OSPF Filtering

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Nov 5th, 2009
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Hi Friends,


It is OSPF Design query on OSPF filtering using DISTRIBUTION-LIST.


I know that DISTRIBUTION-LIST prevents prefix to get into the routing table but donot prevent the LSA or modify the topology table.


Thus, for example if I use DISTRIBUTION-LIST to prevent a prefix 192.168.1.1 in routing table of a RouterA which is in Area 0 and but the same prefix 192.168.1.1 is present in the routing table of RouterB which is in Area1. If RouterB wants to reach 192.168.1.1 the packet is reaching RouterA which is suppose to be the path to reach 192.168.1.1, and blackholed as this prefix is not available in the RIB.


Thus, two thing i want to know here


1. Does the summary LSA generated by ABR checks its routing table before generating summary LSA? I think summary LSA generated by ABR is not depanding on the ABR routing table (Thus because, the blackhole is created in our case). Is this correct?


2. What is the advantage or real time usage of DISTRIBUTION-LIST as it creates inconsistent topology as in our case?


Thanks for your patience reading this bit lengthy explanation


Waiting for your valuable reply


sairam

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CriscoSystems Thu, 11/05/2009 - 20:33
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1. No. LSA's are propagated to neighbors as they are received. An LSA might be blocked because of area type (for instance, type 5 LSA's are blocked from stub areas). The routing table is built from information in the LSA's. The routing table is never consulted before LSA transmission.


2. In the topology you describe, yes it is inconsistent - but that is a weird topology!! Distribute lists can be used for security, traffic management (for instance, maybe the Graphics department is sending dirty emails to Engineering; you can apply a distribute list to the router in Graphics so that it won't have routes to Engineering but will still have routes to the rest of the company).


There are other uses; those are just some examples.


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