R21xx-HP FlexFabric 11900 IP Multicast Configuration Guide

102
Troubleshooting PIM
A multicast distribution tree cannot be correctly built
Symptom
A multicast distribution tree cannot be correctly built because there are no multicast forwarding entries
established on the routers (including routers directly connected with multicast sources or receivers) in a
PIM network.
Analysis
• On a PIM-DM enabled network, multicast data is flooded from the router that directly connects to
the multicast source to the routers that directly connects to the receivers. When the multicast data is
flooded to a router, 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-DM is not enabled on the RPF
interface toward the multicast source, the router cannot create an (S, G) entry.
• 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.