Convert from a multicast address to a HW address

Sometime i may be neccessary to block a Multicast address on a vlan.

This could be acomplished by blocking the hardware address of this vlan. It’s possible to staticly map certain ports for a mac address. So that only these ports are recievers of the address.

So first it’s neccessary to convert the multicast address to a hardwareaddress.

The first fields 0100.5e are reserved by the IANA for such demands.

The range is 0100.5e00.0000 through 0100.5e7f.ffff.

Starting with these first digits, continue to add the last ones, by converting the numbers to hexadecimal numbers.

For example, the multicast address of EIGRP is 224.0.0.10.

This gives:

0 => 00

0 => 00

10 => 0a

So the corresponding mac address for the EIGRP 224.0.0.10 Mmulticast address is 0100.5e 00 00 0a

The switch does snooping for packets on vlans, to determin the recieving ports. First we have do disable ip igmp snooping for te specific vlan:

no ip igmp snooping vlan 10

Then it’s possible to map the certain multicast mac address with certain receiver switchports.

mac-address-table static 0100.5e00.000a vlan 10 int f0/1 f0/2

So now only port f0/1 and f0/2 recieve this multicast address. This is only possible by turning of snooping functionality by the switch.

Source:

http://www.hep.ucl.ac.uk/~ytl/multi-cast/addresstranslation_01.html

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