Broadcast vs Multicast

[vpalania]

Can anyone explain the difference between

1.L2 Broadcast vs L2 MUlticast

2.L3 Broadcast vs L3 MUlticast

Also, how NIC is assigned to look for particular IP Multicast address ?

Thanks,

Vijay

[arvindchari]

The basic advantage between a broadcast and a multicast is that broadcasts reach all devices on the subnet (L3) / local segment (L2) whereas multicasts reach only devices listening to data on that particular multicast address.

An L2 broadcast has an destination address field of all 1's i.e FF FF FF FF FF FF. An L2 multicast (used by L2 protocols such as L2TP for example) is (usually) standards based (i.e. different manufacturers have reserved MAC addresses for their protocols) and all devices that use the protocol are programmed to listen to multicasts for that address.

Similarly and L3 Broadcast has a destination address in the host portion of all 1's. For example if you wanted to broadcast a frame to all computers on the 192.168.192.0 subnet the broadcast address would be 192.168.192.255. The L3 multicast address range as defined by the IANA is 224.0.0.0 - 239.255.255.255.

A host NIC is associated to "listen" to a particular multicast address usually using programs at the application layer (in my experience) to associate / listen to a particular address. CGMP and IGMP are common multicast protocols.

HTH

Please rate posts that help.

Regards

Arvind

[pkhatri]

Hi Vijay,

In the context of Ethernet:

1. A L2 broadcast is sent with a destination MAC address of FF.FF.FF.FF.FF.FF (all ones). A L2 multicast depends on the type of network layer protocol address but in general it will start off with an odd-numbered first octet. For example, IP multicasts appear as 01.00.5E.xx.xx.xx

2. A local L3 broadcast will be address to 255.255.255.255 for IP. A L3 multicast will be address to a destination in the 224.0.0.0-239.255.255.255 range.

A NIC will convert L3 broadcast/multicast addresses to L2 broadcast/multicast addresses and listen for them. For example, the L3 multicast address 224.0.0.5 (used with OSPF) will map to 01.00.5E.00.00.05 which the NIC will listen for.

Hope that helps - pls rate the post if it does.

Paresh

[vpalania]

Hi,

I have read that the most significant bit of the first octet is used to identify whether its a broadcast or multicast.Is it correct?

Also, is '01.00.5E.xx.xx.xx' constant when mappping IP multicast to L2 multicast ? Why is it so ?

Thanks,

Vijay

[pkhatri]

Hi Vijay,

Yes, the MSB of the first octet is set to 1 for multicast addresses. Also, the prefix 01.00.5E is used for all IP to Ethernet MAC address conversions. RFC1112 (which specifies some of the initial work on multicasting by Steve Deering) specifies that this is the prefix that must be used. That is why it is used.

Hope that helps - pls rate the post if it does.

Paresh

[vpalania]

Hi,

Iam bit consfused.If '1' represents multicast, then '0' represents broadcast address.Then, how can a broadcast address be all 1's ?

Thanks,

Vijay

[pkhatri]

Remember that a broadcast is simply a special case of a multicast address. Multicast addresses include multiple hosts. If you extend that to a case where a multicast address includes all hosts, that multicast address is now called a broadcast address. That is why the MSB of the first octet is also 1 for a broadcast address, not zero.

Pls remember to rate posts that help.

Paresh

[vpalania]

Hi,

Then how they are differentiating between multicast and broadcast by using MSB of first octet ?

Thanks,

Vijay

[pkhatri]

All devices (hosts and routers) will listen to the FF.FF.FF.FF.FF.FF MAC address.. there is no other broadcast address to be able to listen to, therefore, there is no reason to make any distinction. There is only one broadcast address so all devices know what it is - they don't need to look at the MSB of the first octet.

Hope that helps - pls rate the post if it does.

Paresh