D-Link DGS-3600 Series User Manual page 246

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xStack DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch
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.
Assert Messages
At times on the PIM-SM enabled network, parallel paths are created from source to receiver, meaning some receivers will receive
the same multicast packets twice. To improve this situation, Assert messages are sent from the receiving device to both multicast
sources to determine which single router will send the receiver the necessary multicast data. The source with the shortest metric
(hop count) will be elected as the primary multicast source. This metric value is included within the Assert message.
PIM-DM
The Protocol Independent Multicast - Dense Mode (PIM-DM) protocol should be used in networks with a low delay (low latency)
and high bandwidth as PIM-DM is optimized to guarantee delivery of multicast packets, not to reduce overhead.
The PIM-DM multicast routing protocol is assumes that all downstream routers want to receive multicast messages and relies
upon explicit prune messages from downstream routers to remove branches from the multicast delivery tree that do not contain
multicast group members.
PIM-DM has no explicit 'join' messages. It relies upon periodic flooding of multicast messages to all interfaces and then either
waiting for a timer to expire (the Join/Prune Interval) or for the downstream routers to transmit explicit 'prune' messages
indicating that there are no multicast members on their respective branches. PIM-DM then removes these branches ('prunes' them)
from the multicast delivery tree.
Because a member of a pruned branch of a multicast delivery tree may want to join a multicast delivery group (at some point in
the future), the protocol periodically removes the 'prune' information from its database and floods multicast messages to all
interfaces on that branch. The interval for removing 'prune' information is the Join/Prune Interval.
PIM-SM-DM
In the PIM-SM, RP is a key point for the first hop of the sender. If the first hop does not have RP information when the sender
sends data out, it will drop the packet and do nothing. Sparse-Dense mode will be useful in this condition. In Sparse-Dense mode,
the packets can be flooded to all the outgoing interfaces and pruning/joining (prune/graft) can be used to control the outgoing
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