OSPF is a link-state database, which means for OSPF to operate properly(build its SPF graph for the topology) there should be no discrepancy between all LSA accoss the domain. If an ABR connected to 2 different Areas (not the backbone area) then this ABR will not be able to advertise its LSAs into area 0 which leads to LSAs not synchronized accross the domain. so here we need the VL in order to make the LSA synchronized. An ABR not connected to area 0 will generate the Summary LSA but it doesnt you them. About the condition of distance vector rule assume we have 2 routers R1 and R2, if R2 advertise a distance to R1 that is larger than the distance that R1 has to reach the destination, than there is a posibility that R2 is using R1 to get to the same destination so R1 say i will not install the route to reach this destination through you ( R2 the next hop) because there is a possibility that you will be routing through me (leading to routing loop) the same concept is applied to L2 routing loops (you need to have a reference point root bridge) i am just trying to keep it simple.
hope this help! Elie ________________________________________ From: [email protected] [[email protected]] on behalf of Chidhu R [[email protected]] Sent: Tuesday, July 31, 2012 9:57 AM To: [email protected] Subject: [OSL | CCIE_RS] OSPF Backbone area rule Hi, Can someone explain me, in OSPF, - Why every area must be connected to area 0 and why those routes in areas which are not connected to area 0 are not being redistributed across the OSPF domain. ( which led virtual link concept i suppose ) - Why any router which is an ABR should be connected to area 0 and why a router which is not connected to area 0 but connected to multiple areas do not summarize the type 3 LSAs. Also please explain one more doubt in the picture attached with this thread. When checked in R5, i am able to see all the routes including the route between R1 and R2 ( *this is not connected to area 0, yet i am able to see the routes and there is no Virtual link configured )* Thanks Chid _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
