xStack
®
DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch CLI Manual
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PROTOCOL INDEPENDENT MULTICAST (PIM) COMMANDS
PIM or Protocol Independent Multicast is a method of forwarding traffic to multicast groups over the network using any
pre-existing unicast routing protocol, such as RIP or OSPF, set on routers within a multicast network. The xStack® DGS-
3600 Series supports three types of PIM, Dense Mode (PIM-DM), Sparse Mode (PIM-SM), and Sparse and Dense Mode
(PIM-SM-DM).
PIM-SM or Protocol Independent Multicast – Sparse Mode is a method of forwarding multicast traffic over the network
only to multicast routers who actually request this information. Unlike most multicast routing protocols which flood the
network with multicast packets, PIM-SM will forward traffic to routers who are explicitly a part of the multicast group
through the use of a Rendezvous Point (RP). This RP will take all requests from PIM-SM enabled routers, analyze the
information and then returns multicast information it receives from the source, to requesting routers within its configured
network. Through this method, a distribution tree is created, with the RP as the root. This distribution tree holds all PIM-
SM enabled routers within which information collected from these router is stored by the RP.
PIM-SM
Two other types of routers also exist with the PIM-SM configuration. When many routers are a part of a multiple access
network, a Designated Router (DR) will be elected. The DR’s primary function is to send Join/Prune messages to the RP.
The router with the highest priority on the LAN will be selected as the DR. If there is a tie for the highest priority, the router
with the higher IP address will be chosen.
The third type of router created in the PIM-SM configuration is the Boot Strap Router (BSR). The goal of the Boot Strap
Router is to collect and relay RP information to PIM-SM enabled routers on the LAN. Although the RP can be statically
set, the BSR mechanism can also determine the RP. Multiple Candidate BSRs (C-BSR) can be set on the network but
only one BSR will be elected to process RP information. If it is not explicitly apparent which C-BSR is to be the BSR, all
C-BSRs will emit Boot Strap Messages (BSM) out on the PIM-SM enabled network to determine which C-BSR has the
higher priority and once determined, will be elected as the BSR. Once determined, the BSR will collect RP data
emanating from candidate RPs on the PIM-SM network, compile it and then send it out on the land using periodic Boot
Strap Messages (BSM). All PIM-SM Routers will get the RP information from the Boot Strap Mechanism and then store it
in their database.
Although Hello packets discover PIM-SM routers, these routers can only join or be “pruned” from a multicast group
through the use of Join/Prune Messages exchanged between the DR and RP. Join/Prune Messages are packets relayed
between routers that effectively state which interfaces are, or are not to be receiving multicast data. These messages can
be configured for their frequency to be sent out on the network and are only valid to routers if a Hello packet has first
been received. A Hello packet will simply state that the router is present and ready to become a part of the RP’s
distribution tree. Once a router has accepted a member of the IGMP group and it is PIM-SM enabled, the interested
router will then send an explicit Join/Prune message to the RP, which will in turn route multicast data from the source to
the interested router, resulting in a unidirectional distribution tree for the group. Multicast packets are then sent out to all
nodes on this tree. Once a prune message has been received for a router that is a member of the RP’s distribution tree,
the router will drop the interface from its distribution tree.
Discovering and Joining the Multicast Group
Two types of distribution trees can exist within the PIM-SM protocol, a Rendezvous-Point Tree (RPT) and a Shortest Path
Tree (SPT). The RP will send out specific multicast data that it receives from the source to all outgoing interfaces enabled
to receive multicast data. Yet, once a router has determined the location of its source, an SPT can be created, eliminating
hops between the source and the destination, such as the RP. This can be configured by the switch administrator by
setting the multicast data rate threshold. Once the threshold has been passed, the data path will switch to the SPT.
Therefore, a closer link can be created between the source and destination, eliminating hops previously used and
shortening the time a multicast packet is sent from the source to its final destination.
Distribution Trees
Register and Register Suppression Messages
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