You could really search the archives for this as its been answered a bunch
of times. Anyway, here's the short story.
A bunch of years ago, it was possible, due to smaller internet routing
tables sizes, to publish your external reachability to your interior gateway
routing protocol, otherwise known as redistribution your BGP into your IGP.
This was done in transit networks that advertised the ability to move
traffic closer to a non local destination. However, if the transit path
across the AS included routers that didn't participate in BGP, and thus
depending on the IGP to forward transit traffic, a situation could occur
where BGP perimeter routers would know about destinations that the IGP
hadn't yet learned. Hence, the AS might accept traffic for destinations
that the interior IGP only routers would have to drop, thus black holing
that traffic. To solve this, a knob was introduced that forced BGP border
routers to verify that the IGP had reachability to a destination before
transit services were advertised for that destination. In essence this
required two complete views of the internet table to be maintained on
perimeter routers and your IGP to handle huge volumes of external
information. As you can imagine, this didn't work well for long and
eventually this practise was abandoned in favour of full mesh IBGP
techniques. At this point, syncronisation is a useless feature that isn't
even supported in many newer implementations.
Pete
*********** REPLY SEPARATOR ***********
On 8/23/2001 at 8:38 AM suleman ibrahim aboo wrote:
>Hi,
>
>Can someone please explain to me about BGP Synchronisation. Why does the
>rule
>apply that states "do not use or advertise a route learned by IBGP to an
>external neighbour until its in the IGP table ". Why would this cause a
>problem. I am not sure I understand the implications.
>
>
>regards
>suleman
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=16966&t=16961
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