15.4.3 IPv4 PIM-SSM

PIM-SSM is extended PIM-SM functionality. PIM-SM and PIM-SSM can run at the same time. The multicast addresses used by PIM-SSM are assigned by the IANA. The Switches allow the address range of multicast addresses (group addresses) for which PIM-SSM runs to be specified by configuration. PIM-SM runs on addresses other than those specified.

Whereas PIM-SM requires multicast forwarding packets when creating multicast entries, PIM-SSM creates multicast forwarding entries by exchanging multicast routing information (PIM-Join), and forwards multicast packets based on the corresponding entries. Note that PIM-SSM does not require a rendezvous point or bootstrap router. This means that packet encapsulation and decapsulation is not needed when multicast packets are forwarded, enabling more efficient multicast forwarding. PIM-SSM runs if IGMPv3 (INCLUDE mode) hosts are connected. The Switch provides measures to enable PIM-SSM from IGMPv2 or IGMPv3 (EXCLUDE mode) hosts.

<Structure of this section>

(1) PIM-SSM messaging support spec

Same as PIM-SM message support specification ("15.4.2 IPv4 PIM-SM (1) PIM-SM message support specification").

(2) Prerequisites for Running PIM-SSM

The following settings are required to configure the Switch:

Settings for each switch

Set the range of group addresses for which PIM-SSM runs.
Switches directly connected to hosts running IGMPv3 (INCLUDE mode)

Set up IGMPv3 for the connected interfaces.
Switches directly connected to hosts running IGMPv2 or IGMPv3 (EXCLUDE mode)

Set up IGMPv2 or IGMPv3 for the connected interfaces.

Set the source address for the group addresses used.

(3) PIM-SSM operation (when the host is in IGMPv3(INCLUDE)

Operation is as follows when the multicast packet distribution server (source address: S1) distributes multicast packets to group 1 (group address: G1).

A request to join the multicast group (IGMPv3 (INCLUDE mode)) is received from hosts.
The switch receiving the join request (IGMPv3 (INCLUDE mode)) sends PIM-Join from the reported group address (G1) and source address (S1) in the direction of the source address (S1, as determined by unicast routing information). In this case, the PIM-Join message contains information about the source address (S1) and group address (G1). Each switch receiving PIM-Join sends PIM-Join hop by hop in the direction of the source address (S1). Switches receiving PIM-Join learn multicast routing information for the source address (S1) and group address (G1).
The multicast packet distribution server (S1) sends multicast packets bound for group 1 (G1). Switches receiving multicast packets forward them based on the multicast forwarding entries generated by the learned multicast routing information.

The following figure shows an overview of PIM-SSM operation.

Figure 15-18: Summary of PIM-SSM Operation

(4) PIM-SSM operation (when hostis IGMPv2 or IGMPv3(EXCLUDE)

Operation is as follows when the multicast packet distribution server (source address: S1) distributes multicast packets to group 1 (group address: G1).

A request to join the multicast group (IGMPv2 or IGMPv3 (EXCLUDE mode)) is received from hosts.
The switch receiving the join request (IGMPv2 or IGMPv3 (EXCLUDE mode)) compares the reported group address (G1) and that set in the configuration. If these group addresses match, a PIM-Join message is sent to the source address (S1) set in configuration over the shortest path (determined by the unicast routing information). In this case, the PIM-Join message contains information about the source address (S1) and group address (G1). Each switch receiving the PIM-Join message sends it hop-by-hop to the source address (S1) over the shortest path. Switches receiving PIM-Join learn multicast routing information for the source address (S1) and group address (G1).
The multicast packet distribution server (S1) sends multicast packets bound for group 1 (G1). Switches receiving multicast packets forward them based on the multicast forwarding entries generated by the learned multicast routing information.

For an outline of the operation of PIM-SSM, see "Overview of the operation of the graphical 15-18 PIM-SSM".

(5) Neighbor Detection

This is the same as PIM-SM("15.4.2 IPv4 PIM-SM (3) Neighbor Detection.

(6) Determining Forwarder

This is the same as PIM-SM("15.4.2 IPv4 PIM-SM (4) Determining Forwarder.

(7) Determining and Working with DR

Same as PIM-SM("15.4.2 IPv4 PIM-SM (5) "Determining and Working with DR".

(8) Notes on Redundant Routes

Same as in PIM-SM("15.4.2 IPv4 PIM-SM (6) Notes on redundant routes).