Counting to Infinity

Discussion in 'Cisco' started by Bob Simon, Jan 31, 2009.

  1. Bob Simon

    Bob Simon Guest

    I'm reading Routers and Routing Basics (CCNA 2 Companion Guide) and
    have a question about RIP route poisoning. If you have two connected
    routers (R1 and R2) and one of R2's interfaces fails, it will
    advertise this network to R1 with a metric of 16.

    The book says that after this route with metric 16 gets installed in
    R1's route table, R1 will forward packets to this network to R2. Is
    this correct? I thought that with an infinite metric, R1 would drop
    the packet. No?
    Bob Simon, Jan 31, 2009
    #1
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  2. Bob Simon

    bod43 Guest

    On 31 Jan, 12:36, Bob Simon <> wrote:
    > I'm reading Routers and Routing Basics (CCNA 2 Companion Guide) and
    > have a question about RIP route poisoning.  If you have two connected
    > routers (R1 and R2) and one of R2's interfaces fails, it will
    > advertise this network to R1 with a metric of 16.
    >
    > The book says that after this route with metric 16 gets installed in
    > R1's route table, R1 will forward packets to this network to R2.  Is
    > this correct?  I thought that with an infinite metric, R1 would drop
    > the packet.  No?


    Yes. Infinite metric routes will I believe result
    in dropped packets.

    I cannot comment about the book you are looking at
    however in my experience many CCNx books are
    perhaps rushed out due to market forces and
    contain more than a few errors.

    While if what you need is to brush up for an exam
    and want focussed material these books have
    their place however there are truely good books about.

    For IP routing:-
    Routing TCP/IP by Jeff Doyle seems to be the one.
    (Second Edition now)

    I have read much of it and it very nice with clear
    (*and complete*) explanations and simply no significant
    errors that I have noticed.

    The RIP RFC is a nice enough read too.
    RFC1058
    bod43, Jan 31, 2009
    #2
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  3. Bob Simon

    Bob Simon Guest

    On Sat, 31 Jan 2009 07:41:30 -0800 (PST), bod43 <>
    wrote:

    >On 31 Jan, 12:36, Bob Simon <> wrote:
    >> I'm reading Routers and Routing Basics (CCNA 2 Companion Guide) and
    >> have a question about RIP route poisoning.  If you have two connected
    >> routers (R1 and R2) and one of R2's interfaces fails, it will
    >> advertise this network to R1 with a metric of 16.
    >>
    >> The book says that after this route with metric 16 gets installed in
    >> R1's route table, R1 will forward packets to this network to R2.  Is
    >> this correct?  I thought that with an infinite metric, R1 would drop
    >> the packet.  No?

    >
    >Yes. Infinite metric routes will I believe result
    >in dropped packets.
    >
    >I cannot comment about the book you are looking at
    >however in my experience many CCNx books are
    >perhaps rushed out due to market forces and
    >contain more than a few errors.
    >
    >While if what you need is to brush up for an exam
    >and want focussed material these books have
    >their place however there are truely good books about.
    >
    >For IP routing:-
    >Routing TCP/IP by Jeff Doyle seems to be the one.
    >(Second Edition now)
    >
    >I have read much of it and it very nice with clear
    >(*and complete*) explanations and simply no significant
    >errors that I have noticed.
    >
    >The RIP RFC is a nice enough read too.
    >RFC1058


    The book I'm using is by Wendell Odom and Rick McDonald published by
    Cisco as part of their Networking Academy Program. It's actually one
    of the best books I've read covering routing basics and it's written
    in an easy to understand style and with lots of examples. I like this
    book and am thinking about getting the next one in the series.

    While I typically find lots of errors in networking books, this one
    has had remarkably few that I've caught. That's why I was surprised
    to find something that really didn't look right. Since you agree that
    routers should not forward packets to destinations with infinite
    metric in the route table, I think I'll write to the authors and see
    what they have to say about this.
    Bob Simon, Jan 31, 2009
    #3
  4. Bob Simon

    Stephen Guest

    On Sat, 31 Jan 2009 06:36:48 -0600, Bob Simon <>
    wrote:

    >I'm reading Routers and Routing Basics (CCNA 2 Companion Guide) and
    >have a question about RIP route poisoning. If you have two connected
    >routers (R1 and R2) and one of R2's interfaces fails, it will
    >advertise this network to R1 with a metric of 16.
    >


    Yes
    >The book says that after this route with metric 16 gets installed in
    >R1's route table, R1 will forward packets to this network to R2. Is
    >this correct? I thought that with an infinite metric, R1 would drop
    >the packet. No?


    Yes

    the more general point is that with Internet standards - go back to
    the source.
    RIP v2 RFC 2453 has a lot of background explanation of why things are
    done - so check there
    http://tools.ietf.org/html/rfc2453

    have a quick hunt for "poison" in the file - there is some stuff about
    this under "split horizon with poison reverse"

    More generally RFCs are usually written to explain things clearly, and
    if you have problems with a text book check the original RFCs.
    --
    Regards

    - replace xyz with ntl
    Stephen, Jan 31, 2009
    #4
  5. Bob Simon

    John Agosta Guest

    "Bob Simon" <> wrote in message
    news:...
    > On Sat, 31 Jan 2009 07:41:30 -0800 (PST), bod43 <>
    > wrote:
    >
    >>On 31 Jan, 12:36, Bob Simon <> wrote:
    >>> I'm reading Routers and Routing Basics (CCNA 2 Companion Guide) and
    >>> have a question about RIP route poisoning. If you have two connected
    >>> routers (R1 and R2) and one of R2's interfaces fails, it will
    >>> advertise this network to R1 with a metric of 16.
    >>>
    >>> The book says that after this route with metric 16 gets installed in
    >>> R1's route table, R1 will forward packets to this network to R2. Is
    >>> this correct? I thought that with an infinite metric, R1 would drop
    >>> the packet. No?

    >>
    >>Yes. Infinite metric routes will I believe result
    >>in dropped packets.
    >>
    >>I cannot comment about the book you are looking at
    >>however in my experience many CCNx books are
    >>perhaps rushed out due to market forces and
    >>contain more than a few errors.
    >>
    >>While if what you need is to brush up for an exam
    >>and want focussed material these books have
    >>their place however there are truely good books about.
    >>
    >>For IP routing:-
    >>Routing TCP/IP by Jeff Doyle seems to be the one.
    >>(Second Edition now)
    >>
    >>I have read much of it and it very nice with clear
    >>(*and complete*) explanations and simply no significant
    >>errors that I have noticed.
    >>
    >>The RIP RFC is a nice enough read too.
    >>RFC1058

    >
    > The book I'm using is by Wendell Odom and Rick McDonald published by
    > Cisco as part of their Networking Academy Program. It's actually one
    > of the best books I've read covering routing basics and it's written
    > in an easy to understand style and with lots of examples. I like this
    > book and am thinking about getting the next one in the series.
    >
    > While I typically find lots of errors in networking books, this one
    > has had remarkably few that I've caught. That's why I was surprised
    > to find something that really didn't look right. Since you agree that
    > routers should not forward packets to destinations with infinite
    > metric in the route table, I think I'll write to the authors and see
    > what they have to say about this.


    Minor point - perhaps even nit-picking, but a 'down' route is never placed
    in the route table.
    One will never see a RIP metric of 16 in the route table.

    If the route has been placed in "hold down," (route table shows "possibly
    down")
    then yes, packets are still forwarded towards the destination until either
    the timer expires
    or the route is re-discovered with a better metric (at the time the route
    was available) from any source, or same or better from the original source
    that was in the FIB.

    What will actually happen in a real operating environment can vary due to
    various
    implementations such as - flash updates, split horizon, enabled/disabled and
    timers.


    -ja
    John Agosta, Jan 31, 2009
    #5
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