Routers learn networks by one of three methods:
- directly attached - this network can directly be talked to through one the router's interfaces
- static route - remote network reachable through another router/gateway is configured by a static configuration entry; static routes need to point to an interface in a common network between the two routers
- routing protocol - remote network reachable through a gateway and route is dynamically exchanged via a routing protocol
do all the static routes settings on the routers necessary? cause there are so many lines which took me quite a while to configure for the connections between servers...
Since you're not using a routing protocol, all remote networks on each router need to be set up with static routes. With only a single upstream router, you can alternatively use that as gateway of last resort.
R0 needs either R1' 192.168.2.1/24 as default gateway or separate routes for 10.90.2.0/28, 172.18.6.0/24 and 192.168.200.0/28.
R1 could use a default route to R2'2 10.90.2.2 (or separate routes for the right-hand leg) but always requires a separate route to 10.1.1.0/24 via R0's 192.168.1.1.
And so on.
for the R0 router, the next hop address should be set to 192.168.1.2 (for the dest. network)rather than addresses such as 10.90.2.2 or 172.18.6.2, why?
R0's gateways need to be interfaces in one of its local networks, so 192.168.1.2 is the next hop for any of R0's routes.
You can't use 10.9.2.2 or 172.18.6.2 because they're not adjacent to R0.
for all the routers, is it necessary to configure static routes and next hop for all the dest. network(which located on other routers) to enable the accessibility between server0 & server2?
Yes. If any of the routers doesn't have a gateway to the destination it'll drop the packet.
does "gateway of last resort" meaningful in this network topology? i don't see a way to use it :(
Each router's gateway of last resort aka default gateway can be used to simplify the routing table as pointed out above. Especially R0 and R3 face only one gateway to all other subnets and that could be their default gateway.