dtimmermdtimmerm Member Posts: 7 ■□□□□□□□□□
setup in GNS3

BDR .1/25---SWITCH--- .2/25 DR

3 routers share a common subnet subnet
Lo0 DR:
Lo0 BDR:


I configured first the DR and BDR (interface,loopback and ospf config)

example of DR config

interface Loopback0
ip address
interface FastEthernet0/0
ip address
duplex auto
speed auto

router ospf 100
network area 0
network area 0

Ok so far so good


I started Wireshark on DRother to see what happens when I configure DRother

what happens:

DRother sends a mcast to with BDR/DR
BDR's hello timer expires and send a hello to, has DR and DRother in it's neighbo list
DR's hello timer expires and send a hello to, has BDR and DRother in it's neighbo list

=>biderectional communication is established with his neighbors

I expected that now and exchange would follow with the DR (DBD,LSR,LSU)

DROTHER --> DR via
DR --> DROTHER unicast to DR

and finally DR via to all routers

but this is not the case.

I see unicast(in both directions) exchanges(DBD, LSupdate) with both DR and the BDR(???) in both directions?? even unicast exchanges(DBD/LSupdate,...) between DR and BDR (while stable)
(30 messages in 2 seconds)??

In this stream I see only 2 messages to 224.0.06?? While the same message is also sent from DROTHER has been sent to

Is this due to GNS3 and wireshark or do I really don't understand how the sync works


  • dtimmermdtimmerm Member Posts: 7 ■□□□□□□□□□

    when situation of above is stable (synced)

    I add new loopback to DROTHER and add the loopback to the OSPF routing process, same area as the rest

    DROTHER --> LSupdate (included: updated LSA1 with 3 links instead of 2)
    DR --> LS update (included: LSA 1 of DROTHER)

    so far so good

    but then

    BDR --> LS update(old LSA type 1 of BDR)* [DROTHER knows this network long time]
    DROTHER --> BDR LS update (included: updated LSA1 of DROTHER with 3 links instead of 2) WTF?(**)
    BDR--> DROTHER LS ack (Ack for **)
    BDR --> LS Ack (Ack for **)
    DROTHER--> (LS Ack)(Ack for *)
    DR --> LS Ack(Ack for *)

    I understand that you won't understand this :)

    But I'm just trying to follow the DBD, Update messages etc via Wireshark

    Like the sequence
    BDR --> LS update(old LSA type 1 of BDR)*
    DROTHER--> (LS Ack)(Ack for *)
    DR --> LS Ack(Ack for *)

    While the network is stable:s

    Route is not aged out,
    why does the BDR send it to 224.0.05
  • dtimmermdtimmerm Member Posts: 7 ■□□□□□□□□□
    what I actually expect is the following

    new router exchanges hello packets (neighbor discovery)
    all other neighbors reply with the new neighbor's router ID in their neighbor list

    ==> bidirectional communication + new router has learnt about DR/BDR via the hello's of the neighbors


    new router communicates with DR via and DR unicast with the new router

    >Exstart, Exchange Loading Full

    and then DR sends a LSA type 2 out to

    or how is it actually going?

    Or does the DR sends out the new LSA 1 out to But of what use is LSA type 2 then.

    I'm confused

  • kryollakryolla Member Posts: 785
    DRother will send LSA1 to DR via .06 and the DR will flood LSA1 and 2 to the whole area via .05. Think about how a router will route packets within an area and you will understand what LSA1 & 2 are for.
    Studying for CCIE and drinking Home Brew
  • dtimmermdtimmerm Member Posts: 7 ■□□□□□□□□□
    that's what I thought but the output in wireshark was quite different

    the LSA 2 seems to be useless as it lists only the attached networks without any further detail.
  • kryollakryolla Member Posts: 785
    your not understanding the OSPF database and how route processing works. Do some more research. LSA 2 are extremely important for intra-area routing
    Studying for CCIE and drinking Home Brew
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