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xStack DGS/DXS-3300 Series Layer 3 Stackable Gigabit Ethernet Switch CLI Manual
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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.
The PIM commands in the Command Line Interface(CLI) are listed below, along with their appropriate parameters, in the
following table.
Command Parameters
enable pim
disable pim
config pim [[ipif <ipif_name 12> | all] {hello <sec 1-18724> |
jp_interval <sec 1-18724> | state [enable |
disable] | mode [dm | sm] | dr_priority
<unsigned_int 0 – 4294967294>}
config pim register_probe_time <value 1-127>
config pim register_suppression_time <value 3-255>
create pim crp group <network_address> rp <ipif_name 12>
delete pim crp group <network address>