From: Gianpietro Lavado (gianpietro1@gmail.com)
Date: Thu Aug 31 2006 - 13:41:00 ART
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
> >
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> > ***********-***********
> >
> > _______________________________________________________________________
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>
> --
> Ivan
>
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