xStack DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch CLI Manual
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PIM
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OMMANDS
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
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.
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.
Discovering and Joining the Multicast Group
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.
Distribution Trees
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.
Register and Register Suppression Messages
Multicast sources do not always join the intended receiver group. The first hop router (DR) can send multicast data without being the
member of a group or having a designated source, which essentially means it has no information about how to relay this information
to the RP distribution tree. This problem is alleviated through Register and Register-Stop messages. The first multicast packet
received by the DR is encapsulated and sent on to the RP which in turn removes the encapsulation and sends the packet on down the
RP distribution tree. When the route has been established, a SPT can be created to directly connect routers to the source, or the
multicast traffic flow can begin, traveling from the DR to the RP. When the latter occurs, the same packet may be sent twice, one type
encapsulated, one not. The RP will detect this flaw and then return a Register Suppression message to the DR requesting it to
discontinue sending encapsulated packets.
