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5252 Views 5 Replies Latest reply: Aug 10, 2009 6:45 PM by Priscilla Oppenheimer RSS

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OSPF Neighbor hello message

Aug 10, 2009 9:00 AM

Praise Ugoji 22 posts since
Jul 10, 2008

Are hello messages exchanged every 4sec between a DR(and BDR) and a fully adjacent router or bewteen all the routers in a particular OSPF area? I mean if there are 6 routers in an OSPF area0 and say router A and B are the DR and BDR, I know exchnage of topology information is between DR and everyother router, but how is the hello message exchanged? i

  • Conwyn 7,907 posts since
    Sep 10, 2008
    Currently Being Moderated
    1. Aug 10, 2009 9:50 AM (in response to Praise Ugoji)
    Re: OSPF Neighbor hello message

    Hi Praise

     

    On detection of a link state, the OSPF router sends a Link State Update (LSU) to the multicast address 224.0.0.6 which is all the OSPF DR/BDRs. The LSU contains several LSAs. After acknowledging the LSU the DR Floods link-state information to all the OSPF routers on the OSPF multicast address 224.0.0.5. Each LSA is acknowledged separately with a LSAck if the LSA is new and therefore added to the Link State Database, otherwise the LSA is ignored. Rather than each router having to form an adjacency with each other router this significantly cuts down on the amount of traffic. DRs in other networks that are connected also receive the LSUs. On receipt of the new LSA the routers recalculate their routing tables.

     

    The LSA has a 30 minute timer that causes the router to send an LSU to everyone on the network once it ages out. This verifies that the link is still valid. If a router receives an LSA with old information then it will send a LSU to the sender to update the sender with the newer information.
    Regards Conwyn
  • JohnB352 31 posts since
    Apr 23, 2009
    Currently Being Moderated
    3. Aug 10, 2009 12:35 PM (in response to Praise Ugoji)
    Re: OSPF Neighbor hello message

    The Hello messages are how routers discover neighbors.  So yes, they are sent from all routers to all routers.

     

    From the Cisco Press book:

     

    As soon as a router has chosen its OSPF RID, and some interfaces come up, the router is
    ready to meet its OSPF neighbors. OSPF routers can become neighbors if they are
    connected to the same subnet (and in some other special cases not covered on the CCNA
    exams). To discover other OSPF-speaking routers, a router sends multicast OSPF Hello
    packets to each interface and hopes to receive OSPF Hello packets from other routers
    connected to those interfaces.

  • Conwyn 7,907 posts since
    Sep 10, 2008
    Currently Being Moderated
    4. Aug 10, 2009 1:36 PM (in response to JohnB352)
    Re: OSPF Neighbor hello message

    Hi Praise

     

    Here we have three router R0 R1 and R2 and R0 is not DR or BDR

     

    r0#show ip ospf nei

    Neighbor ID     Pri   State           Dead Time   Address         Interface
    2.2.2.2           1   FULL/BDR        00:00:35    10.0.0.2        FastEthernet0/0
    3.3.3.3           1   FULL/DR         00:00:34    10.0.0.3        FastEthernet0/0

     

    Here we have three loops LB 1,2 and 3

    So initially router r0 has the fa0/0 int face down. Let us no shut and see what happens

     

    Regards Conwyn

     

    r0#config t
    Enter configuration commands, one per line.  End with CNTL/Z.
    r0(config)#int fa0/0
    r0(config-if)#no shut
    r0(config-if)#
    *Mar  1 00:17:29.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:17:31.139: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
    *Mar  1 00:17:32.139: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
    *Mar  1 00:17:36.247: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:17:36.251: OSPF: Send immediate hello to nbr 3.3.3.3, src address 10.0.0.3, on FastEthernet0/0
    *Mar  1 00:17:36.251: OSPF: Send hello to 10.0.0.3 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:17:36.251: OSPF: End of hello processing
    *Mar  1 00:17:37.967: OSPF: Rcv hello from 2.2.2.2 area 0 from FastEthernet0/0 10.0.0.2
    *Mar  1 00:17:37.971: OSPF: End of hello processing
    *Mar  1 00:17:38.651: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 from LOADING to FULL, Loading Done


    *Mar  1 00:17:39.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:17:42.171: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on FastEthernet0/0from LOADING to FULL, Loading Done


    *Mar  1 00:17:46.187: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:17:46.187: OSPF: End of hello processing
    *Mar  1 00:17:49.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:17:56.227: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:17:56.227: OSPF: End of hello processing
    *Mar  1 00:17:56.687: OSPF: Rcv hello from 2.2.2.2 area 0 from FastEthernet0/0 10.0.0.2
    *Mar  1 00:17:56.687: OSPF: End of hello processing
    *Mar  1 00:17:59.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:18:06.099: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:18:06.099: OSPF: End of hello processing
    *Mar  1 00:18:09.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:18:16.179: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:18:16.179: OSPF: End of hello processing
    *Mar  1 00:18:16.891: OSPF: Rcv hello from 2.2.2.2 area 0 from FastEthernet0/0 10.0.0.2
    *Mar  1 00:18:16.895: OSPF: End of hello processing
    *Mar  1 00:18:19.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:18:26.127: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:18:26.127: OSPF: End of hello processing
    *Mar  1 00:18:29.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:18:36.035: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:18:36.035: OSPF: End of hello processing
    *Mar  1 00:18:36.443: OSPF: Rcv hello from 2.2.2.2 area 0 from FastEthernet0/0 10.0.0.2
    *Mar  1 00:18:36.443: OSPF: End of hello processing
    *Mar  1 00:18:39.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    r0(config-if)#en
    *Mar  1 00:18:45.919: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:18:45.923: OSPF: End of hello processingd
    r0#
    *Mar  1 00:18:49.159: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 10.0.0.1
    *Mar  1 00:18:49.959: %SYS-5-CONFIG_I: Configured from console by consoleno deb
    *Mar  1 00:18:56.239: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 10.0.0.3
    *Mar  1 00:18:56.239: OSPF: End of hello processingug all
    All possible debugging has been turned off
    r0#

  • Priscilla Oppenheimer 54 posts since
    Jul 23, 2008
    Currently Being Moderated
    5. Aug 10, 2009 6:45 PM (in response to Praise Ugoji)
    Re: OSPF Neighbor hello message

    Hello Praise,

     

    OSPF routers send Hello packets out each OSPF-enabled interface every 10 seconds. Not sure why you said 4 seconds. I can never remember myself whether it's 5 seconds as it is with EIGRP Hellos or 10 seconds. But I remember it because 10 has a 0 in it and OSPF has an O in it.

     

    The election of a DR and a BDR avoid all routers becoming adjacent with all other routers. But after the election, all routers do continue to multicast Hellos to the ALLSPFRouters address 225.0.0.5 so they can track neighbors.

     

    So Hello behavior doesn't change, but if a router is a DROther (not DR or BDR), it sends multicast Update packets to the AllDRouters address 224.0.0.6. Only the DR and BRD listen to that address. In addition, the DR floods updates to the DRothers on 224.0.0.5.

     

    Hope that helps.

     

    Priscilla Oppenheimer

    http://www.priscilla.com

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