CANOGA PERKINS CanogaOS Configuration Guide - Manual (Page 180)

CanogaOS Configuration Guide
Proprietary & Confidential Canoga Perkins Metro Ethernet Switches Page 180 of 350
24.3.3 Data Flow from Source to Receivers
1. Sending out Hello Messages
PIM routers periodically send Hello messages to discover neighboring PIM routers. Hello
messages are multicast using the address 224.0.0.13 (ALL-PIM-ROUTERS group). Routers do
not send any acknowledgement that a Hello message was received. A holdtime value determines
the length of time for which the information is valid. In PIM-SM, a downstream receiver must
join a group before traffic is forwarded on the interface.
2. Electing a Designated Router
In a multi-access network with multiple routers connected, one of them is selected to act as a
designated router (DR) for a given period of time. The DR is responsible for sending Join/Prune
messages to the RP for local members.
3. Determining the RP
PIM-SM uses a BootStrap Router (BSR) to originate Bootstrap messages, and to disseminate RP
information. The messages are multicast to the group on each link. If the BSR is not apparent,
the routers flood the domain with advertisements. The router with the highest priority (if
priorities are same, the higher IP address applies) is selected to be the RP. Routers receive and
store Bootstrap messages originated by the BSR. When a DR gets a membership indication from
IGMP for (or a data packet from) a directly connected host, for a group for which it has no entry,
the DR maps the group address to one of the candidate RPs that can service that group. The DR
then sends a Join/Prune message towards that RP.
In a small domain, the RP can also be configured statically.
4. Joining the Shared Tree
To join a multicast group, a host sends an IGMP message to its upstream router, after which the
router can accept multicast traffic for that group. The router sends a Join message to its upstream
PIM neighbor in the direction of the RP.
When a router receives a Join message from a downstream router, it checks to see if a state exists
for the group in its multicast routing table. If a state already exists, the Join message has reached
the shared tree, and the interface from which the message was received is entered in the
Outgoing Interface list. If no state exists, an entry is created, the interface is entered in the
Outgoing Interface list, and the Join message is again sent towards the RP.
5. Registering with the RP
A DR can begin receiving traffic from a source without having a Source or a Group state for that
source. In this case,
the DR has no information on how to get multicast traffic to the RP through a tree. When the
source DR receives the initial multicast packet, it encapsulates it in a Register message, and
unicasts it to the RP for that group. The RP deencapsulates each Register message, and forwards
the extracted data packet to downstream members on the RPT.
Once the path is established from the source to the RP, the DR begins sending traffic to the RP as
standard IP multicast packets, as well as encapsulated within Register messages. The RP
temporarily receives packets twice.