H3C Technologies H3C S12500-X Series Switches - Manual (Page 113)

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• On a PIM-SM enabled network, when a router wants to join the SPT, the router creates an (S, G)
entry 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-SM is not enabled on the RPF interface toward the multicast source, the router
cannot create an (S, G) entries.
• When a multicast router receives a multicast packet, it looks up the existing unicast routing table for
the optimal route to the packet source. The outgoing interface of this route act as the RPF interface
and the next hop acts the RPF neighbor. The RPF interface completely relies on the existing unicast
route and is independent of PIM. The RPF interface must be enabled with PIM, and the RPF neighbor
must be a PIM neighbor. If PIM is not enabled on the RPF interface or the RPF neighbor, the multicast
distribution tree cannot be built correctly, causing abnormal multicast forwarding.
• Because a hello message does not carry PIM mode information, a PIM router cannot identify what
PIM mode its PIM neighbor is running. If the RPF interface on a router and the connected interface
of the router's RPF neighbor operate in different PIM modes, the multicast distribution tree cannot be
built correctly, causing abnormal multicast forwarding.
• The same PIM mode must run on the entire network. Otherwise, the multicast distribution tree cannot
be built correctly, causing abnormal multicast forwarding.
Solution
1. Use display ip routing-table to verify that a unicast route to the multicast source or the RP is
available.
2. Use display pim interface to verify PIM information on each interface, especially on the RPF
interface. If PIM is not enabled on the interfaces, use pim dm or pim sm to enable PIM-DM or
PIM-SM for the interfaces.
3. Use display pim neighbor to verify that the RPF neighbor is a PIM neighbor.
4. Verify that PIM and IGMP are enabled on the interfaces that directly connect to the multicast
sources or the receivers.
5. Use display pim interface verbose to verify that the same PIM mode is enabled on the RPF interface
on a router and the connected interface of the router's RPF neighbor.
6. Use display current-configuration to verify that the same PIM mode is enabled on all routers. For
PIM-SM, verify that the BSR and C-RPs are correctly configured.
Multicast data is abnormally terminated on an intermediate
router
Symptom
An intermediate router can receive multicast data successfully, but the data cannot reach the last-hop
router. An interface on the intermediate router receives multicast data but does not create an (S, G) entry
in the PIM routing table.
Analysis
• If a multicast forwarding boundary has been configured through the multicast boundary command,
and the multicast packets are kept from crossing the boundary, PIM cannot create routing entries for
the packets.
• If an ACL is defined by the source-policy command, and the multicast packets cannot match the
ACL rule, PIM cannot create the routing entries for the packets.