hi ^^ please give me some advice -
i'm trying to build ospf routing protocol. i have four router. two of them(in this case A, B router) are connected to C router, and C router is conneted to D router, which has connection to ISP.
Which router should i set as a Deginated Router ? I'm wondering if C or D should be OSPF DR router..
It doesn't matter, based on the information provided.
The DR (and BDR) have more work to do that other routers and if some of the routers are particularly feeble or busy with other work then it would make sense to assign the DR and BDR roles to more capable units.
Your tolology is so small that it very likely won't matter anyway.
The guidance in the Cisco docs is aimed at Enterprise scale networks where the routers are likely to be quite busy. There is of course no reason for you not to follow best practise no matter how small the network.
designated router is a "per subnet" setting in OSPF.
The designated router is elected when the current DR is lost - and the election is "sticky" so even if a higher priority DR appears, it will only take over when the old DR is disconnected.
Unless some of your routers have lots of OSPF interfaces (say 20 to 50 and up), it doesnt normally matter unless you have a really big network.
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