From: Noor.Yousuf@shell.com
Date: Wed Apr 23 2008 - 03:48:08 ART
Nikhil,
I did some research and managed to find the right answer to this.
R2 is advertising this prefix 200.200.200.0 to R1 and R4 through network
statement. If you look at the BGP table of R1 you will see 200.200.200.0 from
both R2 and R3 but R2 will win the route because it has nothing in the AS-Path
( local to AS - shortest AS-Path). So, now because R1 has the best path
through R2 it will not advertise the same prefix coming from R3(not the best
route) to R2 -- Remember only best routes are advertise to peer.
You have talked about LP, remember both IBGP and EBGP routes keeps the default
LP of 100 at all time until you change it. the bgp table shows only the LP of
ibgp route but when you do show ip bgp 200.200.200.0, you will see that both
are carrying LP of 100. The decesion was made on shortest AS-Path.
I hope this will clear any confusion
Thanks again for the help
Noor
-----Original Message-----
From: Nikhil Engineer [mailto:ccienovice@gmail.com]
Sent: dinsdag 22 april 2008 19:04
To: Yousuf, Noor S SITI-ITIBTAE
Cc: Amir.Tahir@wateen.com; ccielab@groupstudy.com; itsfortarget@gmail.com;
smorris@ipexpert.com; sadiqtanko@gmail.com; Patrick Galligan
Subject: Re: BGP issue
Hi Noor,
I simulated your scenario here is the conclusion i came to:-
R2 is originating the route so this route is preferred and advertised to R1.
Now on R1 you will find two routes one is from R2 and other from R3. R2 will
be preferred cos of highest LP. As this is considered as the best route it
wont be advertised to R2 due to "ibgp-split horizon rule"
This is the only reason you will see that R2 has single route in bgp table.
HTH.
Cheers,
Nikhil E.
On Tue, Apr 22, 2008 at 3:34 PM, Patrick Galligan < pgalligan@gmail.com>
wrote:
I've just done this on a couple of switches:
3750 - iBGP - 3560
On both:
router bgp 200
net 10.0.0.0 mask 255.0.0.0
neigh blah
ip route 10.0.0.0 255.0.0.0 null0
I see both routes in the BGP table on both switches. I see the same
behaviour at customer sites where I'm originating their public block
from both routers to each ISP, with a static pointing to their
firewall/s, and run iBGP between the customer routers.
Noor, have run done any debugs?
eg. deb ip bgp update
and then
clear ip bgp <neighbour address>
Are you running on Dynamips or real routers? What IOS?
On Tue, Apr 22, 2008 at 5:47 PM, Nikhil Engineer < ccienovice@gmail.com>
wrote:
> hi Noor,
>
> As I said before this is due to split horizon.
>
>
> Cheers,
> Nikhil E
>
>
> On 4/22/08, Noor.Yousuf@shell.com < Noor.Yousuf@shell.com> wrote:
> >
>
>
> > Amir,
> >
> > I understand your point, and I know that it is locally originated again
> > and it is in the bgp table but I would only like to know the reason why
the
> > same route from R3 is not in the BGP table anymore?
> >
> > Thanks,
> >
> > -----Original Message-----
> > From: Amir.Tahir/Wateen/Lahore [mailto: Amir.Tahir@wateen.com]
> > Sent: dinsdag 22 april 2008 9:28
> > To: Yousuf, Noor S SITI-ITIBTAE; ccielab@groupstudy.com
> > Cc: itsfortarget@gmail.com; smorris@ipexpert.com; sadiqtanko@gmail.com
> > Subject: RE: BGP issue
> >
> >
> > Dear Noor,
> >
> > When you have advertised 200.200.200.0/24 on R3, why you are advertising
> > same network Prefix on R2.
> >
> > If you have eBGP relationship between R1 & R3, you should learn 200
> > prefix from R3 and should be visible on R2 as well. You don't have to
> > re-advertise it.
> >
> > If you have eBGP relation between R2 & R4, it should be propagated to R4
> > through the AS (Defined on R2 & R1)...
> > If Ip route command confuse you, try to run rip between R1 & R2 to have
> > the NLRI reachibility..
> >
> > As far as you query is concern... you have defined the 200 prefix
> > through Network command on R2 and now its locally advertised. You could
> > verify it through Weight (32768) & AS Path (i).
> >
> >
> > r2#SH IP BGP
> > BGP table version is 8, local router ID is 2.2.2.2
> > 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
> > *> 200.200.200.0 0.0.0.0 0 32768 i
> >
> >
> > If still you are not clear, Please feel free to call me at
> > 0322-400-2647(amir)
> >
> >
> > Regards
> > __________________
> > Amir Tahir
> >
> >
> > -----Original Message-----
> > From: Noor.Yousuf@shell.com [mailto: Noor.Yousuf@shell.com]
> > Sent: Tuesday, April 22, 2008 12:12 PM
> > To: Amir.Tahir/Wateen/Lahore; ccielab@groupstudy.com
> > Cc: itsfortarget@gmail.com; smorris@ipexpert.com; sadiqtanko@gmail.com
> > Subject: RE: BGP issue
> >
> > Let me get clear..
> >
> > R3----EBGP---R1----IBGP-----R2----EBGP----R4
> >
> >
> > from R3 I have advertised the network 200.200.200.0 in BGP
> > no sync
> > network 200.200.200.0 mask 255.255.255.0
> > ip route 200.200.200.0 255.255.255.0 Null0
> >
> > I can see the network on R2 bgp table
> > Now I have advertised the same network in R2
> > no sync
> > network 200.200.200.0 mask 255.255.255.0
> > ip route 200.200.200.0 255.255.255.0 Null0
> >
> > As soon as I have advertised the network in R2 BGP, I cannot see the
> > route coming from R1 in the BGP Table, I thought I should see both
> > routes in BGP table but only one in Routing table.
> >
> > r2#SH IP BGP
> > BGP table version is 8, local router ID is 2.2.2.2
> > 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
> > *> 200.200.200.0 0.0.0.0 0 32768 i
> >
> > Can someone explains why?
> >
> > Thanks,
> >
> >
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>
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