OSPF in fully meshed environment

Discussion in 'Cisco' started by linguafr, Mar 8, 2007.

  1. linguafr

    linguafr Guest

    We have 3 sites areas 0, 1 & 2 in a point-to-point L2 mesh, separate vc
    for each.

    In area 0 and 1 the p2ps terminate at L3 on ASAs
    In area 2 on a router

    At area 0 I have put the L3 interfaces terminating each vc, in the
    remote area, i.e. areas 1 and 2 and ospf is working (except the area
    command doesn't seem to be having an affect). However, I don't know how
    to decide which area to put the L3 interfaces that terminate the p2ps
    between areas 1 and 2. And I'm curious if there's any advice for the
    setup that would limit the need to filter broadcasted routes. i.e. I
    don't want areas broadcasting routes that take the closest area hop.

    Thanks
     
    linguafr, Mar 8, 2007
    #1
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  2. linguafr

    linguafr Guest

    I meant, I don't want areas broadcasting routes that DO NOT take the
    closest area hop.
     
    linguafr, Mar 8, 2007
    #2
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  3. linguafr

    Trendkill Guest

    I'm not sure I understand why you do not want those routes
    advertised. In case a link goes down, would you not want it to route
    the other way? If there is some bandwidth constraint that I don't
    know about, you can still raise the metric on certain routes so it
    will not use them unless it is the only way. If you really want to
    avoid this, it doesn't matter which area those interfaces are in, and
    you can just put distribute lists for the routes you don't want
    advertised.
     
    Trendkill, Mar 8, 2007
    #3
  4. linguafr

    linguafr Guest

    Hi - Thanks for responding. Yes, I'm familiary with prefix-lists. For
    security reasons we have other backup links.

    Can you help with the more urgent question I have about what area to
    place the p2p interfaces between areas 1 & 2??

    Thanks
     
    linguafr, Mar 8, 2007
    #4
  5. linguafr

    Trendkill Guest

    I'm not sure I understand your question. They must go in either area
    1 or 2, and provided they are not in area 0, it will not cause a
    complete reconvergence if these links bounced. Flip a coin! Just
    kidding! Is there something I am missing regarding why it matters
    which one they are going in? Otherwise, just pick...
     
    Trendkill, Mar 8, 2007
    #5
  6. linguafr

    stephen Guest

    if i understand what you trying to do, you are using different areas for the
    different virtual circuits across the WAN.

    simple answer is, dont - put them all in the backbone area.

    If you insist in having areas you can use virtual links - but all that means
    is you "tunnel" the backbone area 0 across the others areas on your WAN -
    which is not good.

    Basically virtual links in your design means it is time to redesign and get
    rid of them.....

    the OSPF topology and "best route" works best within an area - and an area
    can include dozens of OSPF nodes.

    If you must have areas then make each device an ABR and put an area "behind"
    each one - but unless you have hundreds of routers, or some specific
    topology reason to need areas then dont use them.
     
    stephen, Mar 9, 2007
    #6
  7. linguafr

    linguafr Guest

    Hi Stephen

    Thanks for the advice. I think we're stuck with an area at each site
    for the moment anyway. Each site's ABR has a p2p interface in the
    remote area on the other side of the p2p accept for site 2, which I
    arbitrarily just put the p2p interface in area 2. Every site now is
    getting routes from the other two sites, except for site 2 which for
    some reason isn't getting routes from site 1.

    Another question regarding the ospf network type. Initially I didn't
    specify this and all the neighbors are discovered, and, I believe
    they're all designated as BROADCAST. Is there a downside to this? What
    is the requirement or advantage of setting them to say, point-to-point
    non-broadcast?

    Thanks
     
    linguafr, Mar 12, 2007
    #7
  8. linguafr

    linguafr Guest

    I think I missed the crucial point you were making which is that all
    traffic has to traverse the backbone area, so, I can't send traffic
    directly from area 1 to 2?
     
    linguafr, Mar 12, 2007
    #8
  9. linguafr

    stephen Guest

    it sort of can - but the spec for OSPF insists that any ABR should be
    connected to the backbone.

    Cisco have written an OSPF extension into some versions of IOS (and wrote an
    RFC to cover it) that talks about a "fallback" type config where an ABR that
    loses its backbone link can carry on working.

    i think i have seen this happen by accident, but given i dont know which
    version support it, and what you are running......
     
    stephen, Mar 13, 2007
    #9
  10. linguafr

    stephen Guest

    "broadcast" means the hello protocol includes finding a neighbour thru
    multicasts.

    Obviously doesnt work if the interface type doesnt believe in multicast.....

    The downside is a very slight overhead compared to point to point in setting
    up the adj.

    Not worth the extra config effort IMO to go point to point, unless you need
    tight control over exactly what is happening.
     
    stephen, Mar 13, 2007
    #10
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