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CLI-based Configuration Guide - IP Multicast

AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R007

This document describes the principles and configurations of IP multicast, and provides configuration examples.
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Multicast VPN in BGP A-D Mode

Multicast VPN in BGP A-D Mode

Applicable Environment

In a multicast domain (MD)-based MVPN, because the PEs that belong to the same VPN know neither the BGP peers of each other nor multicast source information, the PEs cannot send Join messages to the multicast source to establish a Protocol Independent Multicast-Source-Specific Multicast (PIM-SSM) multicast distribution tree (MDT). Therefore, the share-MDT of the public network cannot use a PIM-SSM tunnel.

MVPN in BGP auto-discovery (A-D) mode is introduced to address this problem. In MVPN in BGP A-D mode, PEs exchange BGP Update packets carrying A-D routes (recording the peers of each PE) to automatically discover the BGP peers of the PEs on a multicast VPN. In this manner, multicast VPN services can be transmitted over a public-network tunnel based on a PIM-SSM MDT.

Related Concepts

BGP A-D MVPN related concepts are as follows:
  • Peer: BGP speakers that exchange messages with each other are called peers.

  • A-D route: is used to discover all peers in the same VPN. This type of route helps implement tunnel setup and control message exchange between peers.

  • BGP update message: is used to exchange routes between BGP peers.

Principles

Currently, two BGP A-D modes, namely, MDT-Subsequent Address Family Identifier (SAFI) A-D and MCAST-VPN SAFI A-D, are supported:
  • In MDT-SAFI A-D mode, a new address family is defined by BGP. In this manner, after VPN instance is configured on a PE, the PE advertises the VPN configuration including the RD and share-group address to all its BGP peers. After a remote PE receives an MDT-SAFI message advertised by BGP, the remote PE compares the share-group address in the message with its share-group address. If the remote PE confirms that it is in the same VPN with the sender of the MDT-SAFI message, the remote PE establishes the PIM-SSM MDT on the public network to transmit multicast VPN services.

  • The principles of MCAST-VPN SAFI A-D are similar to that of MDT-SAFI A-D. That is, the multicast VPN configuration is transmitted through BGP Update packets. The difference is that in MCAST-VPN SAFI A-D mode, a BGP Update packet carries more multicast VPN attributes and information for establishing the public network tunnel. Therefore, the MCAST-VPN SAFI A-D mode is applicable to the next-generation multicast VPN.

The same VPN to which different PEs are added can use the same BGP A-D mode, and the different VPNs to which the same PE is added can use different BGP A-D modes to automatically discover the BGP peers of PEs.
  • Scenario where the same VPN to which different PEs are added can use the same BGP A-D mode in MVPN in BGP A-D mode

    Figure 7-15  Networking diagram of the scenario where the same VPN to which different PEs are added can use the same BGP A-D mode in MVPN in BGP A-D mode

    As shown in Figure 7-15, PE1, PE2, and PE3 belong to VPN RED, and join the share-group G1. The address of G1 is within the SSM group address range. BGP A-D in the same mode is enabled on each PE. In addition, the BGP A-D function is enabled on VPN RED. The site where CE1 resides is connected to Source of VPN RED, and CE2 and CE3 are connected to VPN users. Based on the BGP A-D mechanism, every PE on the network obtains and records information about all its BGP peers on the same VPN, and then directly establishes a PIM-SSM MDT on the public network for transmitting multicast VPN services. In this manner, MVPN services can be transmitted over a public network tunnel based on the PIM-SSM MDT.

    The following takes PE3 as an example to describe service processing in MVPN in BGP A-D mode:
    1. After being configured with the BGP A-D function, PE1, PE2, and PE3 negotiate session parameters, and confirm that both ends support the BGP A-D function. Then, the PEs can establish BGP peer relationships. After receiving a BGP Update packet from PE1 and PE2 respectively, PE3 obtains and records the BGP peer addresses of PE1 and PE2. The BGP Update packets carry the information about the PEs that send packets, such as the PE address and supported tunnel type.

    2. VPN RED is configured on PE3. PE3 joins the share-group G1. PE3 creates a PIM-SSM entry with G1 being the group address and the address of PE1 being the source address and another PIM-SSM entry with G1 being the group address and the address of PE2 being the source address. PE3 then directly sends PIM Join messages to PE1 and PE2 to establish two PIM-SSM MDTs to PE1 and PE2 respectively.

    3. CE3 sends a Join message to PE3. After receiving the Join message, PE3 encapsulates the Join message with the PIM-SSM share-group address, and then sends it to PE1 over the public network tunnel. PE1 then decapsulates the received Join message, and then sends it to the multicast source.

    4. After the multicast data sent by the multicast source reaches PE1, PE1 encapsulates the multicast data with the share-group address, and then forwards it to PE3 over the public network tunnel. PE3 then forwards the multicast data to CE3, and CE3 sends the multicast data to the user.

  • Scenario where the different VPNs to which the same PE is added can use different BGP A-D modes in MVPN in BGP A-D mode

    Figure 7-16  Networking diagram of the scenario where the different VPNs to which the same PE is added can use different BGP A-D modes in MVPN in BGP A-D mode

    As shown in Figure 7-16, PE1 belongs to VPN RED, PE2 belongs to VPN BLUE, and PE3 belongs to both VPN RED and VPN BLUE. BGP A-D in MDT-SAFI mode is enabled on PE1, BGP A-D in MCAST-VPN SAFI mode is enabled on PE2, and both BGP A-D in BGP MDT-SAFI mode and BGP A-D in MCAST-VPN SAFI mode are enabled on PE3. In addition, on PE3, BGP A-D in MDT-SAFI A-D mode is used by VPN RED whereas BGP A-D in MCAST-VPN SAFI A-D mode is used by VPN BLUE. The site where CE1 resides is connected to the multicast source of VPN RED and the site where CE4 resides is connected to the multicast source of VPN BLUE. CE2 and CE3 are connected to VPN users. Based on the BGP A-D mechanism, BGP peers enabled with BGP A-D in the same mode obtain the BGP A-D information from each other, and every PE on the network obtains and records information about all peers on the same VPN and directly establishes a PIM-SSM MDT on the public network for transmitting multicast VPN services. In this manner, MVPN services can be sent over a public network tunnel based on the PIM-SSM MDT.

    The interaction between MVPNs supporting BGP A-D through different BGP A-D address families is as follows:
    1. After being configured with BGP A-D, PE1, PE2, and PE3 negotiate session parameters. Because PE1 and PE2 are configured with BGP A-D in different modes, PE1 and PE2 fail to negotiate session parameters and cannot set up the BGP peer relationship. Because PE3 is configured with BGP A-D in both modes, PE3 can establish BGP peer relationships with PE1 and PE2 respectively. After receiving a BGP Update packet from PE1 and PE2 respectively, PE3 obtains and records the BGP peer addresses of PE1 and PE2. The BGP update packets carry the information about the PEs that send packets, such as the PE address and supported tunnel type.

    2. PE3 encapsulates its BGP A-D information including the address and supported tunnel type in a packet used by MVPN in MDT-SAFI mode and in another packet used by MVPN in MCAST-VPN SAFI mode, and then sends the former packet to PE1 and latter packet to PE2. After receiving the corresponding packet, PE1 and PE2 record the information about PE3.

    3. PE1 and PE3 that belong to the same VPN RED obtain each other's information, and send an (S, G) Join message to each other. Similarly, PE2 and PE3 that belong to the same VPN BLUE obtain each other's peers, and send an (S, G) Join message to each other.

    4. Finally, the multicast traffic of each VPN is forwarded to users attached to CE3 of VPN RED and users attached to CE2 of VPN BLUE respectively.

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Updated: 2019-06-12

Document ID: EDOC1000097181

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