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I'm currently learning IPv6 and they mentioned broadcast is gone in IPv6 and replaced with multicast and unicast.

Are there some situations where multicast or unicast is better over the other in setups?

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  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Jan 5 at 21:05
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"broadcast" was replaced with more specific multicast methods. All-station-broadcast would become a huge mess given the available size of IPv6 LANs -- imagine thousands of nodes broadcasting ARPs looking for each other. (it falls apart in IPv4 already) Multicast Neighbor Discovery limits who hears the requests and answers, in a network with multicast aware hardware.

And there are specific link-local addresses to speak to everyone on the link in the same manner as IPv4 broadcast, without using globally routable addresses.

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  • I'd say the optimization was misguided and resulted in broken solution. Broadcast in practice in LANs rarely is problematic. However allowing clients to arbitrarily create multicast groups that all switches need to track (in software, with states) is very easy to abuse accidentally or purposely. Broadcasting was stateless, simple to do in HW. Tracking essentially 1:source multicast group is complex and scales very poorly.
    – ytti
    Oct 1 '13 at 7:32
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    I see you've never been on a large LAN. IPv6 doesn't change the multicast landscape; everything you complain about, IPv4 hosts have been capable of doing for years. All the switches I know of doing multicast filtering do it entirely in hardware -- mgmt only does IGMP processing.
    – Ricky
    Oct 1 '13 at 20:34
  • Even newest Cisco and Juniper gear snoop in software and populate the tables in software + HW. After snooped, forwarding is blocked/accepted in HW. But there are finite amount of entries in HW, much less than maximum possible amount of groups. So attacker in LAN can congest your snooping and can congest your tables. Of course you can opt out from limited-mcast, and just don't snoop in L2 behaving like normal bcast everything, which is what most do, since the alternative/designed goal of only sending to interests hosts scales poorly.
    – ytti
    Oct 2 '13 at 10:01
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    I'll say it again... An "attacker" can cause the same mess in an IPv4 ONLY network. IPv6 doesn't magically make multicast "broken" -- 'tho it does expose the very limited nature of IPv4 centric multicast hardware.
    – Ricky
    Oct 2 '13 at 17:55
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    History has proven it's a small problem. IPv6's dependence on multicast coupled with traditionally small mcast tables will be a bigger problem, but there are ways to address it.
    – Ricky
    Oct 2 '13 at 20:32
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There is so much different multicast possible in IPv6... are you talking about multicast traffic (for streaming) or generic multicasts?

FF:: is the multicast range....

FF02:: is Link Local scope, with FF02::1 being the equivalent of the Broadcast Address of a subnet in IPv4 FF02::2 is Link Local, All routers FF05:: is Site Local, again ::1 for All Node, ::2 for All Routers...

Basically, IPv6 multicasr permits a better targetting of many types of traffic.

Now, for Using Multicast for streaming, it's a lot like in IPv4, with PIM Sparse Mode (no more Dense Mode), MLD (replacing IGMP), Rendez-Vous Points, etc. There are differences, but not as much as replacing Broadcasts by Scoped Multicasts

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When a node issues a broadcast packet, the packet is replicated to all other hosts on the medium or subnet. So if you have a 32 port switch, and one node issues a broadcast, it goes out each of the 32 ports. This takes away potential bandwidth from everyone else, especially if it is overused.

Multicast requires that interested parties "subscribe" to a multicaster. Multicasted traffic only replicates to previously identified subscribed parties, so there is less bandwidth consumed by unwanted traffic.

So multicast is really the better way to go if it is possible and supported by the software and hardware.

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