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

    OSPF Neighbor hello message

    Praise Ugoji

      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

        • 1. Re: OSPF Neighbor hello message
          Conwyn

          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
          • 2. Re: OSPF Neighbor hello message
            Praise Ugoji

            Conwyn,

            Thanks for the reply. So are you saying hello messages are only sent between a non-DR and a DR router ?Does that  mean that  two non- DR routers do not send hello mesages after the initial two way state have been established beween them?

            • 3. Re: OSPF Neighbor hello message
              JohnB352

              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.

              • 4. Re: OSPF Neighbor hello message
                Conwyn

                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#

                • 5. Re: OSPF Neighbor hello message
                  Priscilla Oppenheimer

                  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