use the same ASM group simultaneously, receiver hosts of the two applications receive traffic from both application sources. This may result in network congestion and malfunction of receiver hosts of the applications.

SSM Model

The SSM model provides service for the data flow from specific sources to a specific group. Receiver hosts can specify the sources from which they want to receive data when they join a group. After joining the group, the hosts receive only the data sent from the specified sources.

The SSM model does not require globally unique group addresses. Each group address must be unique for a multicast source. Different applications on a source must use different SSM groups. Different applications on different sources can reuse SSM group addresses because each pair of source and group has an (S, G). This model saves multicast group address without congesting the network.

Multicast Addresses

To enable multicast sources and group members to communicate, the network must provide network-layer multicast service, which uses IP multicast addresses. Additionally, to enable multicast data to be correctly transmitted on the local physical network, the network must provide link-layer multicast service, which uses multicast MAC addresses. The destination address of a multicast data packet is a group with unknown members but not a specific receiver. Therefore, IP multicast addresses must be mapped to multicast MAC addresses.

IPv4 Multicast Addresses

The Internet Assigned Numbers Authority (IANA) allocates Class D addresses for IPv4 multicast. An IPv4 address is 32 bits long, and the first four bits of a Class D IP address is 1110. Therefore, multicast IP addresses range from 224.0.0.0 to 239.255.255.255. Table 8-2 describes IPv4 multicast addresses.

Table 8-2 Range and description of IPv4 multicast addresses

Class D Address Range

Description

224.0.0.0-224.0.0.255

Permanent multicast group addresses that are reserved by the IANA for routing protocols. The addresses identify a group of network devices and are not used for multicast forwarding. Table 8-3 lists the permanent multicast group addresses.

224.0.1.0-231.255.255.255

233.0.0.0-238.255.255.255

ASM group addresses that are valid on the entire network.

NOTE:

Among the ASM group addresses, 224.0.1.39 and 224.0.1.40 are reserved address, and you are not suggested to use them.

232.0.0.0-232.255.255.255

Default SSM group addresses that are valid on the entire network.

239.0.0.0-239.255.255.255

Administrative multicast addresses that are valid only in the local administrative domain. Different administrative domains can use the same administrative multicast addresses.

Table 8-3 List of permanent multicast group addresses

Permanent Multicast Group Addresses

Description

224.0.0.0

Unassigned

224.0.0.1

All the hosts and routers on a network segment (similar to a broadcast address)

224.0.0.2

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

OSPF designated routers (DRs)

224.0.0.7

Shared tree (ST) routers

224.0.0.8

ST hosts

224.0.0.9

Routing Information Protocol version 2 (RIP-2) routers

224.0.0.11

Mobile agents

224.0.0.12

Dynamic Host Configuration Protocol (DHCP) servers or proxy agents

224.0.0.13

All Protocol Independent Multicast (PIM) routers

224.0.0.14

Resource Reservation Protocol (RSVP) encapsulation

224.0.0.15

Core-based tree (CBT) routers

224.0.0.16

Specified Subnetwork Bandwidth Management (SBM) device

224.0.0.17

All SBM devices

224.0.0.18

Virtual Router Redundancy Protocol (VRRP)

224.0.0.22

IGMPv3 routers

224.0.0.19-224.0.0.21

224.0.0.23-224.0.0.255

Unassigned

IPv6 Multicast Addresses

An IPv6 address is 128 bits long. The IPv6 multicast address format is defined in RFC 4291, as shown in Figure 8-4.

Figure 8-4 IPv6 multicast address format

Compared with an IPv4 multicast address, an IPv6 multicast address has a Group ID field to identify a multicast group.

  • FF: The high-order eight bits are 11111111, indicating that the address is a multicast address. All IPv6 multicast addresses start with FF.

  • Flags: It is 4 bits long and identifies the state of a multicast address.
    Table 8-4 Description of flag values

    Value

    Description

    0

    Permanent multicast group addresses that are reserved by the IANA

    1

    ASM group addresses

    2

    ASM group addresses

    3

    SSM group addresses

    Others

    Unassigned

  • Scope: It is 4 bits long and identifies the scope of a multicast group, for example, whether a multicast group covers nodes in the same network, same site, same organization or any node in the global address space.
    Table 8-5 Description of Scope field values

    Value

    Description

    0, 3, F

    Reserved

    1

    Node/interface-local scope

    2

    Link-local scope

    4

    Admin-Local scope

    5

    Site-local scope

    8

    Organization-local scope

    E

    Global scope

    Others

    Unassigned

  • Group ID: It is 112 bits long and identifies a unique multicast group in the range specified by the Scope field. The Group ID can be permanently or temporarily assigned, depending on the value of the T flag in the Flags field.

Table 8-6 describes the IPv6 multicast address ranges.

Table 8-6 Range and description of IPv6 multicast addresses

Range

Description

FF0x::/32

Reserved group addresses (see Table 8-7).

FF1x::/32 (x is not 1 or 2)

FF2x::/32 (x is not 1 or 2)

ASM group addresses that are valid on the entire network.

FF3x::/32 (x is not 1 or 2)

Default SSM group address range. Addresses in this range are valid on the entire network.

Table 8-7 Commonly used IPv6 multicast addresses

Range

IPv6 Multicast Addresses

Description

Node/interface-local scope

FF01::1

All node or interface addresses

FF01::2

All router addresses

Link-local scope

FF02::1

All node addresses

FF02::2

All router addresses

FF02::3

Unassigned addresses

FF02::4

DVMRP routers

FF02::5

OSPF IGP routers

FF02::6

OSPF IGP DRs

FF02::7

ST routers

FF02::8

ST hosts

FF02::9

RIP routers

FF02::A

EIGRP routers

FF02::B

Mobile agents

FF02::D

All PIM routers

FF02::E

RSVP encapsulation

FF02::1:1

Link name

FF02::1:2

All DHCP proxy agents

FF02::1:FFXX:XXXX

Solicited-node addresses (XX:XXXX indicates the last 24 bits of a node IPv6 address)

Site-local scope

FF05::2

All router addresses

FF05::1:3

All DHCP servers

FF05::1:4

All DHCP relay agents

FF05::1:1000-FF05::1:13FF

Service location

IPv4 Multicast MAC Addresses

When unicast IPv4 packets are transmitted on an Ethernet network, the packets use receiver MAC addresses as destination MAC addresses. However, the destination of a multicast data packet is a group with changeable members but not a specific receiver. Therefore, multicast data packets must use IPv4 multicast MAC addresses on an Ethernet network. IPv4 multicast MAC addresses are link-layer addresses mapped from IPv4 multicast addresses.

As defined by the IANA, leftmost 24 bits of an IPv4 multicast MAC address are 0x01005e, the 25th bit is 0, and the rightmost 23 bits are the same as the rightmost 23 bits of a multicast IPv4 address, as shown in Figure 8-5. Multicast MAC address 01-00-5e-00-01-01 is mapped to IP multicast address 224.0.1.1.

Figure 8-5 Mapping between an IPv4 multicast address and an IPv4 multicast MAC address

The first four bits of an IPv4 multicast address is 1110, mapping the leftmost 25 bits of a multicast MAC address. Only 23 bits of the last 28 bits are mapped to a MAC address. That is, information about 5 bits of the IP address is lost. As a result, 32 multicast IP addresses are mapped to the same MAC address. For example, IP multicast addresses 224.0.1.1, 224.128.1.1, 225.0.1.1, and 239.128.1.1 are all mapped to multicast MAC address 01-00-5e-00-01-01. Address conflicts must be considered in address assignment.

IPv6 Multicast MAC Address

In an IPv6 multicast MAC address, the leftmost 16 bits are 0x3333, and the rightmost 32 bits are mapped to the rightmost 32 bits of an IPv6 multicast address. Figure 8-6 shows the mapping between IPv6 multicast address FF01::1111:1 and an IPv6 multicast MAC address.

Figure 8-6 Mapping between an IPv6 multicast address and an IPv6 multicast MAC address

The figure shows that more IPv6 multicast addresses are mapped to the same multicast MAC address.

IPv4 Multicast Protocols

In IP multicast transmission, the sender only needs to send data to a specified destination address and does not need to know the locations of receivers. It is the responsibility of network devices to forward data from the sender to the receivers. Multicast devices on the network must collect information about senders and duplicate and forward multicast data along the correct path. During multicast development, a complete suite of protocols are introduced.

Table 8-8 lists multicast protocols on an IPv4 network.

Table 8-8 IPv4 multicast protocols

Protocol

Function

Remarks

Internet Group Management Protocol (IGMP)

IGMP manages IPv4 multicast group members and runs on the end of a multicast network (network segments where Layer 3 multicast devices connect to user hosts). Hosts use the IGMP protocol to join or leave multicast groups, and Layer 3 multicast devices use the IGMP protocol to manage and maintain group memberships. IGMP can interact with upper-layer multicast routing protocols.

IGMP has three versions: IGMPv1, IGMPv2, and IGMPv3.

All the three versions support the ASM model. IGMPv3 can be independently used in the SSM model, whereas IGMPv1 and IGMPv2 must be used with SSM mapping.

Protocol Independent Multicast (PIM)

PIM runs on an IPv4 network and sends multicast data flows to multicast devices connected to multicast group members interested in the data flows.

CX11x&CX31x&CX91x series switch modules supports Bidirectional Protocol Independent Multicast (Bidirectional PIM) and Protocol Independent Multicast - Sparse Mode (PIM-SM). Bidirectional PIM applies to the network with multiple multicast sources, and PIM-SM applies to the large network where receivers are widely distributed.

The PIM-SM model differentiates the ASM and SSM models based on multicast addresses in multicast data and protocol packets.
  • If multicast addresses of packets are in the range of SSM group addresses, PIM-SM uses the SSM model to provide multicast services. The PIM-SSM model has a high efficiency and simplifies the multicast address allocation process. It is applicable to the scenario where a group has only one specific source.
  • If multicast addresses of packets are in the range of ASM group addresses, PIM-SM uses the ASM model to provide multicast services.

Multicast Source Discovery Protocol (MSDP)

MSDP is an inter-domain multicast protocol that implements multicast forwarding between PIM-SM domains. Multicast devices in a PIM-SM domain use MSDP to discover multicast sources in other PIM-SM domains, and send information about active sources in other PIM-SM domains to receivers in the local PIM domain.

MSDP is required only when PIM-SM uses the ASM model.

Multiprotocol Border Gateway Protocol (MBGP)

MBGP enables multicast data to be transmitted between multicast sources and receivers in different autonomous systems (ASs).

-

IGMP snooping and IGMP snooping proxy

IGMP snooping enables a switch to create and maintain a Layer 2 multicast forwarding table by listening to IGMP packets exchanged between the upstream Layer 3 device and user hosts. The switch controls forwarding of multicast data packets based on the Layer 2 multicast forwarding table to reduce multicast data flooding on the Layer 2 network.

IGMP snooping proxy enables the switch to substitute for an upstream device to send IGMP Query messages and receive IGMP Report/Leave messages from downstream devices. This function saves bandwidth between the upstream device and local switch.

IGMP snooping is an extension of IGMP on Layer 2 network devices. You can specify the IGMP snooping version on a switch to enable the switch to process IGMP packets of specific versions.

IPv6 Multicast Protocols

Table 8-9 lists multicast protocols on an IPv6 network.

Table 8-9 IPv6 multicast protocols

Protocol

Function

Remarks

Multicast Listener Discovery (MLD)

MLD manages IPv6 multicast group members and runs on the end of a multicast network (network segments where Layer 3 multicast devices connect to user hosts). Hosts use the MLD protocol to join or leave multicast groups, and Layer 3 multicast devices use the MLD protocol to manage and maintain group memberships. MLD can interact with upper-layer multicast routing protocols.

MLD has two versions: MLDv1 and MLDv2.

MLDv2 can be independently used in the SSM model, whereas MLDv1 must be used with SSM mapping.

MLD works on an IPv6 network in the same way IGMP works on an IPv4 network. MLDv1 is similar to IGMPv2, and MLDv2 is similar to IGMPv3.

Protocol Independent Multicast (PIM) for IPv6

PIM (IPv6) runs on an IPv6 network and sends multicast data flows to multicast devices connected to multicast group members interested in the data flows.

The CX11x&CX31x&CX91x series switch modules support Bidir-PIM (IPv6) and PIM-SM (IPv6). Bidir-PIM (IPv6) applies to networks with many multicast sources, and PIM-SM (IPv6) applies to large-scale networks with sparsely distributed group members.

The PIM-SM (IPv6) model differentiates the ASM and SSM models based on multicast addresses in multicast data and protocol packets.
  • If multicast addresses of packets are in the range of SSM group addresses, PIM-SM (IPv6) uses the SSM model to provide multicast services. The PIM-SSM (IPv6) model has a high efficiency and simplifies the multicast address allocation process. It is applicable to the scenario where a group has only one specific source.
  • If multicast addresses of packets are in the range of ASM group addresses, PIM-SM (IPv6) uses the ASM model to provide multicast services.
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Updated: 2019-08-09

Document ID: EDOC1000041694

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