MT Transmission Along a Share-MDT

MT Transmission Along a Share-MDT

1. A VPN instance on a PE sends a VPN multicast message to a multicast tunnel interface (MTI).

2. The PE adds the MTI address as the source address and the share-group address as the group address to the message and converts the message to a multicast data message of the public network, regardless of whether the message is a protocol message or a data message. Figure 7-7 shows the encapsulation format of a public network multicast data message.

3. The PE forwards the multicast data message to a public network instance.

4. The public network instance forwards the message to a public network instance on a remote PE along the share-MDT.

5. The remote PE decapsulates the message, reverts it to a VPN multicast message, and forwards it to a VPN instance.

Figure 7-7 shows the message converting processes. Table 7-1 describes involved fields.

Figure 7-7 Process of converting a VPN multicast message

Major Tasks in the MT Transmission Process

• MTIs exchange Hello messages to set up a PIM neighbor relationship between VPN instances on each PE.

• MTIs exchange other protocol messages to set up a VPN MDT.

• The VPN MDT transmits VPN multicast data.

Multicast protocol message transmission

If a VPN runs PIM-SM:

• MTIs exchange Hello messages to set up PIM neighbors between VPN instances.

• If receivers and the VPN rendezvous point (RP) belong to different sites, receivers send Join messages across the public network to set up a shared tree.

• If the multicast source and the VPN RP belong to different sites, registration must be initiated across the public network to set up a source tree.

In the following example, multicast protocol messages are transmitted along a Share-MDT, the public and VPN networks run PIM-SM, and receivers on the VPN send Join messages across the public network.

As shown in Figure 7-8, the receiver in VPN A belongs to Site 2 and is connected to CE 2. CE 1 is the RP of the VPN group G (225.1.1.1) and belongs to Site 1.

Figure 7-8 Multicast protocol message transmission

The process of transmitting multicast protocol messages along the Share-MDT is as follows:

1. The receiver instructs CE 2 to receive and forward data of the multicast group G. CE 2 creates the (*, 225.1.1.1) entry, and then sends a Join message to the VPN RP (CE 1).

2. The VPN instance on PE 2 receives the Join message sent by CE 2, creates the (*, 225.1.1.1) entry, and specifies an MTI as the upstream interface. The instance then forwards the Join message for further processing. The VPN instance on PE2 considers that the Join message has been sent from the MTI.

3. PE 2 encapsulates the Join message with the address of the MTI on PE 2 as the source address and the share-group address as the group address, and converts the message to a common multicast data message (10.1.2.1, 239.1.1.1) on the public network. PE2 forwards the multicast data packet to the public network instance.

4. The share-MDT forwards the multicast data message (10.1.2.1, 239.1.1.1) to the public network instance on each PE. PEs decapsulate the message and revert it to the Join message sent to the VPN RP. If the VPN RP (CE 1) resides in the site directly connected with a PE, the PE sends the message to its VPN instances for further processing. Otherwise, the PE discards the Join message.

5. After receiving the Join message, the VPN instance on PE 1 considers that the message is received from an MTI. The instance creates the (*, 225.1.1.1) entry, and specifies an MTI as the downstream interface and the interface towards CE 1 as the upstream interface. Then, the instance sends the Join message to the VPN RP.

6. After receiving the Join message from the instance on PE1, CE1 updates or creates the (*, 225.1.1.1) entry. The multicast shared tree across VPNs is thus set up.

Multicast data packet transmission

If a VPN runs PIM-SM:

• When receivers and the VPN RP belong to different sites, the VPN multicast data is transmitted across the public network along a VPN rendezvous point tree (RPT).

• When the multicast source and receivers belong to different sites, the VPN multicast data is transmitted across the public network along a source tree.

In the following example, the public network and VPNs run PIM-SM. VPN multicast data is transmitted across the public network along an SPT.

As shown in Figure 7-9, the multicast source in VPN A sends multicast data to the group G (232.1.1.1). The receiver belongs to Site 2 and connects to CE 2.

Figure 7-9 Multicast data message transmission

The multicast data message transmission process is as follows:

1. The source sends a VPN multicast data message (192.168.1.1, 232.1.1.1) to CE1.

2. CE 1 forwards the VPN multicast data to PE 1 along the SPT. The VPN instance on PE 1 searches for a matching forwarding entry. If the outbound interface of the forwarding entry contains an MTI, the instance forwards the VPN multicast data for further processing. The VPN instance on PE 1 then considers that the Join message is sent out from the MTI.

3. PE 1 encapsulates the VPN multicast message with the address of the MTI on PE 1 as the source address and the share-group address as the group address, and converts the message to a public network multicast data message (10.1.1.1, 239.1.1.1). The message is forwarded to the public network instance.

4. The share-MDT forwards the multicast data message (10.1.1.1, 239.1.1.1) to the public network instance on each PE. Each PE decapsulates it, reverts it to VPN multicast data, and forwards it to a specific VPN instance for further processing. If there is an SPT downstream interface on the PE, the data is forwarded along the SPT. Otherwise, the data is discarded.

5. PE2 searches for the forwarding entry in the VPN instance and sends the VPN multicast data message to the receiver. Transmission of this VPN multicast data message is complete.