Re: Question about BGP Aggregate with AS-SET

Hi,

AS_SET is normally generated when aggregated BGP prefixes (found in
BGP Loc-RIB) have mismatching AS_PATH attributes. There are not
"strict" rules for BGP aggregation, but in general most BGP
implementations *try* to follow section 9.2.4.2 from RFC 1771. One of
the rules there state the following:

  - all tuples of the type AS_SEQUENCE in the aggregated AS_PATH shall
appear in all of the AS_PATH in the initial set of routes to be
aggregated.

In your case, the two prefixes share exactly the same AS_SEQUENCE
tuples and therefore the summarized prefix inherits the same AS_PATH
constructred from *ordered* AS_SEQUENCES.

I would suggest spending some time reading RFC 1771 and the book "BGP
Design and Implementation" to get better understanding of BGP
architecture. It could be really complicated sometimes!

-- 
Petr Lapukhov, petr_at_INE.com
CCIE #16379 (R&S/Security/SP/Voice)
Internetwork Expert, Inc.
http://www.INE.com
2010/4/17 mohmmad imran <imran_mohmmad_at_yahoo.com>:
> Hi Experts,
>
> I have three router in AS 100
>
> R1=Fa0/0-----------------Fa0/0=R2
>                |
>                |
>              Fa0/0
>                |
>                R3
>
>
> R1 is getting the route 112.0.0.0/8 to 119.0.0.0/8 from as-path 54 50 60 and advertsie it to R3
>
> R1 SH IP BGP Output:-
>
> *> 112.0.0.0        1.1.1.254               0             0 54 50 60 i
> *> 113.0.0.0        1.1.1.254               0             0 54 50 60 i
> *> 114.0.0.0        1.1.1.254               0             0 54 i
> *> 115.0.0.0        1.1.1.254               0             0 54 i
> *> 116.0.0.0        1.1.1.254               0             0 54 i
> *> 117.0.0.0        1.1.1.254               0             0 54 i
> *> 118.0.0.0        1.1.1.254               0             0 54 i
> *> 119.0.0.0        1.1.1.254               0             0 54 i
>
>
> R3 SH IP BGP Output:-
>
> *>i112.0.0.0        1.1.1.254               0    100      0 54 50 60 i
> *>i113.0.0.0        1.1.1.254               0    100      0 54 50 60 i
> *>i114.0.0.0        1.1.1.254               0    100      0 54 i
> *>i115.0.0.0        1.1.1.254               0    100      0 54 i
> *>i116.0.0.0        1.1.1.254               0    100      0 54 i
> *>i117.0.0.0        1.1.1.254               0    100      0 54 i
> *>i118.0.0.0        1.1.1.254               0    100      0 54 i
> *>i119.0.0.0        1.1.1.254               0    100      0 54 i
>
>
> If try to aggregate above subnets into one prefix on R1 with:-
>
> router bgp 100
> aggregate-address 112.0.0.0 248.0.0.0 summary-only as-set
>
> I can see that the router is setting up the AS-SET in BGP table, as per below output:-
>
> s>i112.0.0.0        1.1.1.254               0    100      0 54 50 60 i
> *> 112.0.0.0/5      0.0.0.0                      100  32768 {54,50,60} i
> s>i113.0.0.0        1.1.1.254               0    100      0 54 50 60 i
> s>i114.0.0.0        1.1.1.254               0    100      0 54 i
> s>i115.0.0.0        1.1.1.254               0    100      0 54 i
> s>i116.0.0.0        1.1.1.254               0    100      0 54 i
> s>i117.0.0.0        1.1.1.254               0    100      0 54 i
> s>i118.0.0.0        1.1.1.254               0    100      0 54 i
> s>i119.0.0.0        1.1.1.254               0    100      0 54 i
>
> but instead of aggregating all route,if I try to aggregate 112.0.0.0/8 and 113.0.0.0/8 into on 112.0.0.0/7 prefix, router generate the aggregate prefix in BGP table instead of showing the AS-SET it shows the AS-PATH-SEQ on aggregated prefix.
>
> router bgp 100
> aggregate-address 112.0.0.0 254.0.0.0 summary-only as-set
>
> s>i112.0.0.0        1.1.1.254               0    100      0 54 50 60 i
> *> 112.0.0.0/7      0.0.0.0                       100  32768 54 50 60 i
> s>i113.0.0.0        1.1.1.254               0    100      0 54 50 60 i
> *>i114.0.0.0        1.1.1.254               0    100      0 54 i
> *>i115.0.0.0        1.1.1.254               0    100      0 54 i
> *>i116.0.0.0        1.1.1.254               0    100      0 54 i
> *>i117.0.0.0        1.1.1.254               0    100      0 54 i
> *>i118.0.0.0        1.1.1.254               0    100      0 54 i
> *>i119.0.0.0        1.1.1.254               0    100      0 54 i
>
>
> I tried it on two differenet IOS version with same result,I am missing something here?
>
> -Imran
>
>
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