multicast DR

aueddonline

hey guy's bit confused about the DR used for multicast I keep getting little bits of info on it and non of them say what it does starting to mess with my head

dtlokee

You can think of it as the "search engine" in torrents. The senders and the receivers use the RP to locate each other and the actual traffic flows directly between the senders and receivers once they have located one another. In the case of multcast it allows the source to register to the RP saying I am a source so when a receiver sends a join to the RP the source and receiver can now form a shortest path tree between them. A common misconception is the RP stays in the traffic flow, that is not necessarly the case (it may be if the RP is located along the shortest path tree). It is also stated that the RP dosen't need to know it's an RP for this to work but in recent IOS version Cisco enhanced the security of the router now by requiring you to tell the router it will function as a RP (by using the ip pim rp-address command or configuring Auto-rp, bsr, et al) on the router you want to function as a RP or you will get a message something like this "received invald join from...."

HTH

aueddonline

so the DR does the searching? that's it's main job

with the whole RPT and SPT by default it will always switch over to the SPT, yes? unless you configure the 'ip pim spt-threshold (Kbps)' command ?

dtlokee

Not really searching, but it is a place for all the register and join messages to meet up so the receivers can find the sources.

aueddonline

got ya, I think

cheers dtlokee, you make more sence than this book i'm reading

aueddonline

oh one more thing is the DR a IGMP function or a PIM function?

dtlokee

A PIM function, The IGMP join is picked up by the first hop router and forwarded to the DR as a PIM join (or in the case of a multicast source as a PIM Register). The DR then tells the sources and receivers to create a SPT between them.