Troubleshooting Pim Configuration; Failure Of Building A Multicast Distribution Tree Correctly - H3C S7500E Series Operation Manual

Operation Manual – Multicast
H3C S7500E Series Ethernet Switches

5.8 Troubleshooting PIM Configuration

5.8.1 Failure of Building a Multicast Distribution Tree Correctly

I. Symptom
None of the routers in the network (including routers directly connected with multicast
sources and receivers) has multicast forwarding entries. That is, a multicast distribution
tree cannot be built correctly and clients cannot receive multicast data.
II. 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 along the SPT. When the multicast data is flooded to a router, no
matter which router is, it 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 is to 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 router 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
running PIM is unable to know 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 surely fail, causing abnormal multicast forwarding.
The same PIM mode must run on the entire network. Otherwise, the establishment
of a multicast distribution tree will surely fail, causing abnormal multicast
forwarding.
III. Solution
1) Check unicast routes. Use the display ip routing-table command to check
whether a unicast route exists from the receiver host to the multicast source.
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Chapter 5 PIM Configuration