I would like to have more over view to the following:-
1- what is the exact functionality of (DR/BDR) on ospf tobology.
2- what is the point of using (auto-summary) command on eigrp, and when we should use it?
3- also I am alittle bit confused about using Ospf ABR type 3 LSA filtering with different areas, when should be applied?Is any difference between configuring normal OSPF comparing To ABR, what should I consider when configuring Type 3 ABR?
1. One of the major reasons for the DR/BDR concept is to reduce unnecessary flooding. If there wasn't such a concept, each router on a LAN would have to have an adjacency with every other router on a LAN. For example, on a LAN with 10 routers, that would mean 45 adjacencies !! Using a DR/BDR, that same LAN now has only 17 adjacencies. Now, OSPF floods LSAs over adjacencies so the more of those you have, the greater the flooding traffic. And most of this would have been unnecessary since the routers would have already seen the LSAs from some other neighbor.
2. This feature was more of a using in the days of classful routing, where you would not have wanted more specific routes of a classful network from leaking out of that network. These days, it is disabled by default and it should be kept that way !
3. One application of this is when using stub (not totally) areas with multiple ABRs. In such a case, you could filter out some type-3 LSAs from being generated by one of the ABRs. The other ABR would still advertise them and so all traffic for these networks would exit out of this ABR. The ABR on which you did the filtering would still advertise the type-3 default so if the other ABR dies, all traffic will now exit out of this ABR.
As per the functionality about DB/BDR if u have configured OSPF in a BMA network all the DROther routers are going to make adjacency with DR and BDR they will not from adjacency with all the routers on a network this is going to prevent flooding of LSA's.
And as per auto-summary in eigrp it is used to summarize a group of networks and advertise them as a one netowrk as u know if we dnot have auto-summary then we have to do it manually.
An ABR means the router which id having its interfaces in more than one area.
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