ADSL Connection interface mirroring ATM state...?

Discussion in 'Cisco' started by Adam Hill, Nov 7, 2003.

  1. Adam Hill

    Adam Hill Guest

    Is there any way ( on an 837 ) that I can configure a static default
    route via an interface which actually goes down when the ATM does?

    i.e...

    You can do ip route 0.0.0.0 0.0.0.0 dialer<n>

    but the dialer interface is always up ( which seems daft. )

    You can do ip route 0.0.0.0 0.0.0.0 atm0

    but that doesn't seem to work at all. You can enter the command, but
    no actual routing takes place!

    The reason that I want to do this is simple... I want to be able to
    route via a different router ( not backup through the same router )
    in the case that the ADSL goes down. With most other point to point
    interfaces its easy, because the static route disappears when the
    interface goes down. However, not being able to route via an
    interface which can actually go down is a problem.

    On a related note... why doesn't the dialer interface go down when
    the ppp connection does? Is there some logic to this or is it a
    random ciscoism?

    --
    Sig demolished to make way for a hyperspace bypass...
     
    Adam Hill, Nov 7, 2003
    #1
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  2. On Fri, 07 Nov 2003 16:06:39 +0000, Adam Hill wrote:

    >
    > On a related note... why doesn't the dialer interface go down when
    > the ppp connection does? Is there some logic to this or is it a random
    > ciscoism?


    A dialer interfaces is up all the time so you can route traffic to it to
    cause it to dial and establish a connection to forward your traffic on.
    If the interface went down, how would you cause it to come up again?

    On the original question, see
    http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123newft/123t/123t_4/gtpbrtrk.htm
    PBR Support for Multiple Tracking Options.
     
    Martin Gallagher, Nov 8, 2003
    #2
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  3. Adam Hill

    Adam Hill Guest

    Martin Gallagher wrote:

    > On Fri, 07 Nov 2003 16:06:39 +0000, Adam Hill wrote:
    >
    >> On a related note... why doesn't the dialer interface go
    >> down when
    >> the ppp connection does? Is there some logic to this or is it a
    >> random ciscoism?

    >
    > A dialer interfaces is up all the time so you can route traffic to
    > it to
    > cause it to dial and establish a connection to forward your traffic
    > on. If the interface went down, how would you cause it to come up
    > again?


    AFAIR dialer interface state goes up and down with connections on,
    for example, basic rate interfaces. On those, you can define a route
    as "permanent" if you want to have the functionality you describe,
    and you can mark the dialer as "persistent" can you not ( which is
    what you normally do on DSL lines, so the line comes back up if it
    can. ) There are plenty of situations where this behaviour is useful.

    Seems odd, then, that an ATM interface behaves differently, when such
    behaviour would be what you actually want most of the time ( and if
    not you have other mechanisms already. )

    > On the original question, see
    >

    http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123newft/123t/123t_4/gtpbrtrk.htm
    > PBR Support for Multiple Tracking Options.


    Ahh, so they have invented yet another method of doing things then...
    Pity its an early deployment release and doesn't appear to be
    supported on the 837, which is the router I wanted to use.

    Oh well, ta for the link Martin...

    --
    Sig demolished to make way for a hyperspace bypass...
     
    Adam Hill, Nov 9, 2003
    #3
  4. Adam Hill

    Andre Beck Guest

    Adam Hill <> writes:
    >
    > Is there any way ( on an 837 ) that I can configure a static default
    > route via an interface which actually goes down when the ATM does?
    >
    > i.e...
    >
    > You can do ip route 0.0.0.0 0.0.0.0 dialer<n>
    >
    > but the dialer interface is always up ( which seems daft. )


    The interface isn't actually "Up/Up", it's "Up/Up (Spoofing)", also known
    as dormant. This makes no real difference when it comes to routes that
    point to such interfaces, but that is mostly intentional.

    > You can do ip route 0.0.0.0 0.0.0.0 atm0
    >
    > but that doesn't seem to work at all. You can enter the command, but
    > no actual routing takes place!


    It's pretty uncommon to route to a real ATM interface unless there is some
    VC mapping established on that interface, be it Q.2931 style SVCs or
    something other. With PVCs, the better way to set this up is via per-PVC
    subinterfaces anyway. But this still doesn't apply to you - even if you
    don't write so, I assume you actually have a DSL connection that uses
    PPPoE or PPPoA on top of a PVC on top of an ATM cellstream on top of
    DSL (yeah, welcome to the world of modern overhead). So it is pretty
    clear that you *need* a dialer, as your connection is in philosophy
    a dialup line, including authentication.

    You could of course do something like

    int a0.55 point-to-point
    ip addr x.x.x.x m.m.m.m
    pvc MyISP 5/55
    encaps aal5snap
    oam-pvc manage 20
    vbr-nrt 2320 2320 1024
    oam retry 3 3 6

    to establish direct IP on top of a SNAP encapsulated PVC to your ISP, or
    a hell of a lot of similar setups. This even works. It just requires the
    other side to think the same way - which a mass DSL provider doesn't do.
    They will supply you a PVC with PPPoE/PPPoA on it, so you must put another
    layer on top of that PVC, and that is a dialer.

    > The reason that I want to do this is simple... I want to be able to
    > route via a different router ( not backup through the same router )
    > in the case that the ADSL goes down. With most other point to point
    > interfaces its easy, because the static route disappears when the
    > interface goes down. However, not being able to route via an
    > interface which can actually go down is a problem.


    This is a real problem with dialers. When they are not connected, there
    is no way to tell whether they could successfully connect when need be,
    so there is no way to remove the route to them only when necessary. At
    least nothing like that is in a straight setup. In a BRI setup, even when
    all dialer pool BRIs are down, the dialers bound to them still stay dormant
    and this is actually correct (BRIs may be able to successfully dial, even
    when they are currently PHY_DEACTIVATED).

    > On a related note... why doesn't the dialer interface go down when
    > the ppp connection does?


    Because if it would, no route would be there that would ever allow a
    packet to go out that dialer again, triggering it to actually dial.
    This is a general flaw of demand dial routing - you must pretend the
    link to be up in order to get the traffic that forces it to go up.
    If the upgoing suddenly fails, you have a problem.

    > Is there some logic to this or is it a random ciscoism?


    It is quite logical, but this logic comes out of a ciscoism - the one that
    dial profiles are actually interfaces. The interface centric approach that
    Cisco cultivates since their first days doesn't allow to implement it in
    a much different way.

    Now the interesting question: Is there any way around that?

    I don't expect it to work, but at least it would have some logic when a
    dialer interface that is set to persist (always stay up and reconnect
    if it drops for whatever reason) would indeed go Down/Down when not
    connected. As this interface is actively trying to reestablish the L2
    without any trigger from L3, the dormant state and route is not needed,
    so it would consequently be removed when the connection fails.

    Then, there is "dialer redial" which allows you to force a dialer to
    go "Down/Down" when a given number of dial attempts fail, and to stay
    down for a given time. This sounds promising, but at least for my
    tests, the feature appeared FUBAR. Neither did the (indeed reached)
    Down/Down cause the route to disappear, nor did the dialer function
    as expected later - it didn't dial at all. So you might try around
    with that, but expect the worst.

    --
    The _S_anta _C_laus _O_peration
    or "how to turn a complete illusion into a neverending money source"

    -> Andre "ABPSoft" Beck +++ ABP-RIPE +++ Dresden, Germany, Spacetime <-
     
    Andre Beck, Nov 12, 2003
    #4
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