Contents

Configuring multicast routing and forwarding· 1

About multicast routing and forwarding· 1

RPF check mechanism·· 1

Usages of static multicast routes· 3

Multicast forwarding across unicast subnets· 4

Restrictions: Hardware compatibility with multicast routing and forwarding· 5

Restrictions and guidelines: Multicast routing and forwarding configuration· 6

Multicast routing and forwarding tasks at a glance· 6

Prerequisites for multicast routing and forwarding· 6

Enabling IP multicast routing· 6

Configuring static multicast routes· 7

Specifying the longest prefix match principle· 7

Configuring multicast load splitting· 7

Configuring a multicast forwarding boundary· 8

Setting the maximum number of cached unknown multicast packets· 8

Configuring an IPv4 MVPN extranet RPF selection policy· 9

Display and maintenance commands for multicast routing and forwarding· 10

Troubleshooting multicast routing and forwarding· 11

Static multicast route failure· 11

 

Configuring multicast routing and forwarding

About multicast routing and forwarding

Each multicast routing protocol has its own routing table. Multicast routing information in routing entries generated by the multicast routing protocols and statically configured multicast routing entries are summarized in a set of (S, G) and (*, G) entries. All the (S, G) and (*, G) entries form a general multicast routing table. The optimal multicast routing entries in the general multicast routing table are added to the multicast forwarding table to guide multicast data forwarding.

RPF check mechanism

A multicast routing protocol uses reverse path forwarding (RPF) check to ensure the multicast data delivery along the correct path and to avoid data loops.

RPF check process

A multicast device performs the RPF check on a multicast packet as follows:

1.     Chooses an optimal route back to the packet source separately from the unicast, MBGP, and static multicast routing tables.

The term "packet source" means different things in different situations:

¡     For a packet that travels along the SPT, the packet source is the multicast source.

¡     For a packet that travels along the RPT, the packet source is the RP.

¡     For a bootstrap message originated from the BSR, the packet source is the BSR.

For more information about the concepts of SPT, RPT, source-side RPT, RP, and BSR, see "Configuring PIM."

2.     Selects one of the three optimal routes as the RPF route as follows:

¡     If the device uses the longest prefix match principle, the route with the highest subnet mask becomes the RPF route. If the routes have the same mask, the route with the highest route preference becomes the RPF route. If the routes have the same route preference, the unicast route becomes the RPF route. If equal cost routes exist, the route with the highest next hop IP address becomes the RPF route.

For more information about the route preference, see Layer 3—IP Routing Configuration Guide.

¡     If the device does not use the longest prefix match principle, the route with the highest route preference becomes the RPF route. If the routes have the same preference, the unicast route becomes the RPF route. If equal cost routes exist, the route with the highest next hop IP address becomes the RPF route.

The RPF route contains the RPF interface and RPF neighbor information.

¡     If the RPF route is a unicast route or MBGP route, the outgoing interface is the RPF interface and the next hop is the RPF neighbor.

¡     If the RPF route is a static multicast route, the RPF interface and RPF neighbor are specified in the route.

3.     Determines whether the packet arrived at the RPF interface.

¡     If the packet arrived at the RPF interface, the RPF check succeeds and the packet is forwarded.

¡     If the packet arrived at the non-RPF interface, the RPF check fails and the packet is discarded.

RPF check implementation in multicast

Implementing an RPF check on each received multicast packet brings a big burden to the device. The use of a multicast forwarding table is the solution to this issue. When the device creates a multicast forwarding entry for an (S, G) packet, it sets the RPF interface of the packet as the incoming interface of the (S, G) entry. After the device receives another (S, G) packet, it looks up the multicast forwarding table for a matching (S, G) entry.

·     If no match is found, the device first determines the RPF route back to the packet source and the RPF interface. Then, it creates a forwarding entry with the RPF interface as the incoming interface and makes the following judgments:

¡     If the receiving interface is the RPF interface, the RPF check succeeds and the device forwards the packet out of all the outgoing interfaces.

¡     If the receiving interface is not the RPF interface, the RPF check fails and the device discards the packet.

·     If a match is found and the matching forwarding entry contains the receiving interface, the device forwards the packet out of all the outgoing interfaces.

·     If a match is found but the matching forwarding entry does not contain the receiving interface, the device determines the RPF route back to the packet source. Then, the device performs one of the following actions:

¡     If the RPF interface is the incoming interface, it means that the forwarding entry is correct but the packet traveled along a wrong path. The packet fails the RPF check, and the device discards the packet.

¡     If the RPF interface is not the incoming interface, it means that the forwarding entry has expired. The device replaces the incoming interface with the RPF interface and matches the receiving interface against the RPF interface. If the receiving interface is the RPF interface, the device forwards the packet out of all outgoing interfaces. Otherwise, it discards the packet.

Figure 1 RPF check process

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As shown in Figure 1, assume that unicast routes are available on the network, MBGP is not configured, and no static multicast routes have been configured on Device C. Multicast packets travel along the SPT from the multicast source to the receivers. The multicast forwarding table on Device C contains the (S, G) entry, with Port A as the incoming interface.

·     If a multicast packet arrives at Device C on Port A, the receiving interface is the incoming interface of the (S, G) entry. Device C forwards the packet out of all outgoing interfaces.

·     If a multicast packet arrives at Device C on Port B, the receiving interface is not the incoming interface of the (S, G) entry. Device C searches its unicast routing table and finds that the outgoing interface to the source (the RPF interface) is Port A. In this case, the (S, G) entry is correct, but the packet traveled along a wrong path. The packet fails the RPF check and Device C discards the packet.

Usages of static multicast routes

A static multicast route can change an RPF route or create an RPF route.

Changing an RPF route

Typically, the topology structure of a multicast network is the same as that of a unicast network, and multicast traffic follows the same transmission path as unicast traffic does. You can configure a static multicast route for a multicast source to change the RPF route. As a result, the device creates a transmission path for multicast traffic that is different from the transmission path for unicast traffic.

Figure 2 Changing an RPF route

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As shown in Figure 2, when no static multicast route is configured, Device C's RPF neighbor on the path back to the source is Device A. The multicast data from the source travels through Device A to Device C. You can configure a static multicast route on Device C and specify Device B as Device C's RPF neighbor on the path back to the source. The multicast data from the source travels along the path: Device A to Device B and then to Device C.

Creating an RPF route

When a unicast route is blocked, multicast forwarding might be stopped due to lack of an RPF route. You can configure a static multicast route to create an RPF route. In this way, a multicast routing entry is created to guide multicast forwarding.

Figure 3 Creating an RPF route

 

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As shown in Figure 3, the RIP domain and the OSPF domain are unicast isolated from each other. For the receiver hosts in the OSPF domain to receive multicast packets from the multicast source in the RIP domain, you must configure Device C and Device D as follows:

·     On Device C, configure a static multicast route for the multicast source and specify Device B as the RPF neighbor.

·     On Device D, configure a static multicast route for the multicast source and specify Device C as the RPF neighbor.

Multicast forwarding across unicast subnets

Devices forward the multicast data from a multicast source hop by hop along the forwarding tree, but some devices might not support multicast protocols in a network. When the multicast data is forwarded to a device that does not support IP multicast, the forwarding path is blocked. In this case, you can enable multicast forwarding across two unicast subnets by establishing a tunnel between the devices at the edges of the two unicast subnets.

Figure 4 Multicast data transmission through a tunnel

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As shown in Figure 4, a tunnel is established between Device A and Device B. Device A encapsulates the multicast data in unicast IP packets, and forwards them to Device B across the tunnel through unicast devices. Then, Device B strips off the unicast IP header and continues to forward the multicast data to the receiver.

To use this tunnel only for multicast traffic, configure the tunnel as the outgoing interface only for multicast routes.

Restrictions: Hardware compatibility with multicast routing and forwarding

F1000 series	Models	Multicast routing and forwarding compatibility
F1000-X-G5 series	F1000-A-G5, F1000-C-G5, F1000-C-G5-LI, F1000-E-G5, F1000-H-G5, F1000-S-G5	Yes
F1000-X-G3 series	F1000-A-G3, F1000-C-G3, F1000-E-G3, F1000-S-G3	Yes
F1000-X-G2 series	F1000-A-G2, F1000-C-G2, F1000-E-G2, F1000-S-G2	Yes
F1000-9X0-AI series	F1000-9390-AI, F1000-9385-AI, F1000-9380-AI, F1000-9370-AI, F1000-9360-AI, F1000-9350-AI, F1000-9330-AI, F1000-9320-AI, F1000-990-AI, F1000-980-AI, F1000-970-AI, F1000-960-AI, F1000-950-AI, F1000-930-AI, F1000-920-AI, F1000-910-AI, F1000-905-AI	Yes
F1000-C83X0 series	F1000-C8395, F1000-C8390, F1000-C8385, F1000-C8380, F1000-C8370, F1000-C8360, F1000-C8350, F1000-C8330	Yes
F1000-C81X0 series	F1000-C8180, F1000-C8170, F1000-C8160, F1000-C8150, F1000-C8130, F1000-C8120, F1000-C8110	Yes
F1000-7X0-HI series	F1000-770-HI, F1000-750-HI, F1000-740-HI, F1000-730-HI, F1000-720-HI, F1000-710-HI	Yes
F1000-C-X series	F1000-C-EI, F1000-C-HI, F1000-C-XI, F1000-E-XI	Yes
F1000-V series	F1000-E-VG, F1000-S-VG	Yes
SecBlade IV	LSPM6FWD8, LSQM2FWDSC8	Yes

 

F100 series	Models	Multicast routing and forwarding compatibility
F100-X-G5 series	F100-A-G5, F100-C-G5, F100-E-G5, F100-M-G5, F100-S-G5	Yes
F100-X-G3 series	F100-A-G3, F100-C-G3, F100-E-G3 , F100-M-G3, F100-S-G3	Yes
F100-X-G2 series	F100-A-G2, F100-C-G2, F100-E-G2, F100-M-G2, F100-S-G2	Yes
F100-WiNet series	F100-A80-WiNet, F100-C80-WiNet, F100-C60-WiNet, F100-C50-WiNet, F100-S80-WiNet, F100-A91-WiNet, F100-A81-WiNet	Yes
F100-C-A series	F100-C-A6, F100-C-A5, F100-C-A3, F100-C-A6-WL, F100-C-A5-W, F100-C-A3-W	Yes
	F100-C-A2, F100-C-A1	No
F100-X-XI series	F100-A-EI, F100-A-HI, F100-A-SI, F100-C-EI, F100-C-HI, F100-C-XI, F100-E-EI, F100-S-HI, F100-S-XI	Yes

 

Restrictions and guidelines: Multicast routing and forwarding configuration

The device can route and forward multicast data only through the primary IP addresses of interfaces, rather than their secondary addresses or unnumbered IP addresses. For more information about primary and secondary IP addresses, and IP unnumbered, see Layer 3—IP Services Configuration Guide.

Multicast routing and forwarding tasks at a glance

To configure multicast routing and forwarding, perform the following tasks:

1.     Enabling IP multicast routing

2.     (Optional.) Configuring static multicast routes

3.     (Optional.) Specifying the longest prefix match principle

4.     (Optional.) Configuring multicast load splitting

5.     (Optional.) Configuring a multicast forwarding boundary

6.     (Optional.) Setting the maximum number of cached unknown multicast packets

7.     (Optional.) Configuring an IPv4 MVPN extranet RPF selection policy

Prerequisites for multicast routing and forwarding

Before you configure multicast routing and forwarding, configure a unicast routing protocol so that all devices in the domain can interoperate at the network layer.

Enabling IP multicast routing

About this task

Enable IP multicast routing before you configure any Layer 3 multicast functionality on the public network or VPN instance.

Procedure

1.     Enter system view.

system-view

2.     Enable IP multicast routing and enter MRIB view.

multicast routing [ vpn-instance vpn-instance-name ]

By default, IP multicast routing is disabled.

Configuring static multicast routes

About this task

To configure a static multicast route for a multicast source, you can specify an RPF interface or an RPF neighbor for the multicast traffic from that source.

Restrictions and guidelines

Static multicast routes take effect only on the multicast devices on which they are configured, and will not be advertised throughout the network or redistributed to other devices.

Procedure

1.     Enter system view.

system-view

2.     Configure a static multicast route.

ip rpf-route-static [ vpn-instance vpn-instance-name ] source-address { mask-length | mask } { rpf-nbr-address | interface-type interface-number } [ preference preference ]

3.     (Optional.) Delete all static multicast routes.

delete ip rpf-route-static [ vpn-instance vpn-instance-name ]

You can use the undo ip rpf-route-static command to delete a specific static multicast route or use the delete ip rpf-route-static command to delete all static multicast routes.

Specifying the longest prefix match principle

About this task

You can enable the device to use the longest prefix match principle for RPF route selection. For more information about RPF route selection, see "RPF check process."

Procedure

1.     Enter system view.

system-view

2.     Enter MRIB view.

multicast routing [ vpn-instance vpn-instance-name ]

3.     Specify the longest prefix match principle.

longest-match

By default, the route preference principle is used.

Configuring multicast load splitting

About this task

You can enable the device to split multiple data flows on a per-source basis or on a per-source-and-group basis. This optimizes the traffic delivery.

Restrictions and guidelines

This feature does not take effect on BIDIR-PIM.

Procedure

1.     Enter system view.

system-view

2.     Enter MRIB view.

multicast routing [ vpn-instance vpn-instance-name ]

3.     Configure multicast load splitting.

load-splitting { source | source-group }

By default, multicast load splitting is disabled.

Configuring a multicast forwarding boundary

About this task

You can configure an interface as a multicast forwarding boundary for a multicast group range. The interface cannot receive or forward multicast packets for the group range.

Restrictions and guidelines

You do not need to enable IP multicast before this configuration.

Procedure

1.     Enter system view.

system-view

2.     Enter interface view.

interface interface-type interface-number

3.     Configure the interface as a multicast forwarding boundary for a multicast group range.

multicast boundary group-address { mask-length | mask }

By default, an interface is not a multicast forwarding boundary.

Setting the maximum number of cached unknown multicast packets

About this task

The device caches a multicast packet for a period of time if no matching multicast forwarding entry is found for the packet. If a multicast forwarding entry is established for the packet within the time period, the device forwards the packet. This mechanism prevents the device from mistakenly dropping multicast packets when the multicast forwarding entries for these packets are to be created.

You can set the maximum number of unknown multicast packets that can be cached for an (S, G) entry, in total, or both.

Restrictions and guidelines

As a best practice, set the value in the multicast forwarding-table cache-unknown total command to be far greater than the value in the multicast forwarding-table cache-unknown per-entry command.

Procedure

1.     Enter system view.

system-view

2.     Set the maximum number of unknown multicast packets that can be cached for an (S, G) entry.

multicast forwarding-table cache-unknown per-entry per-entry-limit

By default, the device can cache only one unknown multicast packet for an (S, G) entry.

3.     Set the maximum number of unknown multicast packets that can be cached in total.

multicast forwarding-table cache-unknown total total-limit

By default, the device can cache 1024 unknown multicast packets in total.

Configuring an IPv4 MVPN extranet RPF selection policy

About this task

IPv4 MVPN extranet RPF routing policies are used for IPv4 multicast transmission when multicast sources and receivers are located in different VPNs.

Restrictions and guidelines

The PIM modes in the source VPN instance and the receiver VPN instance must be the same. Only PIM-SM and PIM-SSM are supported.

Multicast packets can only be forwarded between two VPNs. The receiver VPN instance cannot also be the source VPN instance.

In PIM-SM mode, you can configure only one RPF selection policy for a multicast group in a VPN instance.

If an IPv4 MVPN extranet RPF selection policy with only the multicast group address specified is configured in the receiver VPN instance, the multicast traffic for the intra-VPN transmission will be interrupted.

To implement source-specific RPF selection in MVPN extranet, you must configure two MVPN extranet RPF routing policies as follows:

·     In one policy, specify the address of the RP designated to the multicast group that requires inter-VPN multicast communication as the source address.

·     In the other policy, specify the multicast source in the source VPN instance as the source address.

To implement source-and-group-specific RPF selection in MVPN extranet, you must configure two MVPN extranet RPF routing policies as follows:

·     In one policy, specify the address of the RP designated to the multicast group as the source address, and specify the multicast group.

·     In the other policy, specify the multicast source in the source VPN instance as the source address, and specify the multicast group.

·     Make sure the multicast groups in the two policies are the same to avoid inter-VPN multicast transmission failure.

Common Layer 3 multicast supports both the source-PE-based MVPN extranet option and receiver-PE-based MVPN extranet option.

For the source-PE-based MVPN extranet option, if PIM-SM mode is used, the RP of the receiver VPN instance must be configured on the multicast source-side device.

Procedure

1.     Enter system view.

system-view

2.     Enter MRIB view.

multicast routing [ vpn-instance vpn-instance-name ]

3.     Configure an IPv4 MVPN extranet RPF selection policy.

multicast extranet select-rpf [ vpn-instance vpn-instance-name ] { source source-address { mask | mask-length } | group group-address { mask | mask-length } } *

By default, no IPv4 MVPN extranet RPF selection policies are configured.

Display and maintenance commands for multicast routing and forwarding

CAUTION: The reset commands might cause multicast data transmission failures.

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Execute display commands in any view and reset commands in user view.

 

Task	Command
Display information about the interfaces maintained by the MRIB.	display mrib [ vpn-instance vpn-instance-name ] interface [ interface-type interface-number ]
Display multicast boundary information.	display multicast [ vpn-instance vpn-instance-name ] boundary [ group-address [ mask-length | mask ] ] [ interface interface-type interface-number ]
Display multicast fast forwarding entries.	display multicast [ vpn-instance vpn-instance-name ] fast-forwarding cache [ source-address | group-address ] * [ slot slot-number ]
Display DF information.	display multicast [ vpn-instance vpn-instance-name ] forwarding df-info [ rp-address ] [ verbose ] [ slot slot-number ]
Display statistics for multicast forwarding events.	display multicast [ vpn-instance vpn-instance-name ] forwarding event [ slot slot-number]
Display multicast forwarding entries.	display multicast [ vpn-instance vpn-instance-name ] forwarding-table [ source-address [ mask { mask-length | mask } ] | group-address [ mask { mask-length | mask } ] | incoming-interface interface-type interface-number | outgoing-interface { exclude | include | match } interface-type interface-number | slot slot-number | statistics ] *
Display information about the DF list in the multicast forwarding table.	display multicast [ vpn-instance vpn-instance-name ] forwarding-table df-list [ group-address ] [ verbose ] [ slot slot-number ]
Display multicast routing entries.	display multicast [ vpn-instance vpn-instance-name ] routing-table [ source-address [ mask { mask-length | mask } ] | group-address [ mask { mask-length | mask } ] | incoming-interface interface-type interface-number | outgoing-interface { exclude | include | match } interface-type interface-number ] *
Display static multicast routing entries.	display multicast [ vpn-instance vpn-instance-name ] routing-table static [ source-address { mask-length | mask } ]
Display RPF information for a multicast source.	display multicast [ vpn-instance vpn-instance-name ] rpf-info source-address [ group-address ]
Clear multicast fast forwarding entries.	reset multicast [ vpn-instance vpn-instance-name ] fast-forwarding cache { { source-address | group-address } * | all } [ slot slot-number ]
Clear statistics for multicast forwarding events.	reset multicast [ vpn-instance vpn-instance-name ] forwarding event
Clear multicast forwarding entries.	reset multicast [ vpn-instance vpn-instance-name ] forwarding-table { { source-address [ mask { mask-length | mask } ] | group-address [ mask { mask-length | mask } ] | incoming-interface { interface-type interface-number } } * | all }
Clear multicast routing entries.	reset multicast [ vpn-instance vpn-instance-name ] routing-table { { source-address [ mask { mask-length | mask } ] | group-address [ mask { mask-length | mask } ] | incoming-interface interface-type interface-number } * | all }

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	NOTE: ·     When you clear a multicast routing entry, the associated multicast forwarding entry is also cleared. ·     When you clear a multicast forwarding entry, the associated multicast routing entry is also cleared.

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Troubleshooting multicast routing and forwarding

Static multicast route failure

Symptom

No dynamic routing protocol is enabled on the routers, and the physical status and link layer status of interfaces are both up, but the static multicast route fails.

Solution

To resolve the problem:

1.     Use the display multicast routing-table static command to display information about static multicast routes. Verify that the static multicast route has been correctly configured and that the route entry exists in the static multicast routing table.

2.     Check the type of interface that connects the static multicast route to the RPF neighbor. If the interface is not a point-to-point interface, be sure to specify the address for the RPF neighbor.

3.     If the problem persists, contact H3C Support.