The successful establishment of multicast configuration requires both multicast routing and multicast forwarding to be addressed.  Organisations that want to use multicast:

• MUST support MBGP on the edge router
• MUST support PIM-SM on at least the edge router
• MAY require support of MSDP on the edge router

MBGP

Multicast routing within KAREN is achieved using multiprotocol BGP (MBGP) which needs to be supported on the edge device that provides connectivity to KAREN.  MBGP provides multicast route information independently of the unicast routing information that is provided via BGP.

PIM-SM

Multicast forwarding can be achieved by enabling protocol independent multicast sparse mode (PIM-SM) on the edge router.  When this is done you will use the core KAREN router as the rendezvous point (RP) as you will be joining the KAREN multicast domain.

MSDP

Some organisations may want to establish an independent (internal) PIM-SM environment. This option requires multicast source discovery protocol (MSDP) to be supported on edge routers providing separation between the KAREN multicast domain and the organisation's multicast domain.

It is also important to understand that the use of MSDP does not separate the multicast domains completely. By default, the core KAREN router will win the RP election. While there will be two multicast domains there will only be one RP. It is necessary to filter the bootstrap messages from both entering and exiting at the edge router.  Once this is achieved the two multicast domains will have separate RP’s which will build independent forwarding tree’s, allowing the organisation to utilise KAREN’s multicast services as well as their own internal service.

An example of this is the Cisco interface command 'ip pim bsr-border' which configures the interface in question as a PIM domain border and drops PIM-SM bootstrap messages bi-directionally.

More information

What is multicast? | KAREN Technical FAQs

Multicasting and beacons | KAREN wiki

Updated 25 March 2009