1 2 Previous Next 21 Replies Latest reply: Oct 14, 2013 7:22 AM by Darren Starr (CCSI, 4xCCNP, 7xCCNA) RSS

    how many DR and BDR?

    disomnium

      I am pretty much confused about the following......

      1) What exactly DR and BDR are? ...( I already read cisco definition..didn't help..its still vague)

      2)What is the maximum and minimum number of  DR and BDR.. an AREA 0 ( ie  a single area) can facilitate ?

      3) Is there any command to know the DR and BDR in the internal area ?

        • 1. Re: how many DR and BDR?
          Aaron Cary

          1. Here is the explanation of DR and BDR, what they are and how they are determined.

           

          Designated router

          A designated router (DR) is the router interface elected among all routers on a particular multiaccess network segment, generally assumed to be broadcast multiaccess. Special techniques, often vendor-dependent, may be needed to support the DR function on nonbroadcast multiaccess (NBMA) media. It is usually wise to configure the individual virtual circuits of a NBMA subnet as individual point-to-point lines; the techniques used are implementation-dependent.

          Do not confuse the DR with an OSPF router type. A given physical router can have some interfaces that are designated (DR), others that are backup designated (BDR), and others that are non-designated. If no router is DR or BDR on a given subnet, the DR is first elected, and then a second election is held if there is more than one BDR.[11] The DR is elected based on the following default criteria:

          • If the priority setting on a OSPF router is set to 0, that means it can NEVER become a DR or BDR (Backup Designated Router).
          • When a DR fails and the BDR takes over, there is another election to see who becomes the replacement BDR.
          • The router sending the Hello packets with the highest priority wins the election.
          • If two or more routers tie with the highest priority setting, the router sending the Hello with the highest RID (Router ID) wins. NOTE: a RID is the highest logical (loopback) IP address configured on a router, if no logical/loopback IP address is set then the Router uses the highest IP address configured on its active interfaces. (e.g. 192.168.0.1 would be higher than 10.1.1.2).
          • Usually the router with the second highest priority number becomes the BDR.
          • The priority values range between 0 - 255[12], with a higher value increasing its chances of becoming DR or BDR.
          • IF a HIGHER priority OSPF router comes online AFTER the election has taken place, it will not become DR or BDR until (at least) the DR and BDR fail.
          • If the current DR 'goes down' the current BDR becomes the new DR and a new election takes place to find another BDR. If the new DR then 'goes down' and the original DR is now available, it then becomes DR again, but no change is made to the current BDR.

          DR's exist for the purpose of reducing network traffic by providing a source for routing updates, the DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in an area will form a slave/master relationship with the DR. They will form adjacencies with the DR and BDR only. Every time a router sends an update, it sends it to the DR and BDR on the multicast address 224.0.0.6. The DR will then send the update out to all other routers in the area, to the multicast address 224.0.0.5. This way all the routers do not have to constantly update each other, and can rather get all their updates from a single source. The use of multicasting further reduces the network load. DRs and BDRs are always setup/elected on Broadcast networks (Ethernet). DR's can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay or ATM. DRs or BDRs are not elected on point-to-point links (such as a point-to-point WAN connection) because the two routers on either sides of the link must become fully adjacent and the bandwidth between them cannot be further optimized.

          [edit] Backup designated router

          A backup designated router (BDR) is a router that becomes the designated router if the current designated router has a problem or fails. The BDR is the OSPF router with second highest priority at the time of the last election.

           

          2. You can only have 1 DR and 1 BDR in an area. When the DR goes down, the BDR becomes DR and then an election is taken place to see who is new  BDR.

           

          3. show ip ospf interface x - where x is the interface using OSPF

          • 2. Re: how many DR and BDR?
            disomnium

            Thnx Aaron... i came across that definition already... what confused me was the terminology used  as designated router and the definition..


            "A designated router (DR) is the router interface elected among all routers on a particular multiaccess network segment, generally assumed to be broadcast multiaccess."

             

            ...Could u please tell me what is the basis/ground to conclude there is only one DR and BDR in an AREA ??....

             

            As you can see... it says its a router interface (not a router) and then it says on particular multiaccess network segment, does that mean different vlans,subnets and other segments?(if yes then that would make more than one DR and BDR is possible in a single AREA) .. and to make it more confusing it says multiaccess ( in Tod Lamle book it noted multiaccess and mulitpoint is two different things... wonder what is the difference) ...does that again mean that if more multiaccess exits, more DR and BDR exists as well ?

            • 3. Re: how many DR and BDR?
              Aaron Cary

              Look at this just like the root switch situation, where all the switches elect 1 switch to be the main control point for traffic. This just allows you to have a backup, whereas the root switch does not. Multipoint is where 1 exit points to multiple destinations. Multiaccess is where there are multiple points of entry to a network segment. This would be ideal for a mesh network where multiple devices have multiple ways around the network. You still can only have 1 DR and 1 BDR per OSPF area though.

              • 4. Re: how many DR and BDR?
                Nilesh W

                Hi,

                 

                To minimize routing traffice,Routing table laod, CPU processing load  OSPF uses area hierarchical  structure.

                In every area one DR and one BDR selected on the basis of higher IP address or Priority or RID ( Which can be configured )

                Every router in same area having identical database ( LSDB - link state database ).

                 

                Aaron given a fantastic explaination on DR and BDR.

                 

                Regards,

                Nilesh W

                • 5. Re: how many DR and BDR?
                  Scott Morris - CCDE/4xCCIE/2xJNCIE

                  Take a look at the RFC...  RFC 2328 outlines OSPFv2.  Sections 7.3 and 7.4 discuss the DR and BDR.

                   

                  One DR.  One BDR.  Everyone else is called "DROTHER".

                   

                  HTH,

                   

                  Scott

                  • 6. Re: how many DR and BDR?
                    Nilesh W

                    Hi Scott,

                     

                    Right  I have marked wrong remark,Sorry

                    i have to put DROTTHER  inplace of every router.

                     

                    Regards,

                    Nilesh W

                    • 7. Re: how many DR and BDR?
                      disomnium

                      Thanks for the replies guys, I guess that settles this confusion, thanks Aaron for the answer and feed back,.. thanks to Nilesh for the inputs and Scott for the reference.

                      • 8. Re: how many DR and BDR?
                        jneiberger

                        That last statement is incorrect. You only have one DR and BDR per multiaccess segment, not area. There is no particular limitation as far as areas are concerned.

                        • 9. Re: how many DR and BDR?
                          Gerald

                          I am also confused on how many DRs were in an area.  If I have an area which has one cluster of routers on one side and another cluster of routers, connected by a wan link; would each cluster have their own DR and BDR?

                          • 10. Re: how many DR and BDR?
                            Diwakar Sharma -CCNA/CCNP/JNCIA-ER

                            One DR and One BDR in and area

                            • 11. Re: how many DR and BDR?
                              Sey

                              Hi,

                               

                              I can have a couple dozens of DRs and BDRs within an area. Please see comment #8.

                              • 12. Re: how many DR and BDR?
                                mmistretta

                                You can have multiple DR in an area... DR is based on the segment/subnet, not the area...

                                for example:

                                 

                                                Net Link States (Area 0)

                                 

                                 

                                  Routing Bit Set on this LSA

                                  LS age: 586

                                  Options: (No TOS-capability, DC)

                                  LS Type: Network Links

                                  Link State ID: 150.1.25.1 (address of Designated Router)

                                  Advertising Router: 155.5.5.5

                                  LS Seq Number: 80000002

                                  Checksum: 0xCD2

                                  Length: 36

                                  Network Mask: /24

                                        Attached Router: 155.5.5.5

                                        Attached Router: 155.2.2.2

                                        Attached Router: 155.6.6.6

                                 

                                 

                                  Routing Bit Set on this LSA

                                  LS age: 627

                                  Options: (No TOS-capability, DC)

                                  LS Type: Network Links

                                  Link State ID: 150.1.36.3 (address of Designated Router)

                                  Advertising Router: 155.3.3.3

                                  LS Seq Number: 80000001

                                  Checksum: 0xB7CC

                                  Length: 32

                                  Network Mask: /24

                                        Attached Router: 155.3.3.3

                                        Attached Router: 155.6.6.6

                                 

                                 

                                  Routing Bit Set on this LSA

                                  LS age: 592

                                  Options: (No TOS-capability, DC)

                                  LS Type: Network Links

                                  Link State ID: 150.1.44.1 (address of Designated Router)

                                  Advertising Router: 155.5.5.5

                                  LS Seq Number: 80000002

                                  Checksum: 0x883D

                                  Length: 36

                                  Network Mask: /24

                                        Attached Router: 155.5.5.5

                                        Attached Router: 155.4.4.4

                                        Attached Router: 155.6.6.6

                                 

                                 

                                  Routing Bit Set on this LSA

                                  LS age: 644

                                  Options: (No TOS-capability, DC)

                                  LS Type: Network Links

                                  Link State ID: 150.1.56.5 (address of Designated Router)

                                  Advertising Router: 155.5.5.5

                                  LS Seq Number: 80000001

                                  Checksum: 0xD28F

                                  Length: 32

                                  Network Mask: /24

                                        Attached Router: 155.5.5.5

                                        Attached Router: 155.6.6.6

                                • 13. Re: how many DR and BDR?
                                  Ri0N

                                  There can be only one DR and one BDR per multiaccess segment (like Ethernet LAN).  This means that a router can be DR in one segment while being BDR in another.  The DR/BDR concept is used to minimize the number of adjacencies and LSA replication.  Once the DR/BDR has been elected, other routers (called DROthers) will form an adjacency with the DR and BDR only.  Without DR/BDR, n(n-1)/2 adjacencies would be formed.  For example, let's imagine a multiaccess network of 5 routers.  Withouth DR/BDR, 10 adjacencies would be required.  However, with the DR and BDR elected, only 7 adjacencies (two from each DROther to DR/BDR and one between DR and BDR) is needed.  Here the difference is small but would get significantly bigger if more routers were added.

                                   

                                  The DR and BDR are elected through a process based on interface priority and router ID.  The priority value can be manually set between 0 and 255, the default is 1.  The highest priority wins.  Configuring the priority with 0 guarantees that the router can never become DR or BDR.  The configuration is done on interface level with #ip ospf priority <number-value>.

                                   

                                  The router ID is, by default, the highest up/up loopback or regular interface address.  Loopback interface (if configured) is preferred.  The router ID can also be manually defined with the #router-id x.x.x.x  subcommand in router mode.

                                   

                                  Technically you could modify the DR/BDR election process by changing these configuration settings.  However, once the DR and BDR have been elected, they will remain in those roles, until the DR fails.  Even if a router with higher interface priority or router ID joined the segment, it would not become the DR because there is no preemption mechanism.  Similarly, if the DR fails, the current BDR will take its place (a new BDR will also be elected), and when the failed DR comes back online, it will not reclaim its position.

                                  • 14. Re: how many DR and BDR?
                                    BRenES

                                    Very useful explanations!

                                     

                                    Thanks!

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