1 2 Previous Next 18 Replies Latest reply: Sep 10, 2012 11:28 PM by Ahmed Muhi RSS

    OSPF DR/BDR election purpose

    mongolio

      Good day.

       

      Recently i re-red  OSPF chapters from Wendell Odom ROUTE book and based on next quotation question arised -

       

      SPF requires that the LSDB model the topology with nodes (routers) and connections between

      nodes (links). In particular, each link must be between a pair of nodes. When a multiaccess

      data link exists—for instance, a LAN—OSPF must somehow model that LAN so

      that the topology represents nodes and links between only a pair of nodes. To do so,

      OSPF uses the concept of a Type 2 Network LSA.

       

       

      So this is explaining why DR/BDR system were invented, but what i cant understand - why on NBMA network i can simply set ip ospf network-type point to multipoint and everething works well on "multiaccess data link" without electing DR/BDR. So i can make conclusion that purpose of inventing DR/BDR adduced in Odom book is "bogus". So can any1 clarify this or adduce another reason for inventing DR/BDR? Only two i can find are reducing LSA flooding on segment and reducing size of LSDB, is there any others?

        • 1. Re: OSPF DR/BDR election purpose
          Stason

          NBMA is a special case where OSPF emulates a broadcast network.  You can also set it up as p2p or p2mp. The Type 2 LSA was written for broadcast segments, but since NBMA can act sort of like a broadcast segment or sort of like a p2p segment depending on how it is configured, they defaulted NBMA to use the same mechanism as a broadcast segment.  To quote the RFC:

           

          "NBMA mode is the most efficient way to run OSPF over non-

            broadcast networks, both in terms of link-state database

            size and in terms of the amount of routing protocol traffic."

           

          So basically they did it like that due to efficiency.

          • 2. Re: OSPF DR/BDR election purpose
            Joel Shrestha

            Mongolio,

             

            In P2MP network,  you already have a point or Hub to which all the multipoint or spokes reports to. Doesn't it look like a DR/BRD setup?. So, Point to multi-point network itself is like a DR/BDR setup. I think, that's why ospf doesn't require DR/BDR cause your underlying layer 2 network defines who is the HUB here.

             

            Multi-access network allows multiple devices to be on the same subnet or network, but nobody knows who should it talk to, it's like a group of people without a leader,  and if you refer to requirement for SPF, it makes sense to create a point to whom all others on the same network can talk to or report to.

             

            Hope this helps.

             

            Joel

            • 3. Re: OSPF DR/BDR election purpose
              mongolio

              Joel Shrestha wrote:

               

              In P2MP network,  you already have a point or Hub to which all the multipoint or spokes reports to. Doesn't it look like a DR/BRD setup?. So, Point to multi-point network itself is like a DR/BDR setup.

               

              But if set P2MP ospf network type on fullmeshed NBMA network everything works well with every node has full adjacency with every another node and here no analogy with DR/BDR.

               

              So question from  original post is still actual - is purpose of inventing DR/BDR adduced in Wendell Odom ROUTE book bogus or not?

              • 4. Re: OSPF DR/BDR election purpose
                lp4nb

                Information provided by the router lsa in case of, p-to-p is little different from the router lsa for a multiaccess network.

                In p-to-p, router lsa shows the link's interface ip address, and specific subnet the link is connected to. What router is on the other side of this P-to-P link

                can be known just on basis of the information that the router lsa contains.

                 

                But, in case of multiaccess networks, things are little different, router lsa on thses networks tell the interface ip address, but it doesn't tell the specific subnet it is connected to and who else is on the same specific subnet. All this information is provided by the Network LSA, and hence the network lsa is required for ospf to function correctly on a mutliaccess network. Now when network lsa is used, every router can build a map of who are connected to it.

                 

                P-to-M, this network type is similar to point-to-point network type, except, a single subnet is used, and the router lsas, don't tell the subnetwork, but a host address of the interface as a single stub link. The information enough to know what routers are connected to this router, on what interface addresses.

                 

                 

                SPF requires that the LSDB model the topology with nodes (routers) and connections between

                nodes (links). In particular, each link must be between a pair of nodes. When a multiaccess

                data link exists—for instance, a LAN—OSPF must somehow model that LAN so

                that the topology represents nodes and links between only a pair of nodes. To do so,

                OSPF uses the concept of a Type 2 Network LSA.

                 

                 

                Additionally, (I guess!) the snip above, doesn't reason the invention of DR/BDR concept. What it says about is - what the information fed to SPF, should mean ( what a node should repesent, or what a link should represent) in actual topology, so that the ospf make no mistake in judging the map and computing the SPT.

                And as for the multiaccess networks the router lsa are little different and they don't provide all the inputs required by the OSPF to compute SPT. So, the additional information that the router lsa fails to provide is provided by TYPE 2 network lsa.

                 

                Regards.

                • 5. Re: OSPF DR/BDR election purpose
                  mongolio

                  lp4nb wrote:


                  P-to-M, this network type is similar to point-to-point network type, except, a single subnet is used, and the router lsas, don't tell the subnetwork, but a host address of the interface as a single stub link.

                   

                  Rather interesting observation. But new question arised from where than SPF derive information about NBMA network mask if Router LSA

                   

                  don't tell the subnetwork, but a host address of the interface as a single stub link.

                   

                   

                  And with similar approach it possible to describe Multiaccess network aswell (only using Router LSAs) i think, the only consequence that LSDB will significant grow.

                  • 6. Re: OSPF DR/BDR election purpose
                    lp4nb

                    Rather interesting observation. But new question arised from where than SPF derive information about NBMA network mask if Router LSA

                    host routes have a subnet mask, its just /32.

                     

                    And with similar approach it possible to describe Multiaccess network aswell (only using Router LSAs) i think, the only consequence that LSDB will significant grow.

                    .

                     

                     

                    technically a similar approach is possible but that will be unnecsessary burden on the router.

                     

                    In a p-to-m network type the end-points are advertised as host routes, and If the routes are not advertised as host routes, P-to-M net type won't be behaving like a P-to-P. (because if I have 4 routers with 10.1.1.0/29 prefix, and I send packet to any of them and longest match in RT is /29 then the packet will be sent on multiple links, but when using the host routes, this duplicates can be prevented.

                    also, p-to-m is usually used in partial mesh topologies, thus they considerably bring down the lsa size when compared to a full mesh (p-to-M) implementation.

                     

                    Regards

                     

                    P.S. (i answered to best of my understanding on the subject, and any corrections if there is/are, are welcome.)

                    • 7. Re: OSPF DR/BDR election purpose
                      mongolio

                      host routes have a subnet mask, its just /32.

                       

                      You didnt understood me - yes, everything in LSA type 1 points to host routes, but all routers (those who even dont belong to same P2MP network) have known of considered P2MP network in RT with proper mask, but from LSDB in LSA type 1 i can derive only /32 masks - so where real mask stored?

                       

                      P-to-M net type won't be behaving like a P-to-P. (because if I have 4 routers with 10.1.1.0/29 prefix, and I send packet to any of them and longest match in RT is /29 then the packet will be sent on multiple links

                      Here im not sure that you are right, cos OSPF dont participate in packet forwarding, and furthermore RT will have /29 route in it regardless of OSPF network type used.

                      • 8. Re: OSPF DR/BDR election purpose
                        lp4nb

                        the routers in p-to-m still advertise out these routes as host routes to the routers, that are not participating in this p-to-m, but in ospf domain.

                         

                        second portion, I was not confident about that earlier, nor am i now, but if i get some stuff on this thing, i'll post.

                        But I still feel if the routes are not advetised as host routes there could be some duplicates. :-)

                        if you happen to find anything in support or contrary, please post it.

                         

                        Regards

                        • 9. Re: OSPF DR/BDR election purpose
                          mongolio

                          these routes as host routes to the routers,

                          This turn of speech makes me crazy first time i red it.

                           

                          Besides i perfectly understand and furthermore there is no possibility that those routes will be advertised to other worls with any changes cos this is contradict to principes of OSPF.

                          I ask - how all other routers known about MASK of P2MP network - in LSA type 1 only host routers presented and no any notice about mask of network! But how you can explore those routers anyway install in RT NOT host routes but subnet route.

                          • 10. Re: OSPF DR/BDR election purpose
                            lp4nb

                            didn't quite get you,

                             

                            say this is the network R1, R2, R3 in a partial mesh p-to-m

                             

                             

                                                      R2  1.1.1.2

                                            _____/

                            1.1.1.1     /

                                 R1 ---[

                                  |       \______

                                  |                    \

                                  |                    R3  1.1.1.3

                                  |

                                R4

                            10.1.14.4

                             

                            R2  has routes to R1 and R3 as 

                               1.1.1.1/32  via 1.1.1.1

                                1.1.1.3/32  via 1.1.1.1

                             

                            and say R1 is advertising out them to R4,

                            R4

                            --

                             

                            1.1.1.1/32 via 10.1.14.1

                            1.1.1.2/32 via 10.1.14.1

                            1.1.1.3/32 via 10.1.14.1

                            ---------------------------

                            to get the subnet that the ip address belongs to, you can do  a boolean AND between ip and the mask

                            here, for any ip, the subnet will be same as the ip because of its /32 mask. So evry host route is a subnet with a single ip.

                             

                            Regards

                            • 11. Re: OSPF DR/BDR election purpose
                              mongolio

                              Just checked this at GNS3 and can confirm that you absolutely right here. with P2MP to rest of network host prefixes announced instead of subnet. Very interesting observation, tnx!

                              • 12. Re: OSPF DR/BDR election purpose
                                Joel Shrestha

                                Mongolio,

                                 

                                Let me ask you a question, is it not the ospf interface type that decides if the network reqires the DR/BDR or not?. I was just trying to do an analogy with layer 2 network in my last reply. Please let me know if i'm not understanding you correctly.

                                 

                                Joel

                                • 13. Re: OSPF DR/BDR election purpose
                                  wave - CCNP R&S

                                  If you have a partial mesh topology you might select P-2-M because it's possible that all of the routers won't connect to a hub.  In order to use a DR, all routers must connect to it.

                                   

                                  Because P-2-M advertises the host routes you can still keep all of the routers in the same subnet and you don't need frame-relay mappings between the spoke routers.  You only need spoke-to-hub mappings.  (If you're using frame-relay that is.)

                                  • 14. Re: OSPF DR/BDR election purpose
                                    mongolio

                                    ospf interface type that decides if the network reqires the DR/BDR

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