EIGRP Multicast Hellos on Serial Links?

poguepogue Senior MemberMember Posts: 213
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?


Currently working on: CCNA:Security
Up next: CCNA:Voice


  • mattaumattau Senior Member Member Posts: 218
    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

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

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

    IP: tableid=0, s= (Serial0/1), d= (Serial0/1), routed via RIB
    IP: s= (Serial0/1), d= (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.
    CCNP ROUTE - passed 20/3/12
    CCNP SWITCH - passed 25/10/12
    CCNP TSHOOT - passed 11/12/12

  • poguepogue Senior Member Member Posts: 213
    Hmm...I don't why Hellos would be sent multicast, but Updates sent by unicast? I thought both would pick the same method of sending.

    I guess I will have to do some debugs and whatnot to see if Updates go by unicast or not..

    Also, I did not mean keep the neighborship up without dropping, but simply to make sure it is up after the change to static neighbor.


    Currently working on: CCNA:Security
    Up next: CCNA:Voice
Sign In or Register to comment.