Routing Table The show ip mroute command is the most useful command for determining the state of multicast sources and groups from the perspective of the selected router. The output of the command generally represents a part of the multicast distribution tree, with an incoming interface and a list of outgoing interfaces. The following options are available:

summary: Displays a one-line, abbreviated summary of each entry in the IP multicast routing table.
count: Displays statistics about the group and source, including number of packets, packets per second, average packet size, and bits per second.
active: Displays the rate at which active sources are sending to multicast groups. Active sources are those sending at a rate specified in the kbps argument. The default is 4 kbps.

The output of the show ip mroute command in Figure shows a multicast routing table in a PIM-SM environment:

(*, G) entry: Timers, the RP address for the group, and the flags for the group (S is sparse) are listed.
- The incoming interface is the interface toward the RP—if it is Null, the router itself is the RP. The Reverse Path Forwarding (RPF) neighbor is the next-hop address toward the RP. If it is 0.0.0.0, the router is the RP for the group.
- The outgoing interface list (OIL) lists the outgoing interfaces, along with modes and timers.
(S, G) entry: Timers and flags for the entry are listed (T indicates that it is on the SPT; A indicates that it is to be advertised by Multicast Source Discovery Protocol [MSDP]).
- The incoming interface is the interface toward the source S. The RPF neighbor is the next-hop address toward the source. If it is 0.0.0.0, the source is directly attached.
- The OIL lists the outgoing interfaces, in addition to modes and timers.

Content 7.4 Multicast Configuration and Verification 7.4.3 Finding PIM Neighbors When PIM-SM is configured, the first step in verifying proper operation is to check PIM-enabled interfaces and to determine whether the PIM neighbors are correct. You can use the following commands to accomplish this:

show ip pim interface: Displays the information about interfaces configured for PIM.
show ip pim neighbor: Displays the discovered PIM neighbors.
mrinfo: Displays information on multicast routers that are peering with the local router (no address) or with the addressed router.

The show ip pim interface command displays the following information:

Address: IP address of the interface.
Interface: Type and number of the interface configured for PIM.
Ver/Mode: PIM version (1 or 2) that is running on the interface and the mode (dense mode, sparse mode, or sparse-dense mode).
Nbr Count: Number of neighbors on this link.
Query Intvl: Frequency at which PIM hellos and queries are sent (default is 30 seconds).
DR Prior: Priority used in DR election. If all the routers on a multiaccess link have the same priority (default is 1), the highest IP address is a tiebreaker.
DR: IP address of the DR. (On point-to-point links, there are no DRs, so the output shows 0.0.0.0.)

The show ip pim neighbor command displays the following information:

Neighbor Address: IP address of the PIM neighbor.
Interface: Interface where the PIM hello (PIM query in PIMv1) of this neighbor was received.
Uptime: Period of time that this PIM neighbor has been active.
Expires: Period of time (hold time) after which this PIM neighbor is no longer considered active. Receipt of another PIM hello or query resets the timer.
Ver: PIM version that the neighbor is using (1 or 2).
DR Priority: If the neighbor supports this option, the numeric value is shown. If a number is not shown, the neighbor does not support the option.

The sample output of the mrinfo command in Figure displays information about the connected multicast-enabled routers peering with the R1 router.

Content 7.4 Multicast Configuration and Verification 7.4.4 Checking RP Information The RP for a certain multicast group operating in PIM-SM has to be reachable and known to the router. In addition to using a unicast ping, you can use the following commands when troubleshooting RP reachability:

show ip pim rp: Displays, without arguments, RP information on active groups. If the group address or name is provided, only the RP information for the selected group is shown (assuming that it is an active group).
show ip pim rp mapping: Displays the contents of the important group-to-RP mapping cache that contains the information about which RP is active for which group range. This cache is populated by the auto-RP or BSR mechanisms and by static RP assignments. It is very important to check this information to verify that the router possesses the RP mapping information consistent with proper network operation.
show ip rpf: Displays RPF information for the RP or for the source.

The show ip pim rp command just lists all active groups and their associated RPs. This form of the command is becoming obsolete, because it offers limited information. In most cases, you should use the show ip pim rp mapping instead , because it provides details on the actual contents of the group-to-RP mapping cache, such as the following:

IP address of a router that distributed the information—when the source of the information is a local router it either has manual RP configuration or is a source of automatically distributed information.
Mechanism by which this information was determined—auto RP, BSR, or static.
Whether this router is operating as a candidate-RP, mapping agent, or BSR.

The show ip rpf command displays RPF information associated with the specified source address. The specified address does not necessarily have to be a currently active source. In fact, it may be an IP address, including the address of the RP. Specifying the address of the RP is very useful in determining the RPF information for the shared tree. “RPF interface” is the interface in the direction of the source (or RP), while “RPF neighbor” is the address of the next-hop router in the direction of the source (or RP). “RPF type” indicates the source of the RPF information. In the example, unicast indicates that the information was derived from the unicast routing table (in this case, from Open Shortest Path First Protocol [OSPF]). Other RPF types include Distance Vector Multicast Routing Protocol (DVMRP), Multiprotocol Border Gateway Protocol (BGP) extensions for IP multicast, or static. RPF information is essential in multicast routing, and special care has to be taken when inspecting the PIM-SM information because of the possible coexistence of shared trees and SPTs.

Content 7.4 Multicast Configuration and Verification 7.4.5 Checking the Group State If the multicast traffic is not flowing to receivers, the IGMP group membership has to be checked on the leaf routers. The show ip igmp interface command displays information about the selected interface, and the show ip igmp groups command lists the local groups known to the router. Enabling PIM on an interface also enables IGMP operation on that interface. An interface can be configured as dense, sparse, or sparse-dense mode. The mode determines how the router populates its multicast routing table and how the router forwards multicast packets that it receives from its directly connected LANs. You must enable PIM in one of these modes for an interface to perform IP multicast routing. Figures and display sample output for each command.

Content 7.4 Multicast Configuration and Verification