Dual ISP Question

Discussion in 'Cisco' started by J.Cottingim, Apr 4, 2006.

  1. J.Cottingim

    J.Cottingim Guest

    Is there a way to have a router with an ethernet connection to two
    ISP's (and an inside interface of course) fail over from one ISP to the
    other when traffic stops flowing on the first one?

    * Neither ISP shares it's routing table. (it's not an option so don't
    suggest it)
    * Neither interface that faces the ISP's will go down (both ethernets
    attached to a switch)

    I was thinking of something with a route-map to a known "pingable"
    address - force it out primary ISP interface - but how do you make it
    fail-over to the backup ISP.

    Of course I could write a script that runs in conjunction with the
    route-map, but I'd really like to keep it on the router.

    Suggestions??


    ___ ___ ___ ___
    / \/ \ / \/ \
    | ISP A | | ISP B |
    \___/\___/ \___/\___/
    ¦R¦ ¦R¦
    +-+ +-+
    ¦ ¦
    ¦ +--------+ ¦
    +---¦myRouter¦---+
    +--------+
    ¦
    ¦
    +--------------+
     
    J.Cottingim, Apr 4, 2006
    #1
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  2. In article <>,
    J.Cottingim <> wrote:
    >* Neither ISP shares it's routing table. (it's not an option so don't
    >suggest it)
    >* Neither interface that faces the ISP's will go down (both ethernets
    >attached to a switch)


    >I was thinking of something with a route-map to a known "pingable"
    >address - force it out primary ISP interface - but how do you make it
    >fail-over to the backup ISP.


    Some platforms with some versions of IOS support ping-based
    detection of route availability. I do not recall any of the details;
    probably one of Vincent Jones' messages names the technology.

    Which platform and version are you working with?
     
    Walter Roberson, Apr 4, 2006
    #2
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  3. Couldn't you just set up 2 default routes, 1 with a higher
    administrative distance than the other?

    Say,

    ip route 0.0.0.0 0.0.0.0 dialer1
    ip route 0.0.0.0 0.0.0.0 dialer2 220

    I think that might work.

    Walter Roberson wrote:
    > In article <>,
    > J.Cottingim <> wrote:
    > >* Neither ISP shares it's routing table. (it's not an option so don't
    > >suggest it)
    > >* Neither interface that faces the ISP's will go down (both ethernets
    > >attached to a switch)

    >
    > >I was thinking of something with a route-map to a known "pingable"
    > >address - force it out primary ISP interface - but how do you make it
    > >fail-over to the backup ISP.

    >
    > Some platforms with some versions of IOS support ping-based
    > detection of route availability. I do not recall any of the details;
    > probably one of Vincent Jones' messages names the technology.
    >
    > Which platform and version are you working with?
     
    Ken Gallagher, Apr 4, 2006
    #3
  4. J.Cottingim

    Merv Guest

    Merv, Apr 4, 2006
    #4
  5. J.Cottingim

    J.Cottingim Guest

    The weighted routes wouldn't work as interface won't be going down to
    remove the static route.
     
    J.Cottingim, Apr 4, 2006
    #5
  6. J.Cottingim

    J.Cottingim Guest

    This looks like what I'm looking for. - Thanks.
    The command have changed slightly from 12.3(8)T to 12.4

    The command "rtr" has changed to "ip sla monitor"

    I'll try to post an example config when I have it.

    Thanks again
    J.Cottingim
     
    J.Cottingim, Apr 4, 2006
    #6
  7. J.Cottingim wrote:

    > This looks like what I'm looking for. - Thanks.
    > The command have changed slightly from 12.3(8)T to 12.4
    >
    > The command "rtr" has changed to "ip sla monitor"
    >
    > I'll try to post an example config when I have it.
    >
    > Thanks again
    > J.Cottingim


    When I last tried this (a couple of years ago) there were two major gotchas
    you would need to work around, a search for "ping based routing" should get
    you more details:

    1 - NAT translates in effect would not go away, and policy NAT was only
    applied when a new translation was created. Only a problem if the router is
    doing NAT as well as routing.

    2 - Under some conditions (never fully quantified) the response time
    recorder would stop probing a down link, preventing fail back to the
    preferred link.

    Number one could be worked around with a hack, but I never saw number two in
    a repeatable lab setup allowing full characterization, so I don't know if
    it is an avoidable feature or a bug.

    Good luck, have fun, and report back :)

    --
    Vincent C Jones, Consultant Expert advice and a helping hand
    Networking Unlimited, Inc. for those who want to manage and
    Tenafly, NJ Phone: 201 568-7810 control their networking destiny
    http://www.networkingunlimited.com
     
    Vincent C Jones, May 8, 2006
    #7
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