Re: AS SET

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