about DR election and flooding LSA packets in OSPF?

Hey guyz i forgot something very important about single-area OSPF. After establishing adjacency with directly connected routers by sending and receiving hello packets with required same values such as area ID, LSA packets are exchanged between adjacent routers. However,then, additionally DR is elected in order to flood LSAs between all others once more.

So if we exchange LSA packets including route and link-state information between all other directly connected routers, why do we need to flood LSA packets by DR "second time"?

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Comments

Msafiri

A router will form adjacencies with its neighbors and with the DR and BDR, when a link state changes it will send LSAs to neighbors and DR, the DR will then send the LSA to all routers, this ensures that all routers receive the LSA even those that were not neighbors of the router the LSA originally came from.

It also reduced the amount of bandwidth consumed by routing updates.

thedrama

Msafiri wrote: »

A router will form adjacencies with its neighbors and with the DR and BDR, when a link state changes it will send LSAs to neighbors and DR, the DR will then send the LSA to all routers, this ensures that all routers receive the LSA even those that were not neighbors of the router the LSA originally came from.

It also reduced the amount of bandwidth consumed by routing updates.

Would you explain with more details?

networker050184

The DR/BDR is only used on multiaccess segments where there is a possibility for forming a neighbor with more than one router.

The DR election is completed before the routers form the neighborship, and before any LSAs are exchanged.

All routers on the multiaccess segment then only form full adjacencies with the DR and BDR. All LSA information is exchanged with these fully adjacent neighbors. The DR also creates the type 2 LSA which is flooded to represent all routers on the segment.

It basically just cuts down on all the traffic needed to ensure everyone has all the complete information.

Msafiri

thedrama wrote: »

Would you explain with more details?

The idea behind this is that routers have a central point of contact for information exchange. Instead of each router exchanging updates with every other router on the segment, every router exchanges information with the DR and BDR. The DR and BDR relay the information to everybody else.

In mathematical terms, this cuts the information exchange from O(n*n) to O(n) where n is the number of routers on a multi-access segment. The following router model illustrates the DR and BDR:

OSPF Design Guide - Cisco Systems

thedrama

networker050184 wrote: »

The DR/BDR is only used on multiaccess segments where there is a possibility for forming a neighbor with more than one router.

The DR election is completed before the routers form the neighborship, and before any LSAs are exchanged.

All routers on the multiaccess segment then only form full adjacencies with the DR and BDR. All LSA information is exchanged with these fully adjacent neighbors. The DR also creates the type 2 LSA which is flooded to represent all routers on the segment.

It basically just cuts down on all the traffic needed to ensure everyone has all the complete information.

wait! As much as i can realise from both of your replies,

Normally, LSAs are exchanged between all directly connected routers then topology table is built for each router. Eventually, when convergence is done, best paths are chosen in order to reach remote networks.
Furthermore, LSAs are also flooded by DR to all other routers only when topology changes occur. If topology changes happen on another router, it is told to DR first and DR does the flood. Right?