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•
When PIM-SM runs on the entire network and when a router joins 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 acts as
the RPF interface and the next hop is 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 surely fails, 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 fails, causing abnormal multicast forwarding.
•
The same PIM mode must run on the entire network. Otherwise, the establishment of a
multicast distribution tree fails, causing abnormal multicast forwarding.
Solution
1.
Use the
display ip routing-table
command to verify that a unicast route from the receiver host
to the multicast source is available.
2.
Use the
display pim interface
command to verify PIM information on each interface,
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.
4.
Verify that PIM and IGMP are enabled on the interfaces directly connecting to the multicast
source and to the receivers.
5.
Use the
display pim interface verbose
command to verify that the same PIM mode is enabled
on the RPF interface and the corresponding interface of the RPF neighbor router.
6.
Verify that the same PIM mode is enabled on all routers in the entire network. Make sure that
the same PIM mode (PIM-SM or PIM-DM) is enabled on all routers. For PIM-SM, verify that the
BSR and RP configurations are correct.
Multicast data 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 data but no corresponding (S, G)
entry is created in the PIM routing table.
Analysis
•
If a multicast forwarding boundary has been configured through the
multicast boundary
command, any multicast packet is kept from crossing the boundary, and no routing entry can be
created in the PIM routing table.
•
In addition, the
source-policy
command filters received multicast packets. If the multicast data
fails to pass the ACL rule defined in this command, PIM cannot create the route entry either.
