Re: AS SET

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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Received on Sun Mar 14 2010 - 20:55:26 ART