OSPF sham-link doesnÄT

-------- Original-Nachricht --------
> Datum: Sun, 14 Mar 2010 22:49:28 +0000
> Von: Muzammil Malick <malickmuz_at_gmail.com>
> An: Ronnie Angello <ronnie.angello_at_gmail.com>
> CC: Nadeem Rafi <nrafia_at_gmail.com>, Cisco certification <ccielab_at_groupstudy.com>
> Betreff: Re: AS SET

> Ahh brilliant. Thanks Guys!
>
> On 14 March 2010 22:12, Ronnie Angello <ronnie.angello_at_gmail.com> wrote:
>
> > What happens if/when the specific prefix is lost?
> >
> > It can lead to loops if the prefixes that are being summarized are
> > learned from several eBGP peers, rather than originating within the AS
> > of the advertising router. That's why you would use AS-SET.
> >
> > Ronnie
> >
> > On Sun, Mar 14, 2010 at 4:55 PM, Muzammil Malick <malickmuz_at_gmail.com>
> > wrote:
> > > Ok I am still confused though,
> > >
> > > I fully understand the example but what do you mean by more than a
> single
> > > exit point. More than one aggregating routers?
> > >
> > > The example states
> > >
> > > In any closed network, this aggregate information propagates through
> BGP
> > and
> > > back to one of the ASs that the *as-set* lists. This propagation
> creates
> > the
> > > possibility of a loop. The loop detection behavior of BGP notes its
> own
> > AS
> > > number in the *as-set* of the aggregate update and drops the
> aggregate.
> > This
> > > action prevents a loop.
> > >
> > > This is as I understood.
> > >
> > > If Router C were to start advertising the summary to Router A with an
> AS
> > > SET, Router A would see its own AS in the route and drop it.
> > > If Router C does not include AS SET and advertises to A then A will
> have
> > a a
> > > specific 160.20.0.0/16 and a summary 160.0.0.0/8 with no AS info.
> > > What is the harm in this situation?
> > >
> > > On 14 March 2010 20:31, Nadeem Rafi <nrafia_at_gmail.com> wrote:
> > >
> > >> this will come into action, if you have more than single exit point.
> > >>
> > >> See this excellent example.
> > >>
> > >>
> > >>
> >
> http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094826.shtml
> > >>
> > >>
> > >> On Sun, Mar 14, 2010 at 10:40 PM, Muzammil Malick
> <malickmuz_at_gmail.com
> > >wrote:
> > >>
> > >>> Thanks
> > >>>
> > >>> but how could this cause a loop?
> > >>> If the local router has a more specific route to a destination that
> is
> > >>> covered by the summary then it will use that.
> > >>>
> > >>> R1 ----------- R2
> > >>>
> > >>> If R1 advertises 10.0.0.1/24 to R2 and then R2 advertises back a
> > summary
> > >>> for 10.0.0.0/16 without AS info. R2 has now covered many other
> > prefixes
> > >>> which may have come
> > >>> from other AS' and even the prefix from R1, however if R1 has more
> > >>> specific routes it will use these. If it doesn't have more specific
> > info
> > >>> then this will be its only route anyway so
> > >>> how can a loop be formed?
> > >>>
> > >>>
> > >>>
> > >>> On 14 March 2010 19:21, Nadeem Rafi <nrafia_at_gmail.com> wrote:
> > >>>
> > >>>> If i am wrong please correct me.
> > >>>>
> > >>>> when bgp is summarized then information for the routes
> > >>>> being summarized is lost. And summarizing AS is considered to be
> > originator
> > >>>> of this new route. All the AS afterwards dont have knowledge of
> routes
> > >>>> being summarized that from which AS they belong. And this can led
> from
> > >>>> non-optimal routing to routing loops, because of this as-set is
> used
> > to feed
> > >>>> with more information about routes being summarized.
> > >>>>
> > >>>>
> > >>>>
> > >>>> HTH
> > >>>>
> > >>>> On Sun, Mar 14, 2010 at 10:12 PM, Muzammil Malick <
> > malickmuz_at_gmail.com>wrote:
> > >>>>
> > >>>>> Hi
> > >>>>>
> > >>>>> Could someone please help me out and explain why you would include
> AS
> > >>>>> information in a bgp summary (aggregate-address 10.0.0.0
> 255.255.0.0
> > >>>>> summary-only as-set)
> > >>>>>
> > >>>>> I know what it does and how to use it but why from a design
> > perspective
> > >>>>> would you want to send AS information as apart of a summary.
> > >>>>> Can someone provide an example?
> > >>>>>
> > >>>>>
> > >>>>> Blogs and organic groups at http://www.ccie.net
> > >>>>>
> > >>>>>
> > _______________________________________________________________________
> > >>>>> Subscription information may be found at:
> > >>>>> http://www.groupstudy.com/list/CCIELab.html
> > >
> > >
> > > Blogs and organic groups at http://www.ccie.net
> > >
> > >
> _______________________________________________________________________
> > > Subscription information may be found at:
> > > http://www.groupstudy.com/list/CCIELab.html
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
> >
> > --
> > Ronald Angello
> > Network Architect
> > CCIE 17846
> > CCDP, CCIP, CCNP
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
>
>
>
>

-- 
GMX DSL: Internet, Telefon und Entertainment f|r nur 19,99 EUR/mtl.!
http://portal.gmx.net/de/go/dsl02
Blogs and organic groups at http://www.ccie.net