The first page that introduces any kind of routing. Reading this you should know what a route is and what it's for. In a nutshell it's a pathway between routers. If a router has no route to a destination network packets cannot be sent to anything residing in that network, this includes any and all traffic. It's that simple. If you need more information, google it.
====== Static Routes ======
The easiest form of routing to configure but quickly becomes the most difficult to maintain in anything but small networks. This is because static routing is exactly that, static. You must configure all routes by hand. With networks the size of three routers this isn't a problem, it takes only two commands. Add a 4th router and it becomes much more time consuming. The end routers both need static routes configured and the routes in-between those routers must also have routes configured to get traffic from A to B. Each router added increases the amount of work rather significantly. That said, it is still useful in niche situations -- small networks or networks that don't require communication throughout an entire intranet (collection of networks owned by one entity or org).
===== Example =====
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{{:icnd1:staticroutetop1.png?nolink|}}
Each router knows it's directly connected networks.
* R1: Knows about 1.1.1.0/24
* R2: Knows about 1.1.1.0/24 & 2.2.2.0/24
* R3: Knows about 2.2.2.0/24
Only R2 knows about both networks because it's directly connected to them. A ping from R1 to R3 would fail because neither device has a route to one another.
We can get the above network functional by adding static routes.
===== Configuration =====
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On R1 we'll add a route to the 2.2.2.0/24 network so we can communicate with R3:
R1>en
R1#conf t
R1(config)#ip route 2.2.2.0 255.255.255.0 serial 0/0
R1(config)#do show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Serial0/0
2.0.0.0/24 is subnetted, 1 subnets
S 2.2.2.0 is directly connected, Serial0/0
R3(config)#do show ip route
** CODES OMITTED
Gateway of last resort is not set
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Serial0/0
Note the route codes next to the listed routes. A ping from R1 to R3 at this point, regardless of our added route, will fail. This is because R3 has no route back to R1 -- aka, he has no idea the 1.1.1.0/24 exists or how to get traffic to it. Without a route, all packets to that network will be dropped.
R3 configuration:
R3>en
R3#conf t
R3(config)#ip route 1.1.1.0 255.255.255.0 serial 0/0
R3(config)#do show ip route
** CODES OMITTED
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
S 1.1.1.0 is directly connected, Serial0/0
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Serial0/0
R3#ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/56/84 ms
After adding routes to R1 and R3, the pings are a success!