Troubleshooting Tip: Troubleshooting issues with multicast routing when using PIM in Sparse-Mode

[kaurg_FTNT]

Description	This article describes how to troubleshoot issues with multicast when the firewall is set to operate as a PIM router in Spare-Mode. The article discusses commands and debugs to assist troubleshooting issues with PIM neighborship, multicast routing table and mroute table.
Scope	FortiGate 7.x.
	Example Topology: Rendezvous Point:10.104.0.140 FortiGate configuration should enable the multicast routing and have the required multicast policy to allow multicast traffic. Once routers are configured with Sparse-Mode multicast PIM Hellos are sent by the multicast routers to address 224.0.0.13 to establish neighborship, the default interval is 30 sec which helps maintain the neighborship. If no PIM Hello is received before Hold time (default value is 105 sec) expires the neighborship is lost. Command to check Rendezvous Point mapping quickly. Confirm output with configuration under 'show router multicast'. Check the IGMP Connected Group Membership. Once multicast receivers start sending the IGMPv2 JOIN or IGMPv2. Membership Report group the respective multicast address shows up in IGMP groups. The 'Last Reporter' is the IP of the last receiver that sent the IGMPv2 packet for the respective multicast group (234.5.6.8) via interface 'mgmt'. The multicast receivers will send the IGMPv2 Membership Report when they first join the multicast group and in response to multicast queries. FortiGate will use the route for RP in the unicast routing-table to form IGMP neighborship. The DR (Designated Router) is the router that will forward the PIM join message from the receiver to the RP (rendezvous point). If there is no route in the unicast, routing-table for the RP, then the multicast debug will show that it has failed to get the neighbor for the multicast group. Below is the expected debug when configuration is in place and multicast works. The debug shows IGMP is received followed by PIM-JOIN being sent to RP. Use the below debugs for spare-mode multicast; diagnose ip router pim-sm all enable diagnose ip router pim-sm level info diagnose debug enable PIM JOIN packet sent by FortiGate-1101E to multicast address 224.0.0.13 as seen in Wireshark capture: PIM-SM debugs from FGT-B which is the source/server-side Designated Router (DR) shows that PIM JOIN/PRUNE has been received. In the output of the command 'get router info multicast pim sparse-mode next-hop' the next-hops for all the multicast receivers and source and RP. In the output of the command 'diag ip multicast mroute' the source for the respective multicast group. Note: PIM Sparse-mode only works in VRF=0. NFR has already been raised to support PIM Sparse-mode in multi-VRFs. Additional Information: Command to see if neighborship is active or not: For sparse-mode: 'get router info multicast pim sparse-mode neighbour'. Example: get router info multicast pim sparse-mode neighbour Neighbor Interface Uptime/Expires Ver DR Address Priority/Mode 10.15.0.140 port16 01d03h17m/00:01:17 v2 1 / Related articles:  Technical Tip: How to troubleshoot IGMP and PIM Dense Mode issues Technical Tip: FortiOS Multicast Resource List