3Com S7906E - Switch, Configuration Manual

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1-8
2) If all the C-RPs have the same priority, their hash values are calculated through the hashing
algorithm. The C-RP with the largest hash value wins.
3) If all the C-RPs have the same priority and hash value, the C-RP with the highest IP address wins.
The hashing algorithm used for RP calculation is: Value (G, M, C
i
) = (1103515245 * ( (1103515245 * (G
& M) + 12345) XOR C
i
) + 12345) mod 2
31
. The table below gives the meanings of the values in this
algorithm.
Table 1-1 Values in the hashing algorithm
Value Description
Value Hash value
G IP address of the multicast group
M Hash mask length
C
i
IP address of the C-RP
& Logical operator of “and”
XOR Logical operator of “exclusive-or”
Mod Modulo operator, which gives the remainder of an integer division
RPT establishment
Figure 1-5 RPT establishment in a PIM-SM domain
As shown in Figure 1-5 , the process of building an RPT is as follows:
1) When a receiver joins multicast group G, it uses an IGMP message to inform the directly connected
DR.
2) Upon getting the receiver information, the DR sends a join message, which is hop by hop
forwarded to the RP corresponding to the multicast group.
3) The routers along the path from the DR to the RP form an RPT branch. Each router on this branch
generates a (*, G) entry in its forwarding table. The * means any multicast source. The RP is the
root, while the DRs are the leaves, of the RPT.