Thursday, February 23, 2012

Difference between L2 (layer 2) and L3 (layer 3) multicast

MAC Multicast address generation

Why is multicast dealt with at two layers?
Multicast was initially designed as a layer 3 functionality, where multple hosts from a network could subscribe to a multicast address.
However, the major deficiency in such an approach is that after a router has decided which port to forward packets from a particular multicast to, the IP layer multicast is usually translated to a MAC broadcast on a switch. This results in several inefficiencies. L2 frames are received by host which are not desired. In order to avoid this the switch does IGMP snooping (read over hearing), and marks ports on the switch which are subscribing to that multicast. It then keeps track of the ports by using a destination multicast address that is obtained by conversion of the L3 multicast address to L2 multicast frame address

Conversion of L3 multicast address to L2 MAC address: (based on information on a microsoft website)
"To support IP multicasting, the Internet authorities have reserved the multicast address range of 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for Ethernet and Fiber Distributed Data Interface (FDDI) media access control (MAC) addresses. As shown in Figure above, the high order 25 bits of the 48-bit MAC address are fixed and the low order 23 bits are variable."

Two approaches to implementing multicast on switches:
* Pure IGMP snooping in L2 switches
* Deep packet IGMP inspection in L3 switches


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