Configuring Pim - HP A8800 Configuration Manual

Configuring PIM

Overview
Protocol Independent Multicast (PIM) provides IP multicast forwarding by leveraging unicast 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 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 is subject to 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
routing and
Based on the implementation mechanism, PIM falls into the following types:
Protocol Independent Multicast–Dense Mode (PIM-DM),
•
• Protocol Independent Multicast–Sparse Mode (PIM-SM),
Bidirectional Protocol Independent Multicast (BIDIR-PIM), and
•
Protocol Independent Multicast Source-Specific Multicast (PIM-SSM).
•
To facilitate description, a network comprising PIM-capable routers is referred to as a "PIM domain" in
this document.
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 basic implementation of PIM-DM is as follows:
PIM-DM assumes that at least one multicast group member exists on each subnet of a network, and
•
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, that is, pruned branches resume multicast
forwarding when the pruned state times out and then data is re-flooded down these branches, and
then are pruned again.
When a new receiver on a previously pruned branch joins a multicast group, to reduce the join
•
latency, PIM-DM uses a graft mechanism to resume data forwarding to that branch.
Generally speaking, the multicast forwarding path is a source tree, namely a forwarding tree with the
multicast source as its "root" and multicast group members as its "leaves". Because the source tree is the
shortest path from the multicast source to the receivers, it is also called shortest path tree (SPT).
The operating mechanism of PIM-DM is summarized as follows:
Neighbor discovery
•
SPT building
•
forwarding."
110
"Configuring multicast