EIGRP Multicast vs Unicast

Posted on 2016-11-05
Last Modified: 2016-11-12
If I understand EIGRP Unicast , Routers are configured with Neighbor command and they send queries to whatever router they are configured as neighbor with.

with Multicast, there is no Neighbor command, and routers send their queries to a Multicast adresss
Now does that mean each router own that Multicast address of ?

example if  router RouterA is connected to RouterB, then which router own the ?

Thank you
Question by:jskfan
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Accepted Solution

Predrag Jovic earned 500 total points
ID: 41875581
You are mixing apples and oranges.

Multicast address is address for all EIGRP routers.
It is like radio transmission.
Let's say you set radio receiver on 100.5Mhz - if there is no one broadcasting, sure you will not hear anything. When someone start to broadcast on 100.5Mhz frequency you will hear it.
In case of EIGRP source traffic is unicast address (radio station-1 with address and destination is all EIGRP routers (frequency Every router that is configured to listen on will hear message and if necessary answer. But answer from multicast IP address would not make much sense - it would be unclear who answered on message? So answers are unicast packets.

Here is a proof (wireshark capture of EIGRP traffic) - no one is ever sending from - every router is just receiving on that address (all EIGRP routers are tuned to listen on that frequency).
 Proof is in the pudding
No one own, EIGRP routers are just listening to transmissions there.
When EIGRP neighbors are configured with neighbor command all messages are sent as unicast traffic directly to configured neighbor (that's why all neighbors on specific segment must be configured manually).

Author Comment

ID: 41876099
Radio frequency is somewhere in the AIR.
Router Multicast ip address, it is kind of virtual ip address that might be added to each router when the routes are advertised.
In other words where is the physical location of Multicast address ?
LVL 29

Expert Comment

by:Predrag Jovic
ID: 41876173
In the air. It is not configured on any device.
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Author Comment

ID: 41877617
I am not saying configured.
but when you enable EIGRP on the router, the system will add the Ip address . somewhere.
I know we do not care too much where physically is, but just curious.
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Expert Comment

by:Predrag Jovic
ID: 41877788
I quit.

Author Comment

ID: 41878083
Keep off my Questions, please

Author Closing Comment

ID: 41884700

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