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OSPF LSAs flooding

Hello Experts,

I am confused about the LSA flooding mechanism of OSPF. I have learned that when there is a network change, OSPF routers send LSAs to DR/BDR routers and then DR/DBR distribute LSAs to any additional OSPF router  around as convenient. Today I read that LSAs are actually flooded out all router interfaces except the one where the LSA was received from ( Cisco OSPF command and configuration handbook - page 373 under ip ospf database-filter all out). I still find this difficult to understand and seems to me inefficient. Could you help me to clarify this?

Best Regards,

Robert.

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Hall of Fame Cisco Employee

Re: OSPF LSAs flooding

Hi Rob,

Both statements about the LSA flooding are true. When a router needs to originate a new LSA, it sends it out all its interfaces. However, who is going to receive it depends on the type of interface. If the egress interface is a point-to-point or point-to-multipoint OSPF network type then the LSA will be received directly by the neighboring router and it will propagate it further by flooding it through all remaining interfaces. If the egress interface is a broadcast or nonbroadcast multiaccess OSPF network type then the LSA is first sent to the DR/BDR (224.0.0.6) on the network segment, and the DR will then in turn send the LSA out all its interfaces including the one that has received the LSA (but addressed to a different IP address 224.0.0.5).

I am not sure what exactly do you find ineffective in this procedure. In any case, there are some requirements this procedure has to fulfill:

  • A LSA must be delivered to all routers, thus, it must be flooded through essentially all interfaces.
  • A LSA must not become entrapped in a loop, therefore a router ignores received LSA if it already is present in its link-state database
  • On a multiaccess segment, the DR/BDR must be the first routers that receive a LSA. The remaining routers on the segment are second.

Feel welcome to ask further!

Best regards,

Peter

2 REPLIES 2
Hall of Fame Cisco Employee

Re: OSPF LSAs flooding

Hi Rob,

Both statements about the LSA flooding are true. When a router needs to originate a new LSA, it sends it out all its interfaces. However, who is going to receive it depends on the type of interface. If the egress interface is a point-to-point or point-to-multipoint OSPF network type then the LSA will be received directly by the neighboring router and it will propagate it further by flooding it through all remaining interfaces. If the egress interface is a broadcast or nonbroadcast multiaccess OSPF network type then the LSA is first sent to the DR/BDR (224.0.0.6) on the network segment, and the DR will then in turn send the LSA out all its interfaces including the one that has received the LSA (but addressed to a different IP address 224.0.0.5).

I am not sure what exactly do you find ineffective in this procedure. In any case, there are some requirements this procedure has to fulfill:

  • A LSA must be delivered to all routers, thus, it must be flooded through essentially all interfaces.
  • A LSA must not become entrapped in a loop, therefore a router ignores received LSA if it already is present in its link-state database
  • On a multiaccess segment, the DR/BDR must be the first routers that receive a LSA. The remaining routers on the segment are second.

Feel welcome to ask further!

Best regards,

Peter

Re: OSPF LSAs flooding

Hi Peter,

Thanks for you expedite answer! Now I see it clearly, the router receiving or generating the LSA will flood it out all its OSPF interfaces towards the DR / BDR of every network segment it is attached to (when DR/BDR election applies).

Thanks again, best regards,

Robert.

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