OSPF question about adjacencies

Discussion in 'Cisco' started by Sameer, Nov 12, 2003.

  1. Sameer

    Sameer Guest

    I'm a bit confused about adjacencies. Is the router adjacent only to other
    routers that are directly connected to an interface, or could it be adjacent
    to any router that it gets a hello packet from that contains it's own
    RouterID?

    Also, are hello packets sent out only to directly connected routers, or do
    the directly connected routers also pass this hello packet to all other
    routers?

    Finally, what is the mechanism that allows a DR and BDR to send out link
    state packets on the entire network that makes sure all routers, within the
    OSPF area, receive all the link states?

    -Sameer
     
    Sameer, Nov 12, 2003
    #1
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  2. Sameer

    shope Guest

    "Sameer" <> wrote in message
    news:%3lsb.25$...
    > I'm a bit confused about adjacencies. Is the router adjacent only to

    other
    > routers that are directly connected to an interface, or could it be

    adjacent
    > to any router that it gets a hello packet from that contains it's own
    > RouterID?


    Thats the mechanism, although the spec says the router should only form
    adjacencies when other things match, such as subnet mask, MTU and
    authentication.

    In practice the adjacency is usually over a layer 2 connection - i.e. the 2
    OSPF devices are connected to the same segment - main exception you could
    argue is a virtual link.
    >
    > Also, are hello packets sent out only to directly connected routers, or do
    > the directly connected routers also pass this hello packet to all other
    > routers?


    Hello packets are sent on each attached interface set up to run OSPF -
    unless there are configured neighbours.

    Protocol details are a bit different for a multicast capable network or an
    OSPF point to point style interface.

    Note that on cisco and several other boxes you can set a device to "passive"
    where the hellos dont get generated - this means the interface is part of
    the OSPF database, but you wont form an adjacency - mainly useful for star
    points such as big layer 3 switches where you dont want 50 or more
    adjacencies to the same device over all the alternate paths.
    >
    > Finally, what is the mechanism that allows a DR and BDR to send out link
    > state packets on the entire network that makes sure all routers, within

    the
    > OSPF area, receive all the link states?


    Flooding. Just like all the other OSPF topology info.

    DR description packets are part of the link state database that gets
    replicated within an area.
    >
    > -Sameer

    --
    Regards

    Stephen Hope - remove xx from email to reply
     
    shope, Nov 12, 2003
    #2
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  3. Sameer

    phil~~ Guest

    "Sameer" <> wrote in message news:<%3lsb.25$>...
    > I'm a bit confused about adjacencies. Is the router adjacent only to other
    > routers that are directly connected to an interface, or could it be adjacent
    > to any router that it gets a hello packet from that contains it's own
    > RouterID?


    Hello are sent with a destination 224.0.0.x therefore the packet is not
    forwarded, that's why the adjacency remains on the layer2 connection.
    As stated by the other member the virtual-link is an exception.

    > Also, are hello packets sent out only to directly connected routers, or do
    > the directly connected routers also pass this hello packet to all other
    > routers?


    No, they are not passed

    > Finally, what is the mechanism that allows a DR and BDR to send out link
    > state packets on the entire network that makes sure all routers, within the
    > OSPF area, receive all the link states?
    >
    > -Sameer
     
    phil~~, Nov 13, 2003
    #3
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