From: Alex De Gruiter \(AU\) (Alex.deGruiter@didata.com.au)
Date: Fri Sep 01 2006 - 01:09:15 ART
Hi Angelo,
The key if you want R6 to advertise the path to R4 is to ensure that the
path from R6 to the prefix via R9 is preferred over the path to R4. You
can do this on R6 without affecting the as-path prepend on AS9 (that is,
you can still perform your AS-path prepending and this scenario will be
successful).
As long as R6 believes that the best route to the associated prefixes is
via R9, then R4 will learn 2 x routes to the network. You can do this
with weights; i.e. applying the command "neighbour 172.16.9.9 weight
6000" has the following effect on the network:
R6#sh ip bgp
BGP table version is 5, local router ID is 172.16.5.5
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
* 10.34.19.32/28 192.168.4.4 0 24 1 i
*> 192.168.9.9 0 6000 9 9 9 9 i
You will find that R4's BGP table will look as follows:
R3#sh ip bgp
BGP table version is 3, local router ID is 172.16.3.3
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
* 10.34.19.32/28 192.168.6.6 0 256 9 9 9 9
i
*> 192.168.1.1 0 0 1 i
Thus you have achieved your goal, without compromising your policy on
R9.
You might also want to consider the "bgp bestpath as-path ignore"
command. This will prevent the AS-path from being considered by the BGP
process (this is a hidden command). Installing this on R6 will allow
that router to ignore the AS-path as part of the selection process; this
has the potential of influencing the prefix from R9 as a candidate best
route (thereby allowing advertisement to AS4).
The only problem with this is that the BGP router-id may play a part as
the tie-breaker, meaning that your route from R4 will be preferred over
R9 - therefore, you may still need to apply a route map. This would need
to affect any of the best path selection criteria before router-id
(LOCAL_PREF, weight, MED, origin type, etc.)
Regards,
Alex
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Angelo De Guzman
Sent: Friday, 1 September 2006 11:10 AM
To: Ivan; ccielab@groupstudy.com; Gianpietro Lavado
Subject: Re(2): BGP AS PATH
Thanks!
So without changing the as path that comes from R9. Maybe I should
change my
advertisement out of R9. This way even if R6 prefers these routes
through AS1
it will still advertise these routes to R4 because I did not change any
local
attribute within AS 567. Am I correct?
( I am not able to try this but I guess this will be correct)
Gianpietro Lavado (9/1/06 12:41 AM):
>
>The explanation is: when a router has two BGP possible routes, and
picks one
>of them, it only advertises the preferred one to its neighbors; so in
this
>case, when you activate the prepend, only R6 is holding the two
possible
>routes, and since it prefers the one that comes from AS 1, it will not
>advertise the other one (from AS 9) to AS 24 or to R5.
>When R6 looses the route coming from AS 1 (turning off the R1 peering),
its
>new preferred route is through AS 9, so it starts advertising this
route to
>AS 24 and R5.
>
>HTH
>
>Gianpietro
>
>
>On 8/31/06, Ivan <ivan@iip.net> wrote:
>>
>> AS1 and AS9 announce 10.34.19.32/28 and 10.34.19.0/28 after peering
with
>> R1
>> shut off AS24 see path to this prefixes through AS567 and 2-a
prepends.
>>
>> *> 10.34.19.0/28 192.168.6.6 0 567 9 9
9 i
>> *> 10.34.19.32/28 192.168.6.6 0 567 9 9
9 i
>>
>>
>> > Hi,
>> > R1 AS 1 peers with R2 AS 24
>> > R2 peers with R4 AS 24
>> > R5 AS 256 and R6 AS 256
>> >
>> > R5 peers with R2 in AS 24, and R6 peers with R4 in as 24
>> >
>> > R6 peers to R9 in AS 9
>> >
>> > Both R9 and R1 are advertising the same route.
>> >
>> > When I do as path prepend for the routes coming into R6 (from R9)
>> these
>> > routes dissappears from AS 24.
>> >
>> > Is this normal? If I removed the peering to R1 then the bgp
routes
>> comes
>> > back. Any explanation why R4 is not installaing the BGP routes at
the
>> same
>> > time at R4? I guess it has something to do with the AS Path but I
need
>> more
>> > clarification on this matter.
>> >
>> > ******NO AS Path prepend at R6 yet*******
>> > R4#sh ip bgp
>> > BGP table version is 5, local router ID is 192.168.4.4
>> > 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
>> > * 10.34.19.0/28 192.168.6.6 0 567 9
i
>> > *>i 192.168.1.1 0 100 0 1 i
>> > *> 10.34.19.0/26 192.168.6.6 0 567 9
i
>> > * i 192.168.5.5 0 100 0 567 9
i
>> > * 10.34.19.32/28 192.168.6.6 0 567 9
i
>> > *>i 192.168.1.1 0 100 0 1 i
>> > *> 172.16.80.0/27 192.168.6.6 0 567 7
8 ?
>> > * i 192.168.5.5 0 100 0 567 7
8 ?
>> > R4#
>> > *Aug 31 15:39:29.746: %BGP-5-ADJCHANGE: neighbor 192.168.6.6 Down
Peer
>> > closed th e session
>> > *Aug 31 15:39:29.978: %BGP-5-ADJCHANGE: neighbor 192.168.6.6 Up
>> >
>> > **** With AS path prepend at R6*****
>> > !
>> > access-list 1 permit 10.34.19.0 0.0.0.15
>> > access-list 1 permit 10.34.19.16 0.0.0.15
>> > access-list 1 permit 10.34.19.32 0.0.0.15
>> > !
>> > route-map ADDAS permit 10
>> > match ip address 1
>> > set as-path prepend 9 9
>> > !
>> > ******************************************
>> > R4#sh ip bgp
>> > BGP table version is 8, local router ID is 192.168.4.4
>> > 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
>> > *>i10.34.19.0/28 192.168.1.1 0 100 0 1 i
>> > *>i10.34.19.32/28 192.168.1.1 0 100 0 1 i
>> > *>i172.16.80.0/27 192.168.5.5 0 100 0 567 7
8 ?
>> >
>> > ****Turning off the R1 peering******
>> > R4#sh ip bgp
>> > BGP table version is 14, local router ID is 192.168.4.4
>> > 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
>> > *> 10.34.19.0/28 192.168.6.6 0 567 9
9 9 i
>> > *> 10.34.19.0/26 192.168.6.6 0 567 9
9 9 i
>> > *> 10.34.19.32/28 192.168.6.6 0 567 9
9 9 i
>> > *> 172.16.80.0/27 192.168.6.6 0 567 7
8 ?
>> > * i 192.168.5.5 0 100 0 567 7
8 ?
>> >
>> > TIA,
>> > Angelo
>> >
>> >
>> >
>> > ***********************
>> > No virus was detected in the attachment no filename
>> >
>> > Your mail has been scanned by InterScan MSS.
>> > ***********-***********
>> >
>> >
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