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Troubleshooting PIM
A multicast distribution tree cannot be built correctly
Symptom
None of the routers in the network (including routers directly connected with multicast sources and
receivers) have multicast forwarding entries. That is, a multicast distribution tree cannot be built correctly
and clients cannot receive multicast data.
Analysis
• When PIM-DM runs on the entire network, multicast data is flooded from the first hop router
connected with the multicast source to the last hop router connected with the clients. When the
multicast data is flooded to a router, regardless of which router it is, the router creates (S, G) entries
only if it has a route to the multicast source. If the router does not have a route to the multicast source,
or if PIM-DM is not enabled on the router's RPF interface to the multicast source, the router cannot
create (S, G) entries.
• When PIM-SM runs on the entire network and when a router will join the SPT, the router creates (S,
G) entries only if it has a route to the multicast source. If the router does not have a route to the
multicast source, or if PIM-DM is not enabled on the router's RPF interface to the multicast source, the
router cannot create (S, G) entries.
• When a multicast router receives a multicast packet, it searches the existing unicast routing table for
the optimal route to the RPF check object. The outgoing interface of this route will act as the RPF
interface and the next hop will be taken as the RPF neighbor. The RPF interface completely relies on
the existing unicast route, and is independent of PIM. The RPF interface must be PIM-enabled, and
the RPF neighbor must also be a PIM neighbor. If PIM is not enabled on the router where the RPF
interface or the RPF neighbor resides, the establishment of a multicast distribution tree will surely fail,
causing abnormal multicast forwarding.
• Because a hello message does not carry the PIM mode information, a router that is running PIM
cannot identify what PIM mode its PIM neighbor is running. If different PIM modes are enabled on
the RPF interface and on the corresponding interface of the RPF neighbor router, the establishment
of a multicast distribution tree will fail, causing abnormal multicast forwarding.
• The same PIM mode must run on the entire network. Otherwise, the establishment of a multicast
distribution tree will fail, causing abnormal multicast forwarding.
Solution
1. Use the display ip routing-table command to verify that a unicast route exists from the receiver host
to the multicast source.
2. Use the display pim interface command to verify that PIM is enabled on the interfaces, especially
on the RPF interface. If PIM is not enabled on the interface, use the pim dm or pim sm command to
enable PIM-DM or PIM-SM.
3. Use the display pim neighbor command to verify that the RPF neighbor is a PIM neighbor.
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