4 Replies Latest reply: Mar 2, 2012 12:58 PM by Stephen RSS

    ospf DBD packet exchange

    shanthosh.rk

      Hi CLN,

       

      After the initial exchange of DBD packets after neighbourship formation, how often does the DBD exchange happens? if any new network is say added to the DROTHER router for example, how does it initiate the further advertisements?

       

       

      can someone guide me pl??

       

       

       

      -

      shanthosh

        • 1. Re: ospf DBD packet exchange
          Scott_O'Brien

          if no changes every 30 Min

          if changes details are sent to the DR and the DR floods the changes out in the area 

          • 2. Re: ospf DBD packet exchange
            Matt

            to be fully correct, no DBD is sent unless a new adjacency is built. The link state refresh is sent every 30 min ( i believe its just an LSU packet with whatever LSAs that are being refreshed inside it with the new sequence number )

             

            then like scott said, if a new link is advertised it goes to the DR

            • 3. Re: ospf DBD packet exchange
              lp4nb

              What i think happens is--

               

               

              the topology is like 

                                                                                    |

                                         -----segment--(DROTHER)---R1------------R2

                                                                                    |

               

               

              R1 is DRother for the segment left to it. and ON a new segment R2 comes up

               

               

              1. R1 and R2 exchange hellos, and when they match, DR/BDR is elected if its a mutliaccess.

              2.  Type 2 DBD packet is sent by both routers with Master, More, Init bit set.

                   and this packet has a dbd seq no assigned by the router that generated the packet, and the router with higher router-id is elected

                   as master and other one is slave.

                   the Seq no that the master used in the first dbd packet is used in further exchange.

              3. the database exchange finishes when both slave and master has sent dbd with More bit set to 0.

              4. At this time if any of them has LSA headers with updated seq no that are not in their database, of the header that are not present, the router transitions to loading state and the link state requests are sent for those particular ones. This is where the link state requests and link state updates are sent.

               

               

              IF the neighbor R2 is added to the same existing sengment, the same process is done with both existing DR/BDR.

               

              A router can't be added specifically to a DROTHER, for that segment.

               

              Regards

              • 4. Re: ospf DBD packet exchange
                Stephen

                Hi

                 

                DBDs are used between the master and slave devices to start building the topology table.  Each DBD packet has a sequence number (but only the master can increment it).  The sequence number increments when a link in OSPF changes.

                 

                Once the weve got past the exchange state, the device will then compare the its  database with its neighbors to see if there is any newer info.  If any info is missing, the router (or whatever device it is) sends a LSR to request the missing info.  The LSU packet is the packet that actually exchanges the routing information between devices.  Each device generates an LSA, or multiple LSAs (there are eleven different types), that stipulate the routing information the device currently knows. Each type of LSA is then encapsulated into a LSU and transmitted between devices.  Upon receiving the LSU the LSA is stripped out and inserted into the LSDB.  The LSAck packets are used as a receipt in order to acknowledge receiving the LSU.

                 

                So to answer your question, the once the DBD's are sent, the only thing what happens is LSRs, LSUs, and LSAcks maintain the topology.  If there is ever a problem with this device, then we may need to go through the process of establishing an OSPF adjancency:

                 

                exstart

                exchange

                loading

                full

                 

                 

                Hope this helps,

                 

                Regards,

                Stephen