3Com Switch 8800 Advanced Software V5 Configuration Guide

Troubleshooting PIM Configuration 605
a device, no matter which device is, it creates (S, G) entries only if it has a route
to the multicast source. If the device does not have a route to the multicast
source, or if PIM-DM is not enabled on the device’s RPF interface to the
multicast source, the device cannot create (S, G) entries.
■ When PIM-SM runs on the entire network, and when a device is to join the SPT,
the device creates (S, G) entries only if it has a route to the multicast source. If
the device does not have a route to the multicast source, or if PIM-DM is not
enabled on the device’s RPF interface to the multicast source, the device cannot
create (S, G) entries.
■ When a multicast device 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 device 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 device
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 device, 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.
Solution
1 Check unicast routes. Use the display ip routing-table command to check
whether a unicast route exist from the receiver host to the multicast
source.
2 Check that PIM is enabled on the interfaces, especially on the RPF
interface. Use the display pim interface command to view the PIM
information on each 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 Check that the RPF neighbor is a PIM neighbor. Use the display pim
neighbor command to view the PIM neighbor information.
4 Check that PIM and IGMP are enabled on the interfaces directly connecting
to the multicast source and to the receivers.
5 Check that the same PIM mode is enabled on related interfaces. Use the
display pim interface verbose command to check whether the same PIM
mode is enabled on the RPF interface and the corresponding interface of
the RPF neighbor device.
6 Check that the same PIM mode is enabled on all the devices in the entire
network. Make sure that the same PIM mode is enabled on all the devices:
PIM-SM on all devices, or PIM-DM on all devices. In the case of PIM-SM,
also check that the BSR and RP configurations are correct.