BGP: Why do suppressed routes make it to the RIB?

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Jun 30th, 2010
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Hey all,

  I have a client with a large WAN deployment, currently there is approx 5000 routes recieved for each remote site which is redistributed into EIGRP.


I would like to summarise to reduce the impact on EIGRP when routes are added or withdrawn.  It all seems fine with the exception that the suppressed routers are making it into the RIB.


Relevant config is below:

router eigrp 2
redistribute bgp 2 metric 10000 10 255 1 1500


router bgp 2

aggregate-address 10.70.0.0 255.255.0.0 summary-only


CE1#show ip bgp

   Network          Next Hop            Metric LocPrf Weight Path
*> 10.70.0.0/16     0.0.0.0                            32768 i
s> 10.70.1.0/24     192.168.254.1                          0 1 2 7474 64711 i
s> 10.70.2.0/24     192.168.254.1                          0 1 3 i


So the more specifc routes are suppressed, but they are still in my RIB


CE1#show ip route
B       10.70.0.0/16 [200/0] via 0.0.0.0, 00:18:46, Null0
B       10.70.1.0/24 [20/0] via 192.168.254.1, 00:18:46
B       10.70.2.0/24 [20/0] via 192.168.254.1, 00:18:46


so they get into my EIGRP topology:


CE1#show ip eigrp topology
P 10.70.0.0/16, 1 successors, FD is 258560
        via Redistributed (258560/0)
P 10.70.1.0/24, 1 successors, FD is 258560, tag is 1
        via Redistributed (258560/0)
P 10.70.2.0/24, 1 successors, FD is 258560, tag is 1
        via Redistributed (258560/0)


Hopefilly someone can explain this.


Cheers


Adam

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Mohamed Sobair Wed, 06/30/2010 - 14:10
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Clark,


The aggregation in BGP will not remove the component subnet from the routing table (RIP), whenever aggrgate is configured the More Specific routes will be suppressed from being send out to the neighbors and Only the summary (Aggragted Address) will be sent. In fact , In order for BGP to send the Summary/Aggrgated address, those Components routes should be in the routing table or at least One.



HTH

Mohamed

francisco_1 Wed, 06/30/2010 - 15:41
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when aggregating with bgp there must be a subnet in thr bgp table that is encompassed by the summarised prefix. for example when you issue the aggregate-address 192.168.0.0 255.255.0.0 then at least one subnet like 192.168.1.0/24 must be in the BGP table (loc-RIB) and not the routers routing table (RIB).

In your case you are advertising 10.70.1.0 & 10.70.2.0 using the network statement including the aggregate-address 10.70.0.0 255.255.0.0 summary-only under bgp so hence why both prefixes are appearing as suppressed and as long as they are in the BGP loc-RIB, they will get redistributed in to EIGRP.


iF you do not want it to appear in EIGRP topology table, you could use a route-map & prefix list and match the aggregation route only and use the route map under eigrp when you redistribute BGP -> EIGRP.


Francisco

Mohamed Sobair Thu, 07/01/2010 - 09:39
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Yes, As I said, one of the component subnet must be exist in the routing table (RIP).



The concept should be clear , Apparantly, if the BGP router has subnet X in the BGP table as best, then this route will eventually be installed in the routing table and at this time the aggregate address will be sent to the neighbor.


However, if this route is not used by BGP itself (Marked as valid or Rip-failure), BGP will not install this path for Network x in the routing table and therfore, you cant aggregate this Network.



So the conclusion, the path must be used by BGP and should be in the RIP (routing table).





HTH

Mohamed

clark.adam.p Thu, 07/01/2010 - 19:06
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Hi all,

  Thanks for all for your responses.


After I posted it, I actually had a "duh" moment.  It is kinda obvious.


I will have to do this with eigrp summaries


Cheers


Adam

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