automatic default route propagation into RIP: default metric

Discussion in 'Cisco' started by Ben Low, Dec 19, 2003.

  1. Ben Low

    Ben Low Guest

    Hello All,

    I'd like to find documentation regarding default metrics for
    'default-information originate'd routes. I've looked through the IOS
    12.2 IP Command Reference set and Jeff Doyle's "Routing TCP/IP Vol 1",
    without success.

    I'm observing that the default metric appears to be >= 16, thus is
    unreachable for RIP.

    ISP---A---B---C

    RouterA and RouterB are running OSPF, with RouterA having a static
    default route pointing to ISP propagated into OSPF via the
    default-information originate command (default metric type and metric,
    not that it matters, right?).

    Now RouterB and RouterC run RIP, and the goal is to get C to obtain
    the default route automatically from the OSPF route:

    RouterB:
    O*E2 0.0.0.0/0 [110/1] via 10.0.0.1, 00:00:08, Ethernet0

    RouterB also has the default-information originate command
    (12.1(5)T10), again using the defaults.

    The 0.0.0.0 route is being propagated into the RIP domain, though with
    an unreachable metric:

    RouterB#debug ip rip
    2d23h: RIP: sending v2 update to 224.0.0.9 via Serial0 (10.1.3.193)
    2d23h: RIP: build update entries
    2d23h: 0.0.0.0/0 via 0.0.0.0, metric 16, tag 0
    2d23h: 10.0.0.0/22 via 0.0.0.0, metric 1, tag 0


    I explicitly played with the OSPF def-info orig metric type and
    metric, with the expected results in RouterB's routing table and 'sh
    ip ospf database external' output; however it seems the RIP
    advertisement always has 16 regardless of what the OSPF-originated
    metric or metric-type is.

    I figured I should be able to set RouterC's RIP def-info orig to use a
    suitable metric, though there are no options allowed save the
    route-map (i.e. no metric). About the only thing I could do was set
    the default-metric, which works as-expected, assuming I expected a
    redistribution config item to effect default route propagation. (Doyle
    notes on p714 that the default metric for redistributed routes is 0,
    is this also the default for def-info originated routes??? and for
    RIP, does 0 => 16?)

    So:

    Q1: What is the default metric for RIP def-info originated routes, and
    why is it >=16? i.e. why bother making these routes unreachable??

    Q2: How are you supposed to set the metric for def-info orig routes?
    (route-map?)

    Q3: Would it be true to say the 'default-information originate'
    command is simply a specialised case of redistribution? i.e. all
    options such as default-metric apply?


    Thanks in advance,

    Ben
    Ben Low, Dec 19, 2003
    #1
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  2. > route-map (i.e. no metric). About the only thing I could do was set
    > the default-metric, which works as-expected, assuming I expected a
    > redistribution config item to effect default route propagation. (Doyle
    > notes on p714 that the default metric for redistributed routes is 0,
    > is this also the default for def-info originated routes??? and for
    > RIP, does 0 => 16?)


    I had the same problem a long time ago and I missed that one in the
    beginning, unfortunately because it is the key.

    If you redistribute an ospf route to RIP by a redistribute, it seems
    to get a metric 0. RIP metrics must be in the range 1-15 on the
    other hand. The metric 0 is invalid and is marked as any other
    invalid route (like say metric 16 would have been marked).
    The RIP refuces it.

    Generally, redistribution cannot convert metrics, so it has some
    default values like zero and it is better to set them manually.
    They might be invalid like in this case. This is a general
    concept for all redistributions! (Some protocols like ospf
    on the other hand accept the defaults while eg. rip does not)

    If you had redistributed it by something like this things will change:
    router rip
    redistribute ospf 123 metric 1

    It would be metric 1 on the router A doing the redistribution instead
    of being invalid (zero) and it would propagate to B and C as
    expected.
    --
    Harri
    Harri Suomalainen, Dec 23, 2003
    #2
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  3. Ben Low

    Ben Low Guest

    "Harri Suomalainen" <> wrote in message news:<vTUFb.82$>...
    ....
    > If you redistribute an ospf route to RIP by a redistribute, it seems
    > to get a metric 0. ...
    > The RIP refuses it.

    ....
    > If you had redistributed it by something like this things will change:
    > router rip
    > redistribute ospf 123 metric 1



    Thanks for confirming the situation Harri. I found it strange, but
    RIP's redistribute command doesn't support setting the metric [1];
    thus it appears it's only possible to either set the default-metric or
    use a route map. I suppose, for the reasons you've noted, it's best to
    be in the habit of setting the default-metric...

    [1] http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123tcr/123tip2r/ip2_o1gt.htm#1037093

    Thanks,

    Ben
    Ben Low, Dec 28, 2003
    #3
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