EIGRP Multicast Hellos on Serial Links?

pogue

Hi, I am going through the Cisco Academy CCNP ROUTE Lab Manual, and have noticed during the EIGRP Challenge Lab, that all of my Serial interfaces are using unicast, per the show ip eigrp interfaces detail command. If I am correct, the bottom portion of this output references whether both Hellos and Updates go out by unicast or multicast, correct?

Now, I though that unless you configure neighbors statically, all interfaces, Serial OR Ethernet, will default to multicast?

ALL my serial interfaces are using unicast, per the above command, even the ones where neighbors are discovered automatically.

I am confused as to why I get this result? Don't ask me if it was like this before, as I never paid attention, until a lab instruction told me to disable multicast packets between two routers, but retain the neighborship. I configured a static neighbor, completed the lab, and then attempted to go back and remove the static neighborship to see the interface go back to multicast, which did not work. I checked the other serial interfaces on the other routers, and they were all unicast.

According to the OCG EIGRP CCNA review, all interfaces should use multicast Hellos?

Thanks,

Russ

mattau

R8(config-router)#do sh ip eigrp int det
IP-EIGRP interfaces for process 1
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se0/1 1 0/0 36 0/15 175 0
Hello interval is 5 sec
Next xmit serial <none>
Un/reliable mcasts: 0/0 Un/reliable ucasts: 0/3
Mcast exceptions: 0 CR packets: 0 ACKs suppressed: 2
Retransmissions sent: 0 Out-of-sequence rcvd: 0
Authentication mode is not set
Use unicast

I am not exactly sure what that means either. From my labs I think it means how the updates are sent. In the case of serial links it is unicast to neighbor. Where as ethernet is multicast to tell all neighbors about the update vs sending 100 unicasts which isnt efficient

Serial links just like ethernet unless specified by the "neighbor" commmand are multicast to 224.0.0.10

EIGRP: Sending HELLO on Serial0/1
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
IP: s=160.10.38.3 (Serial0/1), d=224.0.0.10, len 60, rcvd 2, proto=88

with the neighbor command
EIGRP: Sending HELLO on Serial0/1 nbr 160.10.38.3
AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0


IP: tableid=0, s=160.10.38.3 (Serial0/1), d=160.10.38.8 (Serial0/1), routed via RIB
IP: s=160.10.38.3 (Serial0/1), d=160.10.38.8 (Serial0/1), len 60, rcvd 3, proto=88

If you had static neighbors and you disabled them, eigrp goes back to multicasting. If they were still unicast i would check your config to see if the command was completely removed. Remember both sides need the same config

Also there is no way you can say enable static neighbors between 2 routers and keep the neighborship up. By the time you say
neighbor x x x x serial 1/1 on router A the adjacency would have dropped. It would then reform once you enable the other side.