Information Technology Reference
In-Depth Information
With RPF, received multicast packets are forwarded out all other interfaces, allowing the
data stream to reach all segments. If no hosts are members of a multicast group on any of
the router's attached or downstream subnets, the router sends a prune message up the dis-
tribution tree (the reverse path) to tell the upstream router not to send packets for the mul-
ticast group. So, the analogy for PIM-DM is the push method for sending junk mail, and
the intermediate router must tell upstream devices to stop sending it.
PIM-SM
PIM-SM is defined in RFC 2362. PIM-SM assumes that no hosts want to receive multicast
traffic unless specifically requested The RP gathers the information from senders and
makes the information available to receivers. Routers with receivers have to register with
the RP. The end-host systems request multicast group membership using IGMP with their
local routers. The routers serving the end systems then register as traffic receivers with the
RPs for the specified group in the multicast network.
Joining PIM-SM
With PIM-SM, DRs on end segments receive IGMP query messages
from hosts wanting to join a multicast group. The router checks whether it is already
receiving the group for another interface. If it is receiving the group, the router adds
the new interface to the table and sends membership reports periodically on the new
interface.
If the multicast group is not in the multicast table, the router adds the interface to the
multicast table and sends a join message to the RP with multicast address 224.0.0.13 (all
PIM routers) requesting the multicast group.
Pruning PIM-SM
When a PIM-SM does not have any more multicast receiving hosts
or receiving routers out any of its interfaces, it sends a prune message to the RP. The
prune message includes the group to be pruned or removed.
PIM DR
A designated router is selected in multiaccess segments running PIM. The PIM DR is
responsible for sending join, prune, and register messages to the RP. The PIM router with
the highest IP address is selected as the DR.
Auto-RP
Another way to configure the RP for the network is to have the RP announce its services
to the PIM network. This process is called auto-RP. Candidate RPs send their announce-
ments to RP mapping agents with multicast address 224.0.1.39 (
cisco-rp-announce
). RP
mapping agents are also configured. In smaller networks, the RP can be the mapping
agent. The 224.0.1.40 address is used in AUTO-RP-DISCOVERY is the destination address
for me s s age s from the RP mapping agent to dis cover c andidate s. Config ured RP mapping
agents listen to the announcements. The RP mapping agent then selects the RP for a group
based on the highest IP address of all the candidate RPs. The RP mapping agents then
