OSPF ROUTER TYPE

1 ABR
2 ASBR

ABR

Area Border Router (ABR) is a bridge between Backbone and Area off backbone. With correct IP addressing we can summarize routes information on this router.



ASBR


Autonomous System Border Router  (ASBR) is a bridge between OSPF Area with other Routing protocol like EIGRP , RIP, ETC.



 OSPF NETWORK TYPE


1 DR
2 BDR

WHAT IS DR ?

Let me start by showing you what a DR and BDR are.





We have to send some hello’s, become neighbors, send A DBD…check if our LSDB is synchronized and then send a bunch of LSRs and LSUs to get everything up-to-date.

Now look at the picture above. We have a network with 8 OSPF routers connected on a switch. Each of those routers is going to become OSPF neighbors with all of the other routers…sending hello packets, checking the LSDB, etc. etc.



This is what we would get. A full-mesh of OSPF neighbors. Every router will become OSPF neighbors with all the others and exchange routing information. We will have a lot of OSPF packets flying around on this network just to setup and maintain OSPF neighbor adjacencies. Is there any way to make this more efficient?

 

What if all our OSPF routers would just send their stuff to a single OSPF router who will then forward it to all the other OSPF routers? All our OSPF routers will know about all the routing information out there but we will have far less OSPF traffic. This is exactly what we do with a DR (Designated router). Our OSPF routers will only form a full neighbor adjacency with the DR and not with all other routers!

OSPF DR/BDR LAB


WHAT IS BDR ?

Since bad stuff can happen to our networks we want to have backup for our DR. If it crashes the BDR (Backup Designated Router) will take over. All our OSPF routers will only form full neighbor adjacencies with the DR and BDR and not with all other routers. This sounds efficient right?



NOTE : We only use a DR/BDR on a multi-access network. There is no need to do this on a pointto- point link like a PPP connection. There is only one other router on the other side so why select a DR/BDR there? Makes sense right?