111
PIM configuration (available only on the
A5500 EI)
NOTE:
•
The term
router in this document refers to both routers and Layer 3 switches.
•
To facilitate description, the term
PIM domain in this document refers to a network that comprises
PIM-capable routers.
•
The interfaces in this document refer to Layer 3 interfaces in generic sense and Ethernet interfaces
operating in route mode. For more information about the operating mode of the Ethernet interface,
see the
Layer 2—LAN Switching Configuration Guide.
PIM overview
Protocol Independent Multicast (PIM) provides IP multicast forwarding by leveraging static routes or
unicast routing tables generated by any unicast routing protocol, such as routing information protocol
(RIP), open shortest path first (OSPF), intermediate system to intermediate system (IS-IS), or border
gateway protocol (BGP). Independent of the unicast routing protocols that are running on the device,
multicast routing can be implemented as long as the corresponding multicast routing entries are created
through unicast routes. PIM uses the reverse path forwarding (RPF) mechanism to implement multicast
forwarding. When a multicast packet arrives on an interface of the device, it undergoes an RPF check. If
the RPF check succeeds, the device creates the corresponding routing entry and forwards the packet. If
the RPF check fails, the device discards the packet. For more information about RPF, see the chapter
“Multicast routing and forwarding configuration.”
Based on the implementation mechanism, PIM falls into the following modes:
•
Protocol Independent Multicast–Dense Mode (PIM-DM)
•
Protocol Independent Multicast–Sparse Mode (PIM-SM)
•
Bidirectional Protocol Independent Multicast (BIDIR-PIM)
•
Protocol Independent Multicast Source-Specific Multicast (PIM-SSM)
PIM-DM overview
PIM-DM is a type of dense mode multicast protocol. It uses the push mode for multicast forwarding, and
is suitable for small-sized networks with densely distributed multicast members.
The following describes the basic implementation of PIM-DM:
•
PIM-DM assumes that at least one multicast group member exists on each subnet of a network.
Therefore, multicast data is flooded to all nodes on the network. Then, branches without multicast
forwarding are pruned from the forwarding tree, leaving only those branches that contain
receivers. This flood-and-prune process takes place periodically. Pruned branches resume multicast
forwarding when the pruned state times out. Data is then flooded again down these branches, and
then the branches are pruned again.
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