Static Route, Floating Static Route and Default Static Route

Static route is used when one wants to define the routing process manually.

The static route command is as below:

(config)#ip route

On the exit interface of the router part, the interface can be defined as the type of interface (e.g. serial0, fa0/2) or it can be defined as the ip address of the outbound interface of the router. The difference is that when we set it with the type of interface, the route is configured to be directly connected (AD=0), while the option of setting the IP address of the outbound interface will be considered as static route (AD=1)

The example below shows the 2 different configuration, but has actually the same meaning:

(config)#ip route 10.10.10.10 255.255.255.255 172.168.10.10 --> static route AD=1
(config)#ip route 10.10.10.10 255.255.255.255 serial0 --> direct connected AD=0

A floating static route is a route that is used to back up a dynamic protocol. So, the floating static route will take over the routing when the dynamic protocol such as RIP or OSPF goes down for some reason on an interface. The key in setting a floating static route is the same as in configuring a static route, but adding the administrative distance of it. The AD must be higher than the current value of the dynamic protocol. If we use RIP as the protocol we must set the floating route higher than 120. A value lower than 120 will have the opposite effect; the static route will be applied in routing rather than the RIP.

The floating static route can be done with the command:

(config)#ip route

The default static route is used as a gateway of last resort. If the router does not have a routing table of an incoming packet, then the router will send the packet to according to the default static route. If this route is not set, unknown incoming packets will be dropped.


The command for default static route can be shown below:

(config)#ip route 0.0.0.0 0.0.0.0 /

The symbol for default static route will be S* --> AD=0 (directly connected)/1(static)

Sometimes when updating routing policies the routes are not updated directly. A useful way to refresh the changes is to clear the ip route, and let the router discover the table again by applying the command:

RX#clear ip route*

Routing Table Operation

Routers use their routing tables to determine the outgoing route of the incoming packets. The routing table operation goes as follow in the following order:

1. The route with the longest prefix (or the shortest subnet mask) will be prioritized first, despite of its routing protocol. So if there is a RIP route of 172.10.10.0/28 and an OSPF route of 172.10.10.0/29, it will choose the OSPF route when sending outgoing packets because 172.10.10.0/29 (6 hosts) has a longer prefix and thus more specific hosts than the 172.10.10.0/28 (14 hosts) route.

2. If the route has the same prefix length, it will look for its administrative distance (AD) of the connection to the neighboring routes. Administrative distances are distances that are measured based on the reliability of the connection and routing protocol of one router to the other. Routes with a lower AD are considered more reliable and will be chosen over the other routes with a higher AD number.

For example, directly connected routes are considered very reliable and has therefore an AD=0. RIP routes are considered simple and not so reliable and has therefore an AD=120. OSPF are considered more reliable than RIP and is given an AD=110.

3. If (1) and (2) are the same for multiple routes, the router will look for its metrics according to its table. The metric is the cost of a route to go from the source address to the destination address. Metrics are different from each other's routing protocol.

RIP metrics uses hop counts where the path with the lowest hops of routers is considered as the shortest path. OSPF metric uses cost which is the inversely proportional bandwidth of the current connection and gets prioritized. Lower cost means a faster interface with higher speed and gets prioritized. IGRP uses a composite metric, based on a the composite of bandwidth, delay, load, reliability and max. MTU. By default, IGRP chooses its route based on bandwidth and delay only. Lowest composite metric means better connection and gets prioritized.

4. If (1) to (3) has the same values, then equal-cost load sharing will be applied, where the load will be equally shared between the multiple connections.

In any of the 4 cases above, only the best route is shown on the show ip route command. The alternative route is there when the best route is down, but the alternative route configuration will not be shown in the show ip route command.

To show the IP route type the command:
RX#show ip route

It will show the routing configuration. A B[C/D] via E (optional), F, G
where:
A = type of connection (direct, RIP, IGRP etc)
B = the destination group IP address
C = administrative distance
D = metric
E = next hop inbound interface (not the final destination)
F = length of connection
G = interface connected to

for example:

R 172.10.0.0 [120/5] via 10.20.30.40, 0:02:34, serial0

means that the connection uses RIP routing protocol to the destination 172.10.0.0 with the administrative distance of RIP=120 and the metric of 5 hops. 10.20.30.40 is the inline interface of the next hop and the router is already connected for 2 minutes and 34 seconds. The connection is connected to the serial0 of the router.

-- 6 December 2010 --

Routing Protocol vs Routed Protocol

We often hear the term routing protocol and routed protocol. These 2 protocols is sometimes confusing since they sound almost the same but have actually different meanings.

A routing protocol is a protocol that defines how routers should communicate with each other. It will send and receive information from other routers about its connection, metric, etc., in order to make the own router understand how to and where to send packets to and from other routers. The information itself will be updated if there are changes, in order to get the latest configuration system of the network. Examples of routing protocols are RIP, IGRP, EIGRP, and OSPF.

A routed protocol is a protocol that defines the format of a packet and what information it should contain in order to be able to be routed. For example, IP is a routed protocol. In an IP packet, source IP and destination IP should be available in the packet in order to be able to be read by the router, thus can be forwarded. So, a routed protocol is a packet protocol that can be routed by a rounting protocol. Other examples of a routed protocol is IPX, DECnet, Apple Talk, Banyard Vines, and Xerox Network System (XNS).

-- 3 December 2010 --