| Description | The (conceptual) table listing mappings from multicast
group prefixes to the PIM mode and RP address to use for
groups within that group prefix.
Rows in this table are created for a variety of reasons,
indicated by the value of the pimSmGroupMappingOrigin object.
- Rows with a pimSmGroupMappingOrigin value of 'fixed' are
created automatically by the router at startup, to
correspond to the well-defined prefixes of link-local and
unroutable group addresses. These rows are never
destroyed.
- Rows with a pimSmGroupMappingOrigin value of 'embedded' are
created by the router to correspond to group prefixes
that are to be treated as being in Embedded-RP format, as
defined in [RFC3956].
- Rows with a pimSmGroupMappingOrigin value of 'config' are
created and destroyed as a result of rows in the
pimSmSsmRangeTable and pimSmStaticRPTable being created and
destroyed.
- Rows with a pimSmGroupMappingOrigin value of 'bsr' are
created as a result of running the PIM Bootstrap Router
(BSR) mechanism. If the local router is not the elected
BSR, these rows are created to correspond to group
prefixes in the PIM Bootstrap messages received from the
elected BSR. If the local router is the elected BSR,
these rows are created to correspond to group prefixes in
the PIM Bootstrap messages that the local router sends.
In either case, these rows are destroyed when the group
prefixes are timed out by the BSR mechanism.
- Rows with a pimSmGroupMappingOrigin value of 'other' are
created and destroyed according to some other mechanism
not specified here.
Given the collection of rows in this table at any point in
time, the PIM mode and RP address to use for a particular
group is determined using the following algorithm.
1. From the set of all rows, the subset whose group prefix
contains the group in question are selected.
2. If there are no such rows, the behavior is undefined.
3. If there is at least one such row, from the selected
subset of rows, the subset that have the lowest value
of pimSmGroupMappingPrecedence are selected.
4. From the selected subset of rows, the subset that have
the greatest value of pimSmGroupMappingGrpPrefixLength are
selected.
5. If there are multiple selected rows, the subset that have
pimSmGroupMappingActive set to 'true' are selected. There
must be at least one such row.
6. If there are multiple selected rows, the row selected is
implementation dependent; the implementation might or
might not apply the PIM hash function to select the row.
7. The group mode to use is given by the value of
pimSmGroupMappingPimSmMode from the single selected row; the
RP to use is given by the value of
pimSmGroupMappingRPAddress, unless pimSmGroupMappingOrigin is
'embedded', in which case the RP is extracted from the
group address in question using the algorithm in
[RFC3956]. |