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
> >>>>>
> >>>>>
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> >
> >
> >
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>
>
>
> --
> Ronald Angello
> Network Architect
> CCIE 17846
> CCDP, CCIP, CCNP

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Received on Sun Mar 14 2010 - 22:49:28 ART