Multicast vs. Broadcast frames

I guess this one sounds a little foolish, but still: I would like to understand the way devices treat multicast and broadcast frames on a segment.

Let's assume a segment with 2 routers and 2 PCs. Now let's assume that RIPv1 is enabled. Since it broadcasts its routing table, all the rest of the devices on the segment will receive these broadcasts. In the case of RIPv2, multicast is being used. Since the hub (or even a switch that floods multicast) will transmit the information to the rest of the segment, all the devices, PCs and routers, with receive these fames too. So in both cases, all the devices receive these frames. What's the difference, what am I missing here?

Thanks, Yaron

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RIP2 can run "compatibility mode" where it uses broadcasts rather than m/cast.

adevice with a reasonable interface chip can program the chip with the set of multicast MAC addresses it is interested in, and ignore the rest in hardware.

All broadcasts traditionally need the CPU to examine each packet to decide whether to ignore it.

some layer 2 switches can use multicast control to limit where the packets go within a subnet, reducing the average bandwidth overhead per port.

doesnt matter for a few packet / sec for RIPv2, but once you start sending multiple 5 Mbps MPEG2 video on multicast it gets more important......

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Yaron -

On a single segment like you mention, there isn't really that much of a difference - other than the address that a packet is multicast to or broadcast to (multicast will always be in the range of - However, your PC's will typically have to have an application that uses multicast for them to respond / receive those packets. Something like the Nortel Symposium product, which allows certain users to view real-time data on PBX call routing.

When you get to multiple segments or even multiple subnets, then you get into the real guts of multicast and how it works. Because then, communication between nodes and network devices allows a multicast server application (such as an Anti-virus app) to send feeds to only the hosts who need them. A multicast group can exist with a single multicast host on every subnet in a large vlan environment and each of those hosts (and only those multicast hosts) will receive the server's traffic.

This is a quick and dirty high-level overview, but I hope it helps. The Cisco Press books "Routing TCP/IP" volume 1 & 2 are good books with a large section on multicast.

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V. Evans

Thanks for your answer, much appriciated.

V. Evans:


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Thank you stephen, great response.


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CGMP or IGMP snooping on Catalyst Switches.

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