STATIC AND DYNAMIC ROUTING
Assignment Overview
Static Routing

Static routes are commonly used when routing from a network to a stub network. A stub network is a network accessed by a single route. For an example, see Figure 1. Here you see that any network attached to R1 would only have one way to reach other destinations, whether to networks attached to R2 or to destinations beyond R2. Therefore, network 172.16.3.0 is a stub network and R1 is a stub router.

Figure 1. An example of Stub network

Running a routing protocol between R1 and R2 is a waste of resources because R1 has only one way out for sending nonlocal traffic. Therefore, static routes are configured for connectivity to remote networks that are not directly connected to a router. Again, referring to Figure 1, you would configure a static route on R2 to the LAN attached to R1.

In a static routing system, routes through a data network are described by fixed paths (statically). These routes are usually entered into the router by the system administrator. An entire network can be configured using static routes, but this type of configuration is not fault tolerant. When there is a change in the network or a failure occurs between two statically defined nodes, traffic will not be rerouted. This means that anything that wishes to take an affected path will either have to wait for the failure to be repaired or the static route to be updated by the administrator before restarting its journey. Most requests will time out (ultimately failing) before these repairs can be made.



To configure a static route to network 10.10.20.0/24, pointing to a next-hop router with the IP address of 192.168.100.1, type:



ip route 10.10.20.0 255.255.255.0 192.168.100.1



Destination network

10.10.20.0

subnet

255.255.255.0

next-hop

192.168.100.1

A packet from the network 10.10.20.0/24 will be forwarded to 192.168.100.1.



The other option is to define a static route with reference to the outgoing interface which is connected to the next hop towards the destination network.



ip route 10.10.20.0 255.255.255.0 Serial 0/0



Destination network

10.10.20.0

subnet

255.255.255.0

next-hop

Serial interface 0/0 (local exit)

Modifying Statics Routes



There are times when a previously configured static route needs to be modified:



The destination network no longer exists, and therefore the static route should be deleted.

There is a change in the topology, and either the intermediate address or the exit interface has to be changed.

There is no way to modify an existing static route. The static route must be deleted and a new one configured.



To delete a static route, add no in front of the ip route command, followed by the rest of the static route to be removed.



For example, in the previous section, you removed the static route:



ip route 10.10.20.0 255.255.255.0 192.168.100.1



with the following no ip route command:



no ip route 10.10.20.0 255.255.255.0 192.168.100.1



Then we can configure a new static route using a next-hop address or an exit interface:



ip route 10.10.20.0 255.255.255.0 Serial 0/0



Static Routes and Packet Forwarding



Figure 2 and the following steps illustrate the packet-forwarding process with static routes. In this example, R1, R2, and R3 are routing traffic between PC1 and PC3. Only the processing of traffic from PC1 to PC3 is shown. However, the same process is used for traffic from PC3 back to PC1.







Figure 2. Static Routes and Packet Forwarding



The packet forwarding process of a packet from R1 to R3 is as follows:



The packet arrives on the FastEthernet 0/0 interface of R1. R1 does not have a specific route to the destination network, 192.168.2.0/24; therefore,

R1 uses the default static route.

R1 encapsulates the packet in a new frame. Because the link to R2 is a point-to-point link, R1 adds an “all 1s” address for the Layer 2 destination address.

The frame is forwarded out the Serial 0/0/0 interface. The packet arrives on the Serial /0/0 interface on R2.

R2 decapsulates the frame, examines the packet’s destination IP address, and looks for a route to the destination. R2 has a static route to 192.168.2.0/24 out Serial 0/0/1.

R2 encapsulates the packet in a new frame. Because the link to R3 is a point-to-point link, R2 adds an “all 1s” address for the Layer 2 destination address.

The frame is forwarded out the Serial 0/0/1 interface. The packet arrives on the Serial 0/0/1 interface on R3.

R3 decapsulates the frame, examines the packet’s destination IP address, and looks for a route to the destination. R3 has a connected route to 192.168.2.0/24 out FastEthernet 0/1.

R3 looks up the ARP table entry for 192.168.2.10 to find the Layer 2 MAC address for PC3:

If no entry exists, R3 broadcasts an ARP request out FastEthernet 0/0.

PC3 responds with an ARP reply that includes the PC3 MAC address.

R3 encapsulates the packet in a new frame with the MAC address of interface FastEthernet 0/0 as the source Layer 2 address and the MAC address of PC3 as the destination MAC address.

The frame is forwarded out the FastEthernet 0/0 interface. The packet arrives on the NIC interface of PC3.

To learn more about bipartite graphs and graph representations, check the following sites:



Introduction to Static Routing (http://www.youtube.com/watch?v=Sa5XuO9H29M)

Static Routing (http://www.comm.utoronto.ca/~jorg/teaching/itlab/pdf/Ch3_v3.pdf)

Static Routing (http://en.wikipedia.org/wiki/Static_routing)

Static Routing (http://www.techopedia.com/definition/26161/static-routing)

Case Assignment

You are required to review the Case materials and conduct your own. Please refer to Figure 12, please show:



The packet forwarding process if a packet is sent from PC1 to PC2.

The packet forwarding process if a packet is sent from PC2 to PC3.

Assignment Expectations

Your paper should be between two and three pages. You are expected to understand packet forwarding using static routing.



Upload your assignment when you are finished.