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Problems when running IGP IBGP encounter:BGP routing updates only the best entries

BGP recently doing exercises,Do the following topology when encountered some problems,This topology is

Screenshot 2013-08-09 10.07.17 AM

I configured the network segment:

among them

In the AS1 wherein R1R2R3,R4 in the AS4,R5 in the AS5

Declared 5.5.5.5 and 7.7.7.7 Network segment on R5,1.1.1.1 declared on the network segment where the R1。

Run OSPF on R1R2R3,Let AS1 full-mesh but temporarily declared 24 and 35 the two network segments,This time should be able to see on the table for R1 bgp 5.5.5.5 and 7.7.7.7 each two paths in the routing table and do not

Because there is no next hop,This view corresponds to the routing table R1 shows

Obviously,This is because just run IGP (here is running OSPF) is not declared 24 and 35 the two network segments。Now proclaims,Sure enough, the corresponding routing entry has been written,It seems to work correctly:

Look at bgp table:

How each only a rest?

Problems arise here:Redundant where to go。AD is higher IGP covered it?

Obviously without this command can also see not the case。

If this time to turn off the IGP,Then do a static route on R1,Here it is normal;If you run IGP,I want to have next-hop routing will declare 24 and 35 segments,Once declared,Redundant path disappears。

Actually,This is due to a characteristic caused by BGP:

In BGP,Sending routing updates are sent not all entries,It only sends its best path to its neighbors。

then,AS1 in order to form the full-mesh we ran BGP but also learned R2 35,In this time that the R2 R5 IBGP in line with two!

So that we can understand,R2 his own optimal path sent to R1,Results and R3 is the same as the transmission path of the topology map are both lower (the optimal path)

So,At this momentRoute R1 and R3 each have only one of the segments 5 and 7。

Original article written by Gerber drop-off:R0uter's Blog » Problems when running IGP IBGP encounter:BGP routing updates only the best entries

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