When an IPv6 interior gateway protocol is used to build the unicast routing table, the procedure to detect the upstream device
address assumes the address of a PIM neighbor is always same as the address of the next-hop device, as long as they refer
to the same device. However, it may not be the case when a device has multiple addresses on a link.
Two typical situations can lead to this situation for IPv6. The first situation can occur when the unicast routing table
is not built by an IPv6 interior gateway protocol such as multicast BGP. The second situation occurs when the address of an
RP shares a subnet prefix with downstream devices (note that the RP address has to be domain-wide and therefore cannot be
a link-local address).
The routable address hello option allows the PIM protocol to avoid such situations by adding a PIM hello message option that
includes all the addresses on the interface on which the PIM hello message is advertised. When a PIM device finds an upstream
device for some address, the result of RPF calculation is compared with the addresses in this option, in addition to the PIM
neighbor's address itself. Because this option includes all the possible addresses of a PIM device on that link, it always
includes the RPF calculation result if it refers to the PIM device supporting this option.
Because of size restrictions on PIM messages and the requirement that a routable address hello option fits within a single
PIM hello message, a limit of 16 addresses can be configured on the interface.