Table of Contents

Chapter 1 IPv6 Multicast Routing and Forwarding Configuration. 1-1

1.1 IPv6 Multicast Routing and Forwarding Overview. 1-1

1.1.1 Introduction to IPv6 Multicast Routing and Forwarding. 1-1

1.1.2 RPF Mechanism.. 1-2

1.2 Configuration Task List 1-4

1.3 Configuring IPv6 Multicast Routing and Forwarding. 1-4

1.3.1 Enabling IPv6 Multicast Routing. 1-4

1.3.2 Configuration Prerequisites. 1-5

1.3.3 Configuring IPv6 Multicast Load Splitting. 1-5

1.3.4 Configuring an IPv6 Multicast Forwarding Range. 1-5

1.3.5 Configuring the IPv6 Multicast Forwarding Table Size. 1-6

1.4 Displaying and Maintaining IPv6 Multicast Routing and Forwarding. 1-7

1.5 Troubleshooting IPv6 Multicast Policy Configuration. 1-8

1.5.1 Multicast Data Fails to Reach Receivers. 1-8

 

Chapter 1  IPv6 Multicast Routing and Forwarding Configuration

When configuring IPv6 multicast routing and forwarding, go to the following sections for information you are interested in:

l           IPv6 Multicast Routing and Forwarding Overview

l           Configuring IPv6 Multicast Routing and Forwarding

l           Displaying and Maintaining IPv6 Multicast Routing and Forwarding

l           Troubleshooting IPv6 Multicast Policy Configuration

 

 

1.1  IPv6 Multicast Routing and Forwarding Overview

1.1.1  Introduction to IPv6 Multicast Routing and Forwarding

In IPv6 multicast implementations, multicast routing and forwarding are implemented by three types of tables:

l           Each IPv6 multicast routing protocol has its own multicast routing table, such as IPv6 PIM routing table.

l           The multicast routing information of different IPv6 multicast routing protocols forms a general IPv6 multicast routing table.

l           The IPv6 multicast forwarding table is directly used to control the forwarding of IPv6 multicast packets. This is the table that guides IPv6 multicast forwarding. The IPv6 multicast forwarding table is consistent with the IPv6 routing table.

An IPv6 multicast forwarding table consists of a set of (S, G) entries, each indicating the routing information for delivering multicast data from a multicast source to a multicast group. If a router supports multiple IPv6 multicast protocols, its IPv6 multicast routing table will include routes generated by these protocols. The router chooses the optimal route from the IPv6 multicast routing table based on the configured multicast routing and forwarding policy and installs the route entry into its IPv6 multicast forwarding table.

1.1.2  RPF Mechanism

When creating IPv6 multicast routing table entries, an IPv6 multicast routing protocol uses the reverse path forwarding (RPF) to ensure IPv6 multicast data delivery along the correct path.

The RPF mechanism enables routers to correctly forward IPv6 multicast packets based on the multicast route configuration. In addition, the RPF mechanism also helps avoid data loops caused by various reasons.

I. Implementation of the RPF mechanism

Upon receiving an IPv6 multicast packet sent from a multicast source S to an IPv6 multicast group G, the device first searches its IPv6 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 IPv6 multicast forwarding table, the device 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 IPv6 multicast forwarding table, the 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 device 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 device 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 IPv6 multicast forwarding table.

l           If the interface on which the packet actually arrived is the RPF interface, the RPF check is successful and the device 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 device discards the packet.

II. RPF Check

The basis for an RPF check is an IPv6 unicast route. The IPv6 unicast routing table contains the shortest path to each destination subnet. An IPv6 multicast routing protocol does not rely on a particular type of IPv6 unicast routes; instead, it relies on the existing IPv6 unicast routing information in the device to create IPv6 multicast routing entries.

When performing an RPF check, the device searches its IPv6 unicast routing table using the IP address of the “packet source” as the destination address and automatically selects the optimal route as the RPF route. The outgoing interface in the corresponding routing entry is the RPF interface and the next hop is the RPF neighbor. The device considers the path along which the IPv6 multicast packet from the RPF neighbor arrived on the RPF interface to be the shortest path that leads back to the source.

 

 

Assume that IPv6 unicast routes exist in the network, and IPv6 multicast packets travel along the SPT from the multicast source to the receivers, as shown in Figure 1-1. The IPv6 multicast forwarding table on Switch C contains the (S, G) entry, with VLAN-interface 1 as the RPF interface.

Figure 1-1 RPF check process

l           When an IPv6 multicast packet arrives on VLAN-interface 1 of Switch C, as the interface is the incoming interface of the (S, G) entry, the switch forwards the packet to all outgoing interfaces.

l           When an IPv6 multicast packet arrives on VLAN-interface 2 of Switch C, as the interface is not the incoming interface of the (S, G) entry, the switch performs an RPF check on the packet: The switch searches its IPv6 unicast routing table and finds that the outgoing interface to Source (the RPF interface) is VLAN-interface 1. This means the (S, G) entry is correct and packet arrived along a wrong path. The RPF check fails and the packet is discarded.

1.2  Configuration Task List

Complete these tasks to configure IPv6 multicast routing and forwarding:

Task	Remarks
Enabling IPv6 Multicast Routing	Required
Configuring IPv6 Multicast Load Splitting	Optional
Configuring an IPv6 Multicast Forwarding Range	Optional
Configuring the IPv6 Multicast Forwarding Table Size	Optional

 

1.3  Configuring IPv6 Multicast Routing and Forwarding

1.3.1  Enabling IPv6 Multicast Routing

Before configuring any Layer 3 IPv6 multicast functionality, you must enable IPv6 multicast routing.

Follow these steps to enable IPv6 multicast routing:

To do…	Use the Command…	Remarks
Enter system view	system-view	—
Enable IPv6 multicast routing	multicast ipv6 routing-enable	Required Disabled by default

 

1.3.2  Configuration Prerequisites

Before configuring IPv6 multicast routing and forwarding, complete the following tasks:

l           Configure an IPv6 unicast routing protocol so that all devices in the domain are interoperable at the network layer.

l           Configure IPv6 PIM-DM or IPv6 PIM-SM.

Before configuring IPv6 multicast routing and forwarding, prepare the following data:

l           Maximum number of downstream nodes for a single IPv6 multicast forwarding table entry

l           Maximum number of entries in the IPv6 multicast forwarding table

1.3.3  Configuring IPv6 Multicast Load Splitting

With the IPv6 multicast load splitting feature enabled, IPv6 multicast traffic will be evenly distributed among different equal-cost routes.

Follow these steps to enable IPv6 multicast load splitting:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable IPv6 multicast load splitting	multicast ipv6 load-splitting {source | source-group }	Required Disabled by default

 

1.3.4  Configuring an IPv6 Multicast Forwarding Range

IPv6 multicast packets do not travel infinitely in a network. The IPv6 multicast data of each IPv6 multicast group must be transmitted within a definite scope. Presently, you can define an IPv6 multicast forwarding range by specifying boundary interfaces, which form a closed IPv6 multicast forwarding area.

You can configure the forwarding boundary for a specific IPv6 multicast group on all interfaces that support IPv6 multicast forwarding. A multicast forwarding boundary sets the boundary condition for the IPv6 multicast groups in the specified range. If the destination address of an IPv6 multicast packet matches the set boundary condition, the packet will not be forwarded. Once an IPv6 multicast boundary is configured on an interface, this interface can no longer forward IPv6 multicast packets (including those sent from the local device) or receive IPv6 multicast packets.

Follow these steps to configure an IPv6 multicast forwarding range:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter VLAN interface view	interface interface-type interface-number	—
Configure an IPv6 multicast forwarding boundary	multicast ipv6 boundary ipv6-group-address prefix-length	Required No forwarding boundary by default

 

1.3.5  Configuring the IPv6 Multicast Forwarding Table Size

The device maintains the corresponding forwarding entry for each IPv6 multicast packet it receives. Excessive IPv6 multicast routing entries can exhaust the device’s memory and thus result in low performance of the device. You can set a limit on the number of entries in the IPv6 multicast forwarding table based on the actual networking situation and the performance requirements.

If the configured maximum number of IPv6 multicast forwarding table entries is smaller than the current value, the forwarding entries in excess are not immediately deleted; instead they will be deleted by the IPv6 multicast routing protocol running on the device. The device will no longer create new IPv6 multicast forwarding entries until the number of existing IPv6 multicast forwarding entries comes down below the configured value.

If the configured maximum number of downstream nodes for a single IPv6 multicast forwarding entry is smaller than the current number, the downstream nodes in excess are not deleted immediately; instead they must be deleted by the IPv6 multicast routing protocol. The device will no longer create new IPv6 multicast forwarding entries for newly added downstream nodes until the number of existing downstream nodes comes down below the configured value.

Follow these steps to configure the IPv6 multicast forwarding table size:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure the maximum number of entries in the IPv6 multicast forwarding table	multicast ipv6 forwarding-table route-limit limit	Optional 256 by default
Configure the maximum number of downstream nodes for a single IPv6 multicast forwarding table entry	multicast ipv6 forwarding-table downstream-limit limit	Optional 128 by default

 

1.4  Displaying and Maintaining IPv6 Multicast Routing and Forwarding

To do...	Use the command...	Remarks
Display the IPv6 multicast boundary information	display multicast ipv6 boundary [ ipv6-group-address [ prefix-length ] | interface interface-type interface-number ]	Available in any view
Display the information of the IPv6 multicast forwarding table	display multicast ipv6 forwarding-table [ ipv6-source-address [ prefix-length ] | ipv6-group-address [ prefix-length ] | incoming-interface { interface-type interface-number | register } | outgoing-interface { { exclude | include | match } { interface-type interface-number | register } } | statistics | slot slot-id ] * [ port-info ]	Available in any view
Display the information of the IPv6 multicast routing table	display multicast ipv6 routing-table [ ipv6-source-address [ prefix-length ] | ipv6-group-address [ prefix-length ] | incoming-interface { interface-type interface-number | register } | outgoing-interface { { exclude | include | match } { interface-type interface-number | register } } ] *	Available in any view
Display the RPF route information of the specified IPv6 multicast source	display multicast ipv6 rpf-info ipv6-source-address [ ipv6-group-address ]	Available in any view
Clear forwarding entries from the IPv6 multicast forwarding table	reset multicast ipv6 forwarding-table { { ipv6-source-address [ prefix-length ] | ipv6-group-address [ prefix-length ] | incoming-interface { interface-type interface-number | register } } * | all }	Available in user view
Clear routing entries from the IPv6 multicast routing table	reset multicast ipv6 routing-table { { ipv6-source-address [ prefix-length ] | ipv6-group-address [ prefix-length ] | incoming-interface { interface-type interface-number | register } } * | all }	Available in user view

 

 

1.5  Troubleshooting IPv6 Multicast Policy Configuration

1.5.1  Multicast Data Fails to Reach Receivers

I. Symptom

The multicast data can reach some devices but fails to reach the last-hop device.

II. Analysis

If a multicast forwarding boundary is configured on a VLAN interface through the multicast ipv6 boundary command, any multicast packet will be kept from crossing the boundary.

III. Solution

1)         Use the display pim ipv6 routing-table command to check whether the corresponding (S, G) entries exist on the device. If so, the device has received the multicast data; otherwise, the device has not received the data..

2)         Use the display multicast ipv6 boundary command to view the multicast boundary information on the interfaces. .Use the multicast ipv6 boundary command to change the multicast forwarding boundary setting.

3)         In the case of IPv6 PIM-SM, check the BSR and RP information.