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Describing the XSR's IP Multicast Features
Two basic types of MDTs are source and shared trees, described as follows:
•
A source tree is a distribution network with its root at the source and branches forming a
spanning tree through the network to its receivers. Because this tree uses the shortest path
through the network, it is also referred to as a Shortest Path Tree (SPT). Different sources
usually employ different distribution trees.
•
A shared tree has a common root at a chosen site in the network called Rendezvous Point (RP).
Different sources belonging to the same multicast group share the same distribution tree
rooted at the RP.
Source trees consume more memory to store its states than shared trees but can often supply a
better path from source to receivers with less delay.
The XSR generates and maintains MDTs with PIM, a set of popularly used multicast routing
protocols. PIM derives its name from the fact that it is IP routing protocol independent. Although
termed a multicast routing protocol, PIM uses the existing unicast routing table to perform
multicast forwarding via the Reverse Path Forwarding (RPF) check function instead of building
an independent multicast routing table. In this regard PIM can operate in two modes: PIM Dense
Mode (PIM-DM) and PIM Sparse Mode (PIM-SM).
•
PIM-DM uses a fairly simple approach to handle IP multicast routing. Assuming there are
receivers at most locations for the multicast packet stream, PIM-DM starts by flooding
multicast traffic, then stops at each link when an explicit stop request is received.
•
By contrast, PIM-SM assumes relatively fewer receivers, sending multicasts only when
requested to do so. PIM-SM is characterized by a combination of shared and shortest-path
distribution trees. All group participants can use a shared distribution tree. Or the last hop
routers can initiate a switch to shortest-path trees for certain sources when needed (for
example, as data rates or delay requirements warrant, and scale permits).
Since PIM is unicast routing protocol-independent, PIM-SM uses explicit joins to build the
multicast distribution tree that limits multicast traffic to only flow through the joined branches.
And PIM-SM is flexible enough to change from a shared to source-based tree. These advantages
make PIM-SM a better choice for intra-domain multicast service.
Forwarding Multicast Traffic
The XSR forwards multicast traffic as follows:
•
When a multicast packet with source address S is received by a router, the packet will be
forwarded only if it arrived on the interface to which the router would forward a unicast
packet with destination address S. Otherwise, the packet is dropped. This is known as the
Reverse Path Forwarding (RPF) test which is designed to prevent a forwarding loop since
unicast routes are loop-free.
•
Then the router will check the local distribution tree to get a list of outgoing interfaces,
duplicate the packets, and send them to all outgoing interfaces.
Describing the XSR's IP Multicast Features
IGMPv3 is designed to enable each multicast router to learn, for each of its directly attached
networks, which multicast addresses are of interest to the systems attached to those networks.
IGMP version 3 adds the capability for a multicast router to also learn which sources are of
interest to neighboring systems, for packets sent to any particular multicast address. The data
gathered by IGMP is provided to whichever multicast routing protocol (e.g, PIM) is being used by
the router, to ensure multicast packet delivery to all networks with interested receivers.
7-4 Configuring PIM-SM and IGMP
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