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Hi,

OSPF being a link state routing protocol prepares Link State Database. I would like to understand, if OSPF advertises and redistributes routes from its Link State Database table or from its Routing Table.

IMO, its Link State Database table, pls comment.

Thanx

Hi,

It is from the LSDB. Inside an area all the routers must have the same topological view, so LSAs are exchanged and all the routers have the same perspective of the network. 

HTH

Good luck!

Within the ospf flooding domain or an area ospf process exchanges LSA's (LSA type1, LSA type 2) and based on that all the ospf running routers maintain the same copy of the LSDB. Now it is upto the individual router which runs independent spf calculation for selecting the best route to reach a perticular destination.

But when ospf process needs to send an LSA (type 3,4) outside its own area it behaves like a distance vector becuase the other area routers when receives a summary LSA (type3,4) they believe on the ABR which sends those LSA into the area so these routers just need to run spf to determine cost to reach to the ABR that means these routers adds up the cost (what ABR is advertising plus cost to reach to the ABR) to reach to the routes in the other area. So technically its wrong to say aospf is advetising routes because ospf just exchanges LSAs i.e within area type 1 and 2 while from other area type 3 and 4. 

Now if you are considering redistribution of ospf process into some other IGP(RIP,EIGRP etc or other OSPF process), the redistribution will always performs from the ospf routing table i.e "show ip route ospf". In this by default "show ip route ospf" routes and "show ip ospf interface br" routes will be redistributed from ospf process to other IGP and these ospf routes will end up in the database of other IGP on the device on which redistribution is performed. The other IGP domain will then select the best path based on its best path selection procedures.

Hi.

I tend to agree that routing protocol redistribution happens from the routing table and not the RIB.  When the redistribute ospf command is exectued, it instructs the router to redistribute all OSPF learned routes in the routing table.  In addition, the router will grab all connected interfaces that have an associated network statement.

eborgard:

Hi.

I tend to agree that routing protocol redistribution happens from the routing table and not the RIB.  When the redistribute ospf command is exectued, it instructs the router to redistribute all OSPF learned routes in the routing table.  In addition, the router will grab all connected interfaces that have an associated network statement.

 

Agreed! That's my understanding as well.

eborgard:

I tend to agree that routing protocol redistribution happens from the routing table and not the RIB.  When the redistribute ospf command is exectued, it instructs the router to redistribute all OSPF learned routes in the routing table.  In addition, the router will grab all connected interfaces that have an associated network statement.

Thats true however just to add a point here, network statement under ospf process is not a good indication of router interfaces running ospf because there can be cases where ospf is running on interfaces by "ip ospf <process-id> area <area-id>" command. So the interfaces "show ip ospf int br" running ospf are redistributed into other protocol along with "show ip route ospf" routes. 

Suppose now the redistribution is from BGP in to OSPF, where the BGP redistributed prefixes will go first:

LSDB table

or

Routing table

Hi.

I think it's still from the routing table.  By default, when redistributing BGP into any other protocol, only the exteranl routes will be redistributed.  Now when we consider the AD of eBGP vs iBGP, under normal circumstances, only the eBGP routes will be installed in the routing table because all IGP administrave distance is beter than iBGP of 200. Even external EIGRP is beter than iBGP.  So when redistributing BGP into OSPF (which isn't a good idea in real life unless you are doing some serious filtering) the OSPF process should only see the eBGP routes in the routing table based on the default administrative distance values.  Of course, you can force BGP to release the internal routes with the bgp redistribute-internal command, but it should be used with caution.  I've only seen this command used with MPLS VPNs where BGP was used as the PE - CE protocol and the AS numbers were the same.

Cheers.

ashse:

Suppose now the redistribution is from BGP in to OSPF, where the BGP redistributed prefixes will go first:

LSDB table

or

Routing table

BGP routes will land up in the ospf database on the router which is performing the redistribution, and other ospf running routers in ospf domain will have in their database but they can run spf to install these routes in the routing table.

dcancerian:

Thats true however just to add a point here, network statement under ospf process is not a good indication of router interfaces running ospf because there can be cases where ospf is running on interfaces by "ip ospf <process-id> area <area-id>" command. So the interfaces "show ip ospf int br" running ospf are redistributed into other protocol along with "show ip route ospf" routes. 

The command "ip ospf <process-id> area <area-id>" is not supported on switches only on routers :-).  Although irelevant for this discussion.

Guys, 

Just noticed another OSPF behaviour:

While I am redistributing BGP in to OSPF its OSPF neighbor is seeing the route as O IA.

I am doing this test on GNS, don't know if its GNS issue or ... (till now my GNS never misbehaved)

R1(config-router)#do  sro

router ospf 101 vrf VPN_A

 router-id 1.1.1.1

 log-adjacency-changes

 redistribute bgp 3 metric 5 metric-type 1 subnets

R2(config-router)#do sh ip route ospf

     171.2.0.0/32 is subnetted, 1 subnets

O IA    171.2.2.2 [110/15] via 10.10.10.1, 00:29:43, Ethernet0/2

It depends if you are running OSPF in same domain or not. Is the PE and CPE running OSPF with the same process-id?

No sham-links?

Hi ashse,

It looks like you are running same OSPF process-id on both sites, OSPF domain-id formation is based on OSPF process-id, if you have same OSPF process-id creates same domain and route-type will be O IA, if you run OSPF into different process-id creates different domain-id and routes will be external route.

Verify OSPF domain-id using this command:

show ip bgp vpnv4 rd x:x x.x.x.x

and if you want to change OSPF domain-id  manually:

R4(config-router)#router ospf 23 vrf xxxxx
R4(config-router)#domain-id type 0005 value 000000170100

Good Luck.

ashse:

Suppose now the redistribution is from BGP in to OSPF, where the BGP redistributed prefixes will go first:

LSDB table

or

Routing table

It first goes to OSPF database(OSPF always builds the Link State Database before installing the route into routing table), you can see those prefixes as  external prefixes. Now if you are running EIGRP and OSPF on same router and both routing protocols getting same prefixes, EIGRP routes install into routing table because of lower AD but still routes exists into OSPF database.