PIM (S, G) Join/Prune

Multicast receiver joins/leaves a multicast group in PIM (S, G) modes.

On the network shown in Figure 1, CE1 connects to the multicast source, and CE2 connects multicast receivers. CE2 sends PIM (S, G) Join/Prune messages to CE1. This process shows how a multicast member joins and leaves a multicast group.

Figure 1 NG MVPN

Figure 2 Time sequence for joining a multicast group in PIM (S, G) mode
Click to enlarge

According to Figure 2, Table 1 describes the process of joining a multicast group.

**Table 1** Procedure for joining a multicast group
Step	Device	Key Action
	PE1	After PE1 receives a unicast route destined for the multicast source from CE1, PE1 converts this route to a VPNv4 route, adds the Source AS Extended Community and VRF Route Import Extended Community to this route, and advertises this route to PE2. For more information about the Source AS Extended Community and VRF Route Import Extended Community, see MVPN Extended Community Attributes.
	PE2	After PE2 receives the VPNv4 route from PE1, PE2 matches the export VPN target of the route against its local import VPN target: If the two targets match, PE2 accepts the VPNv4 route and stores the Source AS Extended Community and VRF Route Import Extended Community values carried in this route locally for later generation of the BGP C-multicast route. If the two targets do not match, PE2 drops the VPNv4 route.
	CE2	After CE2 receives an IGMP join request, CE2 sends a PIM-SSM Join message to PE2.
	PE2	After PE2 receives the PIM-SSM Join message: PE2 generates a multicast entry. In this entry, the downstream interface is the interface that receives the PIM-SSM Join message and the upstream interface is the P2MP tunnel interface on the path to the multicast source. PE2 generates a BGP C-multicast route based on the Source AS Extended Community and VRF Route Import Extended Community values stored in . The RT-import attribute of this route is set to the locally stored VRF Route Import Extended Community value. NOTE: In the BGP route with MVPN information, the NLRI field is called MVPN NLRI. The routes whose Route type value is 6 or 7 are C-multicast routes. For more information about C-multicast route structure, see MVPN NLRI.
	PE2	PE2 sends the BGP C-multicast route to PE1.
	PE1	After PE1 receives the BGP C-multicast route: PE1 checks the Administrator field and Local Administrator field values in the RT-import attribute of the BGP C-multicast route. After PE1 confirms that the Administrator field value is its MVPN ID, PE1 accepts the BGP C-multicast route. PE1 determines to which VPN instance routing table should the BGP C-multicast route be added based on the Local Administrator field value in the RT-import attribute of the route. PE1 adds the BGP C-multicast route to the corresponding VPN instance routing table and creates a VPN multicast entry to guide multicast traffic forwarding. In the multicast entry, the downstream interface is PE1's P2MP tunnel interface. PE1 converts the BGP C-multicast route to a PIM-SSM Join message.
	PE1	PE1 sends the PIM-SSM Join message to CE1.
	CE1	After CE1 receives the PIM-SSM Join message, CE1 generates a multicast entry. In this entry, the downstream interface is the interface that receives the PIM-SSM Join message. After that, the multicast receiver successfully joins the multicast group, and CE1 can send multicast traffic to CE2.

Figure 3 Time sequence for leaving a multicast group

Figure 3 shows the procedure for leaving a multicast group, and Table 2 describes this procedure.

**Table 2** Procedure for leaving a multicast group
Step	Device	Key Action
	CE2	CE2 detects that a multicast receiver attached to itself leaves the multicast group.
	PE2	PE2 deletes the corresponding multicast entry after this entry ages out. Then, PE2 generates a BGP Withdraw message.
	PE2	PE2 sends the BGP Withdraw message to PE1.
	PE1	After PE1 receives the BGP Withdraw message, PE1 deletes the corresponding multicast entry and generates a PIM-SSM Prune message.
	PE1	PE1 sends the PIM-SSM Prune message to CE1.
	CE1	After CE1 receives the PIM-SSM Prune message, CE1 stops sending multicast traffic to CE2.