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2356 Views 18 Replies Latest reply: Sep 10, 2012 11:28 PM by Ahmed Muhi RSS 1 2 Previous Next

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OSPF DR/BDR election purpose

Mar 6, 2012 7:45 PM

mongolio 55 posts since
Jan 22, 2009

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?

  • Stason 14 posts since
    Jan 24, 2011
    Currently Being Moderated
    1. Mar 6, 2012 6:10 AM (in response to mongolio)
    Re: OSPF DR/BDR election purpose

    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.

  • Joel Shrestha 132 posts since
    Jun 20, 2011
    Currently Being Moderated
    2. Mar 6, 2012 11:04 AM (in response to mongolio)
    Re: OSPF DR/BDR election purpose

    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

  • lp4nb 520 posts since
    Jun 27, 2009
    Currently Being Moderated
    4. Mar 6, 2012 9:51 PM (in response to mongolio)
    Re: OSPF DR/BDR election purpose

    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.

  • lp4nb 520 posts since
    Jun 27, 2009
    Currently Being Moderated
    6. Mar 7, 2012 8:11 AM (in response to mongolio)
    Re: OSPF DR/BDR election purpose

    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.)

  • lp4nb 520 posts since
    Jun 27, 2009
    Currently Being Moderated
    8. Mar 8, 2012 2:23 AM (in response to mongolio)
    Re: OSPF DR/BDR election purpose

    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

  • lp4nb 520 posts since
    Jun 27, 2009
    Currently Being Moderated
    10. Mar 8, 2012 4:03 PM (in response to mongolio)
    Re: OSPF DR/BDR election purpose

    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

  • Joel Shrestha 132 posts since
    Jun 20, 2011
    Currently Being Moderated
    12. Mar 9, 2012 9:53 AM (in response to mongolio)
    Re: OSPF DR/BDR election purpose

    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

  • wave - CCNP R&S 74 posts since
    May 31, 2011
    Currently Being Moderated
    13. Mar 9, 2012 10:01 AM (in response to mongolio)
    Re: OSPF DR/BDR election purpose

    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.)

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