What is multicast?

Simply put, when you need to send data to a “group” of device and NOT ALL ( that’s broadcast) and NOT ONE ( that Unicast), that where we need multicast.

Why should we care? Or What problem multicast actually solve ?

We should, cause that the only way to send traffic to “group of devices”, that is more effective vs Broadcast, think of broadcast as uninvited guest, if that guest end up at your door, you will feel upset, in terms of Network, if device see a traffic that is not destined for him, he will just drop it!, but after processing it, that sheer waste of network bandwidth and device resources!, so yea Multicast is still relevant.

You know lot of application uses multicast, IP TV, video conferencing, Netflix, YouTube, see now you will care….You Tube!

what range of IP is multicast ?

What you need to know about multicast wrt to CCIE DC?

lets cover basics

You need to know multicast, primary in context of VxLAN, I will write about VxLAN as well in separate post.

PIM- Protocol independent multicast, why independent ?, it can work as overlay on any underlay routing protocol like OSPF, EIGRP, BGP. Remember that PIM cannot work standalone, you need unicast routing protocol as underlay for PIM to work on top of it.

PIM have 2 flavours

Dense Mode- No one use this, it basically “Flood” ( think of broadcast and how bad that can be!), multicast traffic everywhere and then expect host to “Pure” out of multicast stream if they are not interested!, see I am already feeling like uninvited guest :), now as of today, most of vendors don’t even support this mode, but you should still know what it is and how it work.

Imagine above network, now if we are using PIM in Dense mode, the way it work is, when server is ready to stream multicast traffic, it send it to First hop router (FHR) R1 and R1 will just send that traffic to all the router in PIM enable network (R2/3/4), now let say if only host 5/6 are interested in the multicast traffic, and they should “Only” get it but in Dense mode all host get the traffic and later they need to “Prune” themself out of it, the other issues with dense mode is, all router will maintain (S,G) entry [ source and group], this will create scaling problem, imagine 1000 of servers and 100,000 host, router will run out of memory, if they keep track of all state info on S,G entry. So basically, no one use this.

Sparse Mode- Now this mode is useful, why ?, you see name itself says it “Sparse” meaning “less”, so here we have an optimization in PIM protocol, how?, we use something called as Rendezvous point(RP), so how does this RP help, it help by not keeping (S,G) state entries, instead it keep (*,G), * meaning “Any source” and G is Multicast group. Now in Sparce mode we have 2 sub modes – ASM( any source multicast) and SSM ( source specific multicast)


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