PLANET WGS3-2820 - Manual (Page 299)

User’s Manual of WGS3-2820/WGS3-5220
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4.7.5 PIM-DM
Dense mode PIM initiates forwarding state in routers when a source begins to send. A source does not give any prior
notifications to the network when it sends multicast datagrams to a group G. If a receiving router does not already have a
forwarding entry, it creates it for the source and group G. This forwarding entry is called a (S,G) entry. It includes the following
contents: source address, group address, the incoming interface, a list of outgoing interfaces, a few flags and a few timers.
The incoming interface for (S,G) is determined by an RPF lookup in the unicast routing table. The (S,G) outgoing interface list
contains interfaces that have PIM routers present or host members for group G.
If a router creates a (S,G) entry with an empty outgoing interface list after receiving a multicast datagram, it must trigger a
PIM-Prune message toward the source S. This type of entry is called a negative cache entry. Negative cache entries can be
found on leaf routers with no local group members, or on routers where prune messages were received from downstream
routers that caused the outgoing interface list to become NULL.
Dense mode PIM routers send periodic Hello messages out of each interface and keep track of neighbors based on received
Hello messages. The Hello message has a Holdtime field that tells the neighbor to delete neighbor information if it is not
refreshed before expiration.
Dense-mode PIM assumes that when a source starts sending, all downstream systems want to receive multicast datagrams.
Initially, multicast datagrams are flooded to all areas of the network. If some areas of the network do not have group members,
dense-mode PIM will prune off the forwarding branch by setting up prune state. The prune state has an associated timer, hich
on expiration will turn into forward state, allowing data to go down the branch previously in prune state.
The prune state contains source and group address information. When a new member appears in a pruned area, a router can
``graft'' toward the source for the group, turning the pruned branch into forward state. The forwarding branches form a tree
rooted at the source leading to all members of the group. This tree is called a source rooted tree.
The broadcast of datagrams followed by pruning of unwanted branches is often referred to as a broadcast-and-prune cycle,
typical of dense mode protocols. The broadcast-and-prune mechanism in dense mode PIM uses a technique called reverse
path forwarding (RPF), in which a multicast datagram is forwarded if the receiving interface is the one used to forward unicast
datagrams to the source of the datagram.
Compared with multicast routing protocols with built-in topology discovery mechanisms (e.g. DVMRP with its own RIP-like
``unicast'' routing protocol), dense mode PIM has simplified design, and is not hard-wired into a specific type of topology
discovery protocol. However, such simplification does incur more overhead and cause broadcast-and-prune to occur on some
links that could be avoided if sufficient topology information is available, e.g. to decide whether each interface leads to any
downstream neighbors for a particular source. We chose to accept the additional overhead in favor of the simplification and
flexibility gained by not depending on a specific type of topology discovery protocol.