BGP AS-SET with route aggregation

Unanswered Question
Feb 26th, 2009

hi guys

i am trying to configure as-et with bgp aggregation but sounds somthing missing

in the attached topology

R0 AS 1

R1 AS 10

R2 as 20

all Ebgp..

i have advertised route from R1 then aggregate it

normally apear in all peers

als i did advertised a route in R2 THEN i aggregated this route in R1

this way R2 recive this aggregate as well

with as set i put it as 20 in the aggregate this way i blocked the summary from being seen by AS 20 only nice

the problem is

if i am tring to block the summary route advertised and aggregated in R1 useing as-et i get the whole summary route droped from R1 routing table and from others as well !! why?

for example

in R1 i have the summary

now i blocked this summry from being sent to R2 but R0 now leaking the summry to R2

what i am trying to do is to add as 20 to the summary route this way even if R0 will leak it R2 will drop becuase it will see its AS in the as path of the summry route logical good

i did the following and the route get droped from the source R1 where i did the config and the summary and advertisment !!

i as-path access-list 1 permit ^20$

route-map map1

match as-path 1

Router bgp 10

aggregate-address summary-only as-set advertise-map map1

then after this the route /16 disapear ???

any idea

note: i just need it like this is for study perpuses not a practical case

Thank you

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Overall Rating: 3.5 (2 ratings)
pkurdziel Thu, 02/26/2009 - 20:34

I was working on one of Narbik's advanced workbook tonight and it's similar to what you are asking. See the attachment

FYI, the question is truncated....

1. r2 should aggregate all the networks in address space...........

2. Aggregation should be configured such that R1 in AS 100 is the only AS that receives the aggregate route, R3 and future peer neighbors should NOT receive the aggregate route.

Marwan ALshawi Thu, 02/26/2009 - 23:30

Hi thanks for this information

but i did somthing like this and worked with me

if i want to apply my case to the yours

the oute is in R2 in your case and aggregated into bgp

and there is requirement to send only specific route to R1 and no aggregate but becuase R1 has link to R3 r3 sending this aggregate

with AS-set i want to pretend that AS of R1 is in the path by adding it through the advertise map

note: this description based on your case not my topology

thanks again

Harold Ritter Fri, 02/27/2009 - 20:20


Can you try removing the aggregate-address command and reapplying it.


Marwan ALshawi Sat, 02/28/2009 - 03:32

actually i did alot of times

no luck

i just wanna know why when i do it like this the aggregate route disapear from R1 the one which dose the aggregation and then of course will disapear from others?

i think smothing worng but i cant find !!

thank you

Harold Ritter Sat, 02/28/2009 - 04:54


This should work. I just tried a quick test as follow.

router bgp 2

no synchronization

bgp log-neighbor-changes

aggregate-address as-set summary-only advertise-map map1

neighbor remote-as 1

no auto-summary


ip as-path access-list 1 permit ^1$


route-map map1 permit 10

match as-path 1


Here are the results:

R2#sh ip bgp

BGP table version is 4, local router ID is

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 32768 1 i

s> 0 0 1 i


As you can see, the summary does get generated.

Can you post your full config for the R1.


Harold Ritter Sat, 02/28/2009 - 17:15


Thanks for the additional information. I was a bit confused as I thought that the more specific routes to create were advertised by AS 20.

The reason why it doesn't work is that your route-map map1 matches on AS 20 but this ASN doesn't exist in any of the more specific routes as they are originated in AS 10.

The purpose of the advertise-map is to select from the more specific routes, which ones will be selected to create the aggregate. In your case, none of the more specific routes qualify, hence the aggregate not being created.


Marwan ALshawi Sat, 02/28/2009 - 17:40


i think i realized that but i wanted a confirmation

thanks for that,

i asked becuase i was reading a ccie lab case

and in the bgp

there was the following whihc i did not agree but they did it like this ( it is internetworkexpert)

there is a BB router in AS 54 ( 2 of them )

and lets say R1 in AS200 and R2 in AS 100

forst they asked to let AS 54 see R1 as in AS 100

this done

now R1 is summrizing route not from R2 but it should be denied from going to R2

becuase R1 and R2 connected to BB AS 54 now AS 54 is leaking the summary to R2

the solusion for this is to make as i asked add R2 AS number to the summary route that R1 make it which is AS 100

but the contradiction here first we made R1 AS 200 to apear to AS54 as AS 100 (above) thats mean we will not need this step becuase AS 54 will see it as from AS 100

AM i right in this ?

also if want to make it with advertise map will not work becuase it is as the answer you have given to me !!

thanks again

Harold Ritter Tue, 03/03/2009 - 08:53


I can't really comment on the requirements directly as I haven't seen them but from what you are telling me the scenario seems a bit confused.


Marwan ALshawi Tue, 03/03/2009 - 15:06


it is confusing you are CCIE and you know CCIE asks only complex things :)

anyway you confirmed to me what i have seen i mean the more specific prefixes need to be on the router that doing the aggregation from at least one AS not the local to use as set with a route map

in other words if i have the routes advertised by R1 and aggregated by R1 i cant add other any other AS in the as-set

in the above what is meant exactly

lets say you have tow routers R1 and R2

R1 AS 10 and R2 as 20

both of them connected to a router lets say R4 and R5

R4 and R5 are in one AS lets say AS 50

now in the case R1 havs route and has been aggregated this is fine

in the begining one of the requrement asked R1 should apear to AS 50 as it is in AS 20 !

done with local-as command

now AS 50 see any routes from R1 as its coming from AS 20 not the original one which is 10

other requremnt asked to filter out the aggrgate from being sent from R1 AS 10 to R2 AS 20 filter fine

but the issue here as they asked later that AS 50 is leaking the aggregate as both connected to it they solved it with as-set by adding as 20 to the aggrgate

there to problems first one is as you mentioned above the route need to be coming fron AS 20 at least one spicific route to make the as-set


why we doing this as in the begning we made R1 apear as in AS 20 this way AS 50 will send the aggregate to R2 as from AS 20 and thus will be droped because R2 in AS 20

sorry about that but i would like to share it with you :)

if you agree with me let me



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