Troubleshooting guide

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ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 11 Troubleshooting Layer 3 Network Connections
Troubleshooting Layer 3 IP Multicast Switching
Dense Mode
In dense mode, a switch router assumes that all other switch routers want to forward multicast packets
for a group. Therefore, interfaces with PIM dense mode enabled receive the multicast feed as soon as a
single user requests one. That segment will continue to receive the multicast until it times out. If a
Catalyst 8500 receives a multicast packet and has no directly attached members or PIM neighbors
present, a prune message is sent back to the source. Subsequent multicast packets are not flooded to this
pruned branch. PIM builds source-based multicast distribution trees. PIM dense mode is most useful
when:
• The senders are receivers are in close proximity to one another
• There are fewer senders than receivers
• Multicast traffic volume is high
• The stream of multicast traffic is constant
Sparse Mode
In sparse mode, a switch router assumes that other switch routers do not want to forward multicast
packets for a group, unless there is an explicit request for the traffic. When hosts join a multicast group,
the directly connected switch routers send PIM join messages to the rendezvous point (RP). The RP
keeps track of multicast groups. Hosts that send multicast packets are registered with the RP by that
host's first-hop switch router. The RP then send joins toward the source. At this point, packets are
forwarded on a shared distribution tree. When the data stream begins to flow from sender to RP to
receiver, the switch routers in the path optimize the path, automatically, to remove any unnecessary hops.
Sparse mode assumes that no hosts want the multicast traffic unless they specifically ask for it.
Sparse mode PIM is optimized for environments where there are many multipoint data streams and each
multicast stream goes to a relatively small number of LANs in the internetwork. PIM sparse mode is
most useful when:
• There are few receivers in a group
• Senders and receivers are separated by WAN links
• The type of traffic is intermittent
There are two types of rendezvous points: statically configured and Auto-RP.
A statically configured PIM rendezvous point (RP) address is used by first-hop switch routers to send
Register packets on behalf of source multicast hosts. The RP address is also used by switch routers on
behalf of multicast hosts that want to become members of a group. These switch routers send Join and
Prune messages toward the RP. A single RP can be configured for all multicast groups or a subset of the
Class D address range as described by the access-list pointer.
Auto-RP automates the distribution of group-to-RP mappings in a PIM network. This feature has the
following benefits:
• It is easy to use multiple RPs within a network to serve different group ranges.
• It allows load splitting among different RPs and arrangement of RPs according to the location of
group participants.
• It avoids inconsistent, manual RP configurations that can cause connectivity problems.
Multiple RPs can be used to serve different group ranges or serve as hot backups of each other. To make
Auto RP work, a Layer 3 enabled ATM switch router must be designated as an RP-mapping agent, which
receives the RP-announcement messages from the RPs and arbitrates conflicts. The RP-mapping agent
then sends the consistent group-to-RP mappings to all other switch routers. Thus, all switch routers
automatically discover which RP to use for the groups they support.