04-06-2012 01:41 AM
Kindly help me to understand what is designated router in multicast term? and what is it use for?
do Designated router is the router that facing directly to the host (source/receiver)?
Many thanks for your explanation.
04-07-2012 12:48 AM - edited 04-07-2012 12:55 AM
DR are used to avoid loops, duplicate joins, etc.
The DR is like a next hop to get not only to the multicast source, this is the next hop to get to the Multicast Tree and to a multicast router called RP, this is not the source, but have the multicast tree control capability, and often will be near to the source.
Under multicast there is two main features or topology, Shared Tree, and Source Based Tree. I suppose you have heard about PIM Sparse Mode, and PIM Dense Mode. Sparse use a Shared Tree, and Dense a Source Tree. PIM is a multicast routing protocol that every multicast router should have enabled.
The objetive of these trees are to use the less bandwidth along your network, and build a loop free topology.
Every receiver, client, host or as you want to call this, receives the same voice, video or data stream than other clients. But sources, only transmit or flood 1 flow into the network. Multicast routers have the work to get this single flow reach every client, so splitting this flow as needed, but saving bandwidth and network resources, so if there is no clients multicast flow will be dropped.
Clients dont speak PIM, these only use IGMP to join the multicast stream whenever they want to receive this, this could be a radio station, video or anything else.
Clients send a igmp join request for a multicast stream (this is a multicast packet), this join request is processed only by the IGMP/PIM DR router in that network link, this avoid duplicated join requests to be forwarded up into the tree. If there is several multicast routers in the same network and all process and forward the joins there will be lot of duplicates.
This DR router, will encapsulate the igmp joing into a pim packet, and will forward this to the next hop pointing to the RP router for that multicast stream, this will be another DR in the multicast path or tree up to the RP.
As soon RP receives the IGMP Join requests into a PIM packet, register the client and start flooding the multicast stream to the client across the multicast tree.
These trees are used to send traffic from the source to the receivers, but also from the receivers or clients to the router called Rendezvous Point or RP. This RP have the responsability to maintain the multicast tree.
This way, multicast traffic is not always flooded across your network, this is usually flooded on demand, or on a a flood and prune basis. DR points to the RP, and RP control the multicast flows, clients joins, source traffic, etc.