ripv2 multicast

[ohassairi]

    hello

suppose i have one switch and many RIPv2 routers connected to it.

ripv2 destination address is 224.0.0.9 that is mapped to 0100:5e00:0009

normally the switch will send this frame to all ethernet ports              

is there a way to take benefit from the multicast address so that the frame will be sent only to the RIP routers.

i know it is possible to map staticlly the mac address to some ports but is there any dynamic solution also?

thanks

[Peter Paluch · Accepted solution]

Hello,

Ideally, if the routers used IGMP to subscribe into RIPv2-routers group 224.0.0.9, that would be the easiest dynamic solution, assuming IGMP Snooping is activated on the switch. However, I have seen no router to send IGMP Join for any router-to-router dynamic protocol that uses multicast addressing, so this option is not really relevant.

Even more, though, Cisco Catalyst switches running IGMP Snooping actually take this fact into account, and they ignore IGMP Snooping protection for any multicast group which gets mapped to the MAC address range 0100.5e00.00XX because that range corresponds to the link-local multicast group 224.0.0.X in which routers do not use the IGMP subscriptions, as explained above. Any multicast traffic mapped to MAC range 0100.5e00.00XX will be flooded by Catalyst switches, regardless of IGMP Snooping.

So I am afraid there is no way of using the multicast addressing in RIPv2 to limit the scope of flooding its messages. However, even though the packets are flooded, the multicast addressing has its advantage here: devices not interested in processing RIPv2 messages will not even process these frames because their NICs are not listening to the MAC of 0100.5e00.0009.

Best regards,

Peter

[ohassairi]

thanks Mr Peter.

[ohassairi]

i think it will be interresting if cisco add a new feature for well known dynamic routing multicast addresses.

every time the switch sees a frame with 0100:5e00:0009 it will add the source interface to one multicast group because sure the source is a ripv2 router. and it will froward frames with this address to members of this group.

but routers with passive interfaces will not be able to receive these frames!