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OSPF Database vs. Routing Table

I was reading a couple of the threads and have an OSPF question. OSPF advertises the state of it's links to neighboring OSPF routers, correct? Someone asked if there was a way to filter route advertisements and I believe the answer was that you can stop a route from being put in the routing table but can't stop it from being in the OSPF database (presumably because all the OSPF routers are to have the same picture of the network). What's the difference between the routing table and the database? I understand that the routing table is the listing of available routes to destinations, no problem with that. I'm wondering if the router maintains both a database of known advertisements AND a routing table (just another database) or if the router really only maintains one database but marks certain routes as being unavailable if they are "filtered" out of the routing table.

2 REPLIES
Anonymous
N/A

Re: OSPF Database vs. Routing Table

It's my understanding that the OSPF maintains separate databases, namely, the routing table and the topology database. These are separate items, and exist in separate memory spaces. The route table tells the router where to forward packets. The topology database is used to calculate a new route when there is a link state change announced.

Kelly

Gold

Re: OSPF Database vs. Routing Table

Each routing protocol in a Cisco router maintains its own internal table, and installs routes from that into the routing table proper. OSPF is no exception--it has an internal ospf database (show ip ospf data), from which it installs routes in the routing table. Filtering options are very protocol dependant--in ospf's case, you can filter between the protocol database and the routing table using a distribute list in. You _can_ filter type 3 LSAs only at the abr in newer ospf code, but other than this, you can't filter anything in the ospf database.

:-)

Russ

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