In our second example we looked at configuring routing statically. Static routing really involves nothing more than telling a router about a non-connected network, and the next-hop address to reach it (or local interface on which to forward the packet). While it may be a little more work than using dynamic routing, static routes are a quick and effective way to configure routing, especially on small networks.
On larger internetworks that consist of many routers and networks, static routing will probably not be the best option. This is because of what transpires when a router or network becomes unreachable – with static routing, a router won’t be able to do anything to find another path through the internetwork. With dynamic routing, when a path becomes unavailable, a router will find out about it from other routers, and will attempt to use a different path, if one exists. That’s not to say that dynamic routing protocols are the solution to all of life’s routing problems. In fact, they can sometimes cause just as many problems as they solve, as we’ll see a little later in this chapter.
Even before our router is configured with static routes, it has a routing table. Each routed protocol (like IP or IPX) maintains its own routing table. To view the IP routing table on a Cisco router, use the show ip route command, as shown below.
RouterA#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set
C 10.0.0.0/8 is directly connected, Serial0
C 192.168.1.0/24 is directly connected, Ethernet0
Before any static (or dynamic) routes exist, a routing table will show entries for only directly connected networks. In this case, the output of show ip route shows two connected networks, 10.0.0.0/8 and 192.168.1.0/24. In both cases, the fact that they are directly connected is clear according to the message following the network ID. However, the letter “C” that precedes the entries also identifies the networks as being directly connected. The codes shown at the beginning of the command output describe how a router knows about a network in its routing table.
Adding a static route isn’t terribly difficult at all. The command to add a static IP route is ip route. The complete syntax of the command is:
ip route [destination network] [mask] [next hop address OR exit interface] [administrative distance] [permanent]
Not all of the options listed above need to be entered. In this case, we’ll enter only the destination network, subnet mask, and next-hop address information. We’ll discuss the other options shortly. Let’s assume that our network is configured as shown in the figure below. In it, RouterA is directly connected to networks 192.168.1.0/24 and 10.0.0.0/8. We’ll need to add a static route on RouterA, telling it that network 172.16.0.0/12 can be reached via the next hop address of 10.0.0.2.
Figure: Router A and Router B interconnecting 3 networks.
RouterA#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#ip route 172.16.0.0 255.240.0.0 10.0.0.2
RouterA(config)#^Z
RouterA#sh ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set
C 10.0.0.0/8 is directly connected, Serial0
C 192.168.1.0/24 is directly connected, Ethernet0
S 172.16.0.0/12 [1/0] via 10.0.0.2
Notice that a new entry has been added to the routing table, and is preceded by an “S”. This designates the route as static. In this case, network 172.16.0.0/12 is accessible via the next-hop address, which is interface S0 on Router B – 10.0.0.2. If you’re setting up static routes and you notice that the route just added doesn’t appear after issuing the show ip route command, it likely means that the next-hop address you specified cannot be contacted. In order for the route to appear in the table, the next-hop address must be accessible. If you want to add a static route and have it appear in the routing table regardless of whether it’s available, add the permanent keyword to the end of the ip route command, as shown below.
RouterA(config)#ip route 172.16.0.0 255.240.0.0 10.0.0.2 permanent
RouterA(config)#^Z
RouterA#sh ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set
S 172.16.0.0/12 [1/0] via 10.0.0.2
C 192.168.1.0/24 is directly connected, Ethernet0
In the example above, I unplugged the S0 interface on RouterB. Notice how network 10.0.0.0/8 no longer appears as directly connected, since the address 10.0.0.2 cannot be reached. However, the use of the permanent keyword keeps the new entry for network 172.16.0.0/12 in the routing table, even though the next hop address is unavailable.
After adding the static route to network 172.16.0.0/12 on RouterA, the next step is to add a static route to network 192.168.1.0/24 on RouterB.
RouterB#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterB(config)#ip route 192.168.1.0 255.255.255.0 10.0.0.1
RouterB(config)#^Z
RouterB#sh ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set
C 10.0.0.0/8 is directly connected, Serial0
C 172.16.0.0/12 is directly connected, Ethernet0
S 192.168.1.0/24 [1/0] via 10.0.0.1
In order to remove a static route, use the “no” version of the ip route command, followed by the network address and subnet mask of the route you wish to remove. For example, to remove the static route to network 192.168.0.0/24 from RouterB, you would enter:
RouterB(config)#no ip route 192.168.0.0 255.255.255.0
Recall that another option for the ip route command allows you to add what is known as an administrative distance. An administrative distance is simply a number that designates how reliable or “trustworthy” the information about a particular route source is considered to be. For example, a directly connected interface is always considered the most trustworthy, and has an administrative distance of 0. A static route is also considered trustworthy, since an administrator will have manually defined it, after all. Static routes are assigned an administrative distance of 1 by default. The lower an administrative distance, the more trustworthy a route is considered to be. Consider the routing table entry below. The administrative distance of this route is 1, as shown by the value in the first portion of the square brackets.
S 192.168.1.0/24 [1/0] via 10.0.0.1
Routes learned from dynamic routing protocols are also assigned default administrative distances. But how are administrative distances used? Well, assume that an administrator has defined a static route to a network, and a route to the same network is also learned from a routing protocol. In cases where this happens, the router will “keep” or use the entry with the lower administrative distance. By default, a static route will always beat an entry learned dynamically. The list below provides the default administrative distances associated with routes learned in different ways.
Directly connected interface 0
Static Route 1
EIGRP 90
IGRP 100
OSPF 110
RIP 120
Unknown 255
To set the administrative distance of a static route to 50, use the ip route command as follows:
RouterB(config)#ip route 192.168.0.0 255.255.255.0 10.0.0.1 50
RouterB(config)#^Z
RouterB#sh ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set
C 10.0.0.0/8 is directly connected, Serial0
C 172.16.0.0/12 is directly connected, Ethernet0
S 192.168.1.0/24 [50/0] via 10.0.0.1
While static routing may be a great deal of work to configure and maintain, there are two scenarios where static routing will commonly be used. The first is on “stub” networks, a network from which there is only one exit router to the rest of an internetwork. The second is in demand-dial routing environments, which are looked at in more detail in Chapter 11.
