From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Tue Jul 19 2005 - 03:17:25 GMT-3
Godswill,
Yes, the problem synchronization is designed to prevent is the
dropping of traffic by non-BGP speaking routers in the transit path. In
order to prevent this the logic of synchronization is that if the route
is installed in the IGP table all routers in the transit path (which
should logically be running IGP) will have a route to the final
destination. The solution is as you said, to suppress bestpath
selection (and hence advertisement) if an IGP match is not found for the
iBGP learned prefix.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Godswill Oletu
> Sent: Monday, July 18, 2005 2:13 PM
> To: ccie2004@excite.com; ccielab@groupstudy.com
> Subject: Re: BGP synchronization
>
> I am trying to follow your logic, but I am finding it difficult to
> reconcile
> this line...
>
> ".....Synch is supposed to address the issue of a router in the middle
of
> your network which is not running IBGP and hence does not know how to
get
> to
> a particular network that your IBGP routers are aware of....."
>
> Is that the issue that Synchronization really addressed?
>
> I think with BGP Synchronization, when an IBGP peer gets an update for
a
> route from another IBGP peer, it looks into its local IGP routing
table,
> if
> that route is present, he is free to advertise it to other BGP Peers,
if
> otherwise, he will refrain from advertising it. In this scenerio, all
> internal routers do not have to necessary run IBGP.
>
> To fulfill the requirements of BGP Synchronization, your IGP should be
> aware
> of all the routes in your network, so that when these routes become
> available to BGP, it can accept them and send them out to external BGP
> neighbors.
>
> eg
>
> interface fastethernet0/0
> ip address 120.1.1.1 255.255.255.0
> !
> interface fastethernet0/1
> ip address 10.1.1.1 255.255.255.0
> !
> ip route 50.0.0.0 255.0.0.0 120.1.1.1
> !
> router bgp 120
> network 120.1.1.0
> network 10.1.1.0
> network 50.0.0.0
> network 60.0.0.0
> network 70.0.0.0
> !
> end
>
> You see that, BGP will not advertise networks 60.0.0.0 & 70.0.0.0,
because
> according to the limited configuration above, there is no reference to
> them
> in the router's routing table, but if you disable syncronization, BGP
will
> accept these networks and advertise them, despite the fact that none
of
> your
> routers know how to reach those networks.
>
> You can see that, if your BGP router advertise networks 60.0.0.0 &
> 70.0.0.0,
> there will be problem if traffic is sent to you destined for those
> networks,
> I believe this is the very problem BGP synchronization is setup to
> prevent.
>
> Others can contribute, so that we can all learn, if this is not what
your
> question is about, you can throw in more light.
>
> my 0.2
>
> Note:
> There are many other methods of making routes available to your BGP
> router,
> the network statement used above is just one of them.
>
> ----
> Godswill Oletu
>
>
> ----- Original Message -----
> From: <ccie2004@excite.com>
> To: <ccielab@groupstudy.com>
> Sent: Monday, July 18, 2005 2:01 PM
> Subject: BGP synchronization
>
>
> > Hi All, I just came up with a question on BGP which I have been
asking
> > myself but just can't seem to get a handle on. It has to do with BGP
> > Synchronization. I know this has been beaten to death and newer IOS
> > versions have it disabled however my question is a combination of
the
> > underlying issue that Synch is supposed to address and Best
Practises.
> > Synch is supposed to address the issue of a router in the middle of
your
> > network which is not running IBGP and hence does not know how to get
to
> a
> > particular network that your IBGP routers are aware of. BGP Best
> practises
> > say that never redistribute your EBGP learnt routes into your
interior
> > routing protocol. Thinking along those lines and if I am right how
> exactly
> > would you get reachability across your network. Would you use
default
> > routes on your non-BGP speaking routers or are there any other
design
> type
> > fixes that I am missing. thx
> >
> > _______________________________________________
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> >
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