A question about ospf

Discussion in 'Cisco' started by aaabbb16@hotmail.com, Aug 5, 2007.

  1. Guest

    If there are only two routers on the broadcast neighborship.
    Is it point to point? Does it still select DR and BDR? if not,
    does it work like point --point (non broadcast neighborship)?

    TIA,
    /st
     
    , Aug 5, 2007
    #1
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  2. <> ha scritto nel messaggio
    news:...
    > If there are only two routers on the broadcast neighborship.
    > Is it point to point? Does it still select DR and BDR? if not,
    > does it work like point --point (non broadcast neighborship)?
    >
    > TIA,
    > /st


    Hi,

    It is always "broadcast" by default and "works" as such, so there is DR/BDR
    election and Type2 LSAs.
    Since this is a waste of LSA database resources, you can force OSPF to treat
    the link as Point-to-Point.

    Regards,
    Gabriele
     
    Gabriele Beltrame, Aug 5, 2007
    #2
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  3. Merv Guest

    Also the adjacency will take longer to form than necessary as there is
    a 40 second wait time on braodcast media to ensure that all routers
    that could become DR or BDR are detected.
     
    Merv, Aug 5, 2007
    #3
  4. Guest

    On 8 5 , 5 29 , "Gabriele Beltrame" <> wrote:
    > <> ha scritto nel messaggionews:...
    >
    > > If there are only two routers on the broadcast neighborship.
    > > Is it point to point? Does it still select DR and BDR? if not,
    > > does it work like point --point (non broadcast neighborship)?

    >
    > > TIA,
    > > /st

    >
    > Hi,
    >
    > It is always "broadcast" by default and "works" as such, so there is DR/BDR
    > election and Type2 LSAs.
    > Since this is a waste of LSA database resources, you can force OSPF to treat
    > the link as Point-to-Point.
    >
    > Regards,
    > Gabriele


    Thanks Gabriele and Merv.
    Some books and Cisco training books usually use two routers to explain
    OSPF from powerup state to full state and not mention it is point to
    point or broadcast. e.g. down-->init->exstart-->exchange state --
    >loading state-->

    full state. if it is broadcast neighbors,which state for dr/bdr
    selection? (exstart decide who is master/slave, means master start to
    excange DBD first )
    or it is true for any neighborships.

    Second question is that all relate to OSPF packets use multicast
    (224...5 and 224...6). Is it true? because from this URL (http://
    www.netacad.lania.mx/reetoling/OSPF_single_area.ppt#435,13,Hello
    Protocol exchange process) "step4 Routers send unicast reply hello
    packets
    back router A with their own information" Is it only unicast packets
    for all OSPF
    related?

    TINA,
    /st
     
    , Aug 5, 2007
    #4
  5. <> ha scritto nel messaggio
    news:...
    > Thanks Gabriele and Merv.
    > Some books and Cisco training books usually use two routers to explain
    > OSPF from powerup state to full state and not mention it is point to
    > point or broadcast. e.g. down-->init->exstart-->exchange state --
    >>loading state-->

    > full state. if it is broadcast neighbors,which state for dr/bdr
    > selection? (exstart decide who is master/slave, means master start to
    > excange DBD first )
    > or it is true for any neighborships.


    Database sync always uses master/slave relationship, either on
    multi-access and point-to-point networks
    DR might happen to be the master but it is not a must.

    The DR election happens between after the routers have become adjacent
    (2way just after init) but before the begin the exstart phase (of course)

    >
    > Second question is that all relate to OSPF packets use multicast
    > (224...5 and 224...6). Is it true? because from this URL (http://
    > www.netacad.lania.mx/reetoling/OSPF_single_area.ppt#435,13,Hello
    > Protocol exchange process) "step4 Routers send unicast reply hello
    > packets
    > back router A with their own information" Is it only unicast packets
    > for all OSPF
    > related?


    hello packets are multicasted to 224.0.0.5 on all broadcast capable
    media (Broadcast,Point-to-point,point-to-multipoint)
    hello packets are unicasted on non broadcast capable media (NBMA and
    point-to-multipoint Non Broadcast) (you need to statically configure
    neighbors)

    LSA acks can be either multicasted or unicasted (e.g. delayed acks and
    direct acks; it depends on medium of course)
    LSA Updates too can be either multicasted or not. (DR always multicast
    to 224.0.0.5 and others (including BDR) always multicast to 224.0.0.6)
    >
    > TINA,
    > /st


    Regards,
    Gabriele
     
    Gabriele Beltrame, Aug 5, 2007
    #5
  6. stephen Guest

    <> wrote in message
    news:...
    > On 8 5 , 5 29 , "Gabriele Beltrame" <> wrote:
    > > <> ha scritto nel

    messaggionews:...
    > >
    > > > If there are only two routers on the broadcast neighborship.
    > > > Is it point to point? Does it still select DR and BDR? if not,
    > > > does it work like point --point (non broadcast neighborship)?

    > >
    > > > TIA,
    > > > /st

    > >
    > > Hi,
    > >
    > > It is always "broadcast" by default and "works" as such, so there is

    DR/BDR
    > > election and Type2 LSAs.
    > > Since this is a waste of LSA database resources, you can force OSPF to

    treat
    > > the link as Point-to-Point.
    > >
    > > Regards,
    > > Gabriele

    >
    > Thanks Gabriele and Merv.
    > Some books and Cisco training books usually use two routers to explain
    > OSPF from powerup state to full state and not mention it is point to
    > point or broadcast. e.g. down-->init->exstart-->exchange state --
    > >loading state-->

    > full state. if it is broadcast neighbors,which state for dr/bdr
    > selection?


    this isnt part of the exchange as such, it happens when "hello" 1st starts
    on 1 of the routers.

    once 1 of them "elects" itself (since both are unlikely to start the
    interface simultaneously), it stays that way as the election result is
    sticky, and the elected interface needs to go down to force another.

    the only case where election may start when an adjacency is coming up is
    when you have a LAN with a break in it, and the 2 interfaces are on
    different sides - the 2 routers both get an interface elected, and you have
    a resolution to fix that.

    (exstart decide who is master/slave, means master start to
    > excange DBD first )
    > or it is true for any neighborships.
    >
    > Second question is that all relate to OSPF packets use multicast
    > (224...5 and 224...6). Is it true? because from this URL (http://
    > www.netacad.lania.mx/reetoling/OSPF_single_area.ppt#435,13,Hello
    > Protocol exchange process) "step4 Routers send unicast reply hello
    > packets
    > back router A with their own information" Is it only unicast packets
    > for all OSPF
    > related?


    the gold rule with RFCs is "check the source".

    RFC 2328 is the standard and it is fairly clearly written - so check that.

    if you must use a book, then "John Moy - Anatomy of a routing protocol" is
    now pretty old, but the author was involved in producing the protocol.....
    >
    > TINA,
    > /st

    --
    Regards

    - replace xyz with ntl
     
    stephen, Aug 5, 2007
    #6
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