Router Does Not Forward Multicast Packets …

Background Information
When troubleshooting multicast routing, the primary concern is the source address. Multicast has a concept of Reverse Path Forwarding check (RPF check). When a multicast packet arrives on an interface, the RPF process checks to ensure that this incoming interface is the outgoing interface used by unicast routing to reach the source of the multicast packet. This RPF check process prevents loops. Multicast routing does not forward a packet unless the source of the packet passes a reverse path forwarding (RPF) check. Once a packet passes this RPF check, multicast routing forwards the packet based only upon the destination address.

Like unicast routing, multicast routing has several available protocols, such as Protocol Independent Multicast dense mode (PIM-DM), PIM sparse mode (PIM-SM), Distance Vector Multicast Routing Protocol (DVMRP), Multicast Border Gateway Protocol (MBGP), and Multicast Source Discovery Protocol (MSDP). The case studies in this document walk you through the process of troubleshooting various problems. You will see which commands are used to quickly pinpoint the problem and learn how to resolve it. The case studies listed here are generic across the protocols, except where noted.

Router Does Not Forward Multicast Packets to Host Due to RPF Failure
This section provides a solution to the common problem of an IP multicast Reverse Path Forwarding (RPF) failure. This network diagram is used as an example.

In the figure above, multicast packets come into E0/0 of Router 75a from a server whose IP address is 1.1.1.1 and sends to group 224.1.1.1. This is known as an (S,G) or (1.1.1.1, 224.1.1.1).

Diagnose the Problem
Hosts directly connected to Router 75a receive the multicast feed, but hosts directly connected to Router 72a do not. First, issue the show ip mroute 224.1.1.1 command to see what is going on with Router 75a. This command examines the multicast route (mroute) for the group address 224.1.1.1:

75a#show ip mroute 224.1.1.1
IP Multicast Routing Table Flags: D - Dense, S - Sparse, C - Connected, L - Local, P - Pruned
R - RP-bit set, F - Register flag, T - SPT-bit set, J - Join SPT
M - MSDP created entry, X - Proxy Join Timer Running
A - Advertised via MSDP
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 224.1.1.1), 00:01:23/00:02:59, RP 0.0.0.0, flags: D
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Ethernet0/1, Forward/Sparse-Dense, 00:01:23/00:00:00
(1.1.1.1, 224.1.1.1), 00:01:23/00:03:00, flags: TA
Incoming interface: Ethernet0/0, RPF nbr 0.0.0.0
Outgoing interface list:
Ethernet0/1, Forward/Sparse-Dense, 00:01:23/00:00:00
Since the router is running PIM dense mode (we know it is dense mode because of the D flag), ignore the *,G entry and focus on the S,G entry. This entry tells you that the multicast packets are sourced from a server whose address is 1.1.1.1, which sends to a multicast group of 224.1.1.1. The packets are coming in the Ethernet0/0 interface and are forwarded out the Ethernet0/1 interface. This is a perfect scenario.

Issue the show ip pim neighbor command to see whether Router 72a is showing the upstream router (75a) as a PIM neighbor:

ip22-72a#show ip pim neighbor
PIM Neighbor Table Neighbor Address Interface
Uptime Expires Ver Mode 2.1.1.1
Ethernet3/1 2d00h 00:01:15 v2
From the show ip pim neighbor command output, the PIM neighborship look good.

Use this show ip mroute command to see whether Router 72a has good mroute:

ip22-72a#show ip mroute 224.1.1.1
IP Multicast Routing TableFlags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,
T - SPT-bit set, J - Join SPT, M - MSDP created entry,
X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
U - URD, I - Received Source Specific Host Report, Z - Multicast Tunnel
Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 224.1.1.1), 00:10:42/stopped, RP 0.0.0.0, flags: DC
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Ethernet3/1, Forward/Dense, 00:10:42/00:00:00
Ethernet3/2, Forward/Dense, 00:10:42/00:00:00 (1.1.1.1, 224.1.1.1), 00:01:10/00:02:48, flags:
Incoming interface: Ethernet2/0, RPF nbr 0.0.0.0
Outgoing interface list:
Ethernet3/1, Forward/Dense, 00:01:10/00:00:00
Ethernet3/2, Forward/Dense, 00:00:16/00:00:00 ip22-72a#
You can see from the show ip mroute 224.1.1.1 command that the incoming interface is Ethernet2/0, while Etheret3/1 is expected.

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