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( PART B )
OSPF Experiment
Student Name:
Student ID:
Network Architectures and Services
Dr Avgoustinos Filippoupolitis
School of Computing and Mathematical Sciences
Table of Contents

Overview.. 4
Objective. 4
Experimental Setup. 4
Network Scenarios. 6

4.1        No_Areas. 6
4.2        Q4_No_Areas_Failure. 10

Result Analysis. 13

5.1        Network Routes. 13
5.2        IP addresses. 15
5.3        Routing Tables. 16
5.4        Link State Database. 18
5.5        OSPF Traffic. 19

Recommendations. 20
Conclusion. 20
References. 21

List of Figures
Figure 1: slip8_gtwy routers placement 5
Figure 2: Router connections with PPP_DS3. 5
Figure 3: OSPF interface cost setting. 6
Figure 4: IP unicast traffic flow configuration. 7
Figure 5: Traffic flow path (B–>H, Red) & (A–>C, Blue). 7
Figure 6: Routing protocol selection. 8
Figure 7: Exporting routing tables. 8
Figure 8: Auto-assigning IP addresses. 9
Figure 9: Exporting IP addresses. 9
Figure 10: Exporting link state database and routing tables. 10
Figure 11: Routing report for IP traffic flows. 10
Figure 12: Failure recovery component 11
Figure 13: Network with failure recovery component 11
Figure 14: Setting failure between a network link. 12
Figure 15: Setting failure timing. 12
Figure 16: IP traffic flow from RouterA–>RouterC (No_Areas scenario). 13
Figure 17: IP traffic flow from RouterB–>RouterH (No_Areas scenario). 14
Figure 18: IP traffic flow from RouterA–>RouterC (Q4_No_Areas_Failure scenario). 14
Figure 19: Assigned IP addresses. 15
Figure 20: Assigned IP addresses. 16
Figure 21: Routing table (No_Areas Scenario). 17
Figure 22: Routing table (Q4_No_Areas_Failure scenario). 17
Figure 23: Router Link State Table (No_Areas Scenario). 18
Figure 24: Router link state table (Q4_No_Areas_Failure scenario). 19
Figure 25: OSPF sent traffic comparison. 20
1.  Overview
Internet Protocol (IP) traffic is governed by the rules established by routing protocols for instance Open Shortest Path First (OSPF), Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) [1]. The purpose of a routing protocol is to efficiently and optimally transfer data packets between network nodes and set communication rules for data transfer. There are two broad categories routing protocols of IP network 1) Interior Gateway Protocol (IGP). 2) Exterior Gateway Protocol (EGP). IGP is responsible for communication within a single autonomous network whereas EGP is utilised for exchanging communication information between various autonomous networks.    Interior Gateway Protocol is further divided in to two types; link state routing protocol and distance-vector routing protocol. OSPF is based on former type in which each router is solely responsible to describe the status of its neighbouring routers to the entire network [2]. Each router’s neighbourhood information is then gathered in a specialised way of flooding and this process forms a link state database which is used to calculate appropriate path between routers. Hence, shortest path is calculated using each path cost assigned by the network and forms destination link state tables to carry IP packets to the next router and ultimately to the destination router.  OSPF also takes care of the network in case of failure in some paths by diverting the traffic to other paths.
2.  Objective
The main objective of this experiment is to analyse OSPF routing protocol in terms of selecting the most appropriate and shortest path from source to destination. It is also important to observe its routing tables and link state database in case of failure in a network path. Therefore, it is about analyzing overall performance metrics of OSPF protocol. There are two scenarios simulated in Riverbed Modeler to see the effect of failure in network path.
3.  Experimental Setup
Two scenarios No_Areas and Q4_No_Areas_Failure are set up and simulated using OSPF protocol in Riverbed Modeler. There are eight slip_gtwy routers connected in certain geometry with point to point link. Second scenario is just a duplicate of the first scenario with only an introduction of network failure in one of the path. Office environment is selected to perform the experiment.
Upto eight serial line interfaces can be maintained by slip8_gtwy routers at specific data rate. These devices are IP based gateway routers in which IP data packets are routed to the desirable destination from the source according to the IP address. These routers support RIP and OSPF protocols to dynamically generate the routing tables and adaptively choose routes between source and destination [3]. In order to route a data packet, this node takes a fixed amount of time which is determined by its IP attribute.
PPP_DS3 link is used to connect slip8_gtwy routers in the experiment. PPP_DS3 stands for point-to-point protocol digital signal three, which is used to connect two nodes in an IP network and having data rate of 44.736 Mbps.
Eight slip8_gtwy routers are placed after selecting from object palette Cisco section. They are named Router A through H as shown in Fig. 1. There are connected using PPP_DS3 link selected from link models in object palette as shown in Fig. 2.
Figure 1: slip8_gtwy routers placement
Figure 2: Router connections with PPP_DS3
4.  Network Scenarios
4.1    No_Areas
After placement of routers, renaming them and connecting them with point to point link, the next step is to assign weights to the links between different routers. It is accomplished by assigning specific interface cost after selecting the link between routers. Interface costs are configured by selecting the appropriate link, go to “Protocols” menu, select OSPF, then “configure interface cost” option, and then assigning the interface cost explicitly to some value as shown in Fig. 3. Interface cost is also called the metric of the said protocol which governs intelligent routing decision by the protocol. The formula of finding this metric is dividing OSPF reference bandwidth is happened to be 100 Mbps by link bandwidth used for interfacing (in this case PPP_DS3, bandwidth of 44.736 Mbps). However, as we are assigning cost to every interface, it will override the cost calculated by formula.
There are 5 links (AàD, DàE, CàE, DàF, EàG) which are assigned the interface cost of 5. It is done by selecting all the links by keep pressing select button and right clicking on them, and the follow the above mentioned procedure to set interface cost. Three links (AàB, AàC, BàC) are assigned interface cost of 20 whereas three links (FàG, FàH, GàH) have interface cost of 10.
Figure 3: OSPF interface cost setting
After setting the interface cost for the whole network, the next step is to create traffic flow which means configuring the source and destination nodes among which communication will take place. IP unicast traffic flow is selected because it is a point to point network. IP unicast means a source is able to send its data to only single destination at a time, while contrary to it multicasting is a process of sending data packets from one source to multiple destinations in an instance. Traffic flows are configured by following the steps 1) selecting routers A and C 2) go to Traffic menu 3) go to “Create Traffic Flow” and selecting IP Unicast 4) then selecting the second radio button as shown in Fig. 4. Repeat the process after selecting RouterB and RouterH. Therefore, two traffic flow paths have been created (RouterAàRouterC & RouterBàRouterH) as shown in Fig. 5.
Figure 4: IP unicast traffic flow configuration
Figure 5: Traffic flow path (B–>H, Red) & (A–>C, Blue)
Furthermore, routing protocol for the experiment is configured as OSPF which will set all the rules and decisions governed by OSPF for this experiment. In order to select the routing protocol for the simulation, go to Protocols menu, select IP and then Routing sections, and finally select Configure Routing Protocols. A window as shown in Fig. 6 will open to choose one from a pool of routing protocols. OSPF has been selected in this case.
Figure 6: Routing protocol selection
In order to communicate with each other, routers need to have IP addresses. IP addresses are identity of the network devices through which they recognize each other. In this experiment, IP addresses are assigned automatically by selecting Autoassign IP Addresses through menu ProtocolsàIPàAddressing.
Routing tables and link state database are two important information of OSPF protocol in order to analyze its working and routing schemes. To view routing tables for the specific routers and their link between routers, routing tables have to be exported by selecting the specific routers (in this case Router A and B) and then going to menu ProtocolsàIPàRoutingàExport Routing Tables as shown in Fig.7.
Figure 7: Exporting routing tables
Figure 8: Auto-assigning IP addresses
In order to access routing table and link state database after the simulation runs, there is need to set them up by right clicking on RouterA, select attributes and then set “Link State Database” and “OSPF Routing Tables” to “Export at end of simulation” option. In this way, both these items can accessible to view and analyze after the simulation as shown in Fig. 10. However, in order to analyze routing table and link state database, it is necessary to know IP addresses of network nodes and link. To retrieve IP addresses of the network, click configure/Run DES button on the toolbar, go to IP option in Global Attributes, and set IP Interface Addressing Mode to Auto Addressed/Export as depicted in Fig. 9. It is then possible to see IP addresses from GDF file.
Figure 9: Exporting IP addresses
Figure 10: Exporting link state database and routing tables
After running the simulation for 10 minutes, to view the IP traffic flows for the configured routes there is need to follow certain steps. Choose option ProtocolsàIPà Demands and then “Display Routes for Configured Demands” in order to view both RouterAàRouterC and RouterBàRouterH routes. Results will be discussed in next section.
Figure 11: Routing report for IP traffic flows
4.2      Q4_No_Areas_Failure
Another scenario Q4_No_Areas_Failure is created by duplicating No_Areas scenario which means all the configurations of No_Areas scenario are kept the same. The new scenario has a Failure added to it by dragging it from the object palette in to the scenario workspace as shown in Fig. 12 & Fig. 13. The reason of creating this scenario with a failure is to analyze OSPF routing decisions in both scenarios; with and without failure added.
Figure 12: Failure recovery component
Figure 13: Network with failure recovery component
The failure is configured by right clicking on it and selecting Attribute option. Number of rows is configured to be 1 and failure is added from RouterD to RouterE link as shown in Fig. 14. Moreover, 180 seconds is selected for the failure to be activated after that time as shown in Fig. 15.
Figure 14: Setting failure between a network link
Figure 15: Setting failure timing
5.  Result Analysis
5.1   Network Routes
Simulation has been run for 10 minutes and network routes for the configured demand have been obtained as shown in following figures. There were two traffic flows selected, one is RouterAà RouterC and RouterBàRouterH. In No_Areas scenario, to send data packets from RouterA to RouterC, the path goes from RouterA to RouterD, then to RouterE and finally to destination that is RouterC as shown in Fig. 16. As it is understood that this network is running OSPF protocol and this routing protocol finds the shortest path from source to destination in terms of cost. The route from AàDàEàC is the shortest with total interface cost of 5+5+5=15 which is less than other paths (AàC=20 and AàBàC=40). On the other hand, traffic flow configured between RouterB to RouterH can have two paths because of equal path cost. As shown in Fig. 17, path is routed from BàAàDàFàH having total cost of 20+5+5+10=40. There is another path BàCàEàGàH having the same cost of 40 which could be another option.
Figure 16: IP traffic flow from RouterA–>RouterC (No_Areas scenario)
Figure 17: IP traffic flow from RouterB–>RouterH (No_Areas scenario)
When looking at the other scenario Q4_No_Areas_Failure, there is an addition of failure element and it has been configured on the path from RouterDàRouterE, so no traffic can flow from this link. As shown in Fig. 18, route between RouterA to RouterC is directly from A to C since this link has the least cost of 20 because it cannot take path having link of DàE, while other path from AàBàC has a cost of 40.
Figure 18: IP traffic flow from RouterA–>RouterC (Q4_No_Areas_Failure scenario)
5.2    IP addresses
In order to clearly understand and comment on routing table and link state database, it is important to know the IP addresses of the routers and their interfaces first. IP addresses of the network can be accessed from GDF file in the project folder. Figure 19 and 20 represents the IP addresses of all the routers in this experiment as well as their interfaces IP addresses. For example, let’s take RouterA and RouterD, IP address is assigned to RouterA whereas RouterD has been assigned IP address and their direct interface has the IP address of as depicted in Fig. 19.
Figure 19: Assigned IP addresses
Let’s take another example for better understanding, RouterC has been assigned IP address and IP address is assigned to RouterE whereas IP address is assigned to their direct interface as shown in Fig. 20. This is true for all the interfaces between routers.
Figure 20: Assigned IP addresses
5.3    Routing Tables
Routing tables represent set of rules for the traffic flow in an IP network. Routings tables for RouterA are analysed for both scenarios and validated the OSPF routing schemes. Analyzing the routing table shown in Fig. 21 for No_Areas scenario, for example take row 3 having destination IP address of which belongs to the subnetwork between RouterC and RouterE. The next hop for this is which is IP address for RouterD, which means total link cost would be 15; AàD=5+DàE=5+EàC=5. Take another example of row 9 having destination IP address of which belongs to the subnetwork of RouterE and RouterG. IP address of next hope node is which belongs to RouterD and total link cost of this traffic flow is 15 which means data packet will move to RouterD from RouterA which costs 5, DàE=5, and finally to destination EàG which costs 5.
Figure 21: Routing table (No_Areas Scenario)
It is time to compare both scenarios routing table since there is a failure link between RouterD and RouterE in second scenario. If we compare row 3, in scenario No_Areas the cost is 15 as discussed above, however in Q4_No_Areas_Failure scenario, the cost is 25 due to the DàE link failure because no traffic can flow through this link, so alternative shortest path is from AàC which is 20 and then CàE as shown in Fig. 22. Now let’s compare row 4 which is link between RouterD and RouterE, total cost of this traffic flow is 10 in scenario No_Areas whereas there is no information about it in the second scenario since it is failed link. It is observed that there are several different routing paths in second scenario due to single failure link which made OSPF to make routing decisions again to calculate shortest path between source and destination.
Figure 22: Routing table (Q4_No_Areas_Failure scenario)
5.4    Link State Database
Link state database is collection of each router’s information which it receives from other routers in the form of LSAs (Link State Advertisements). Each router transmits LSA which is information about its neighbours and in this way information about all the routers are saved in the form of database which is helpful for OSPF to apply shortest path first algorithm to make routing decisions.
This part is to analyze different between link state databases of both scenarios. The main point is to compare advertisement messages of RouterD and RouterE to RouterA because of the failure between RouterD and RouterE in the second scenario. As shown in Fig. 23, belongs to RouterD in row 1, it has received advertisement from RouterA ( having a direct link ( with RouterD, advertisement from RouterE ( have a direct link ( with RouterD, and advertisement from RouterF ( having subnetwork ( with RouterD. Similarly, IP address on row 20 belongs to RouterE, it received advertisements from RouterC, RouterD and RouterG.

Figure 23: Router Link State Table (No_Areas Scenario)
Now if we compare link state database of Fig. 23 with Fig.24 link state database of second scenario, it can be seen that RouterE ( has received advertisements from only RouterC ( and RouterG ( as shown in Fig.24 (Row 44-47) whereas there is no advertisement from RouterD because of the link failure between them. On the other hand, RouterD ( has only received advertisements from RouterA ( and RouterF ( as shown in Fig. 24 (Row 39-42).

Figure 24: Router link state table (Q4_No_Areas_Failure scenario)
5.5    OSPF Traffic
Fig. 25 depicts the traffic sent in bits/sec during the simulation runs for 10 minutes in both scenarios. As it is clear that there is a spike of traffic sent in the beginning in both the scenarios, this is due to the fact that OSPF algorithm gathers information about the whole network and maps the network which helps to make intelligent routing decisions. However, there is a difference in graphs of another spike after 180 seconds of simulation in Q4_No_Areas_Failure scenario due to the initialization of failure after 180 seconds. After the failure between RouterD and RouterE takes effect, OSPF protocol again gathers information about the network and forms a new link state database to make routing decisions again.
Figure 25: OSPF sent traffic comparison
6.  Recommendations
In order to avoid forwarding tables to not change their state time and again, it is better to use multiple routing protocols (in addition to OSPF) to increase the convergence of the network and in this way it is a better way to utilize the available bandwidth.
7.  Conclusion
OSPF protocol has been analyzed in terms of finding the shortest path from source to destination and the changes in its routing table and link state database when failure is added in the network. Two scenarios are simulated using Riverbed modeller and observed that paths between source and destination changes if there is failure in a path in order to find the shortest path and also routing table and link state database changes to accommodate protocol to make different decisions as compared to the scenario without failure path.
8.  References
[1] Buriol, L.S., França, P.M., Resende, M.G. and Thorup, M., 2003. Network design for OSPF routing. In Proceedings of Mathematical Programming in Rio (pp. 40-44).
[2] Vetter, B., Wang, F. and Wu, S.F., 1997, October. An experimental study of insider attacks for OSPF routing protocol. In Network Protocols, 1997. Proceedings., 1997 International Conference on (pp. 293-300). IEEE.
[3] Aboelela, E., 2003. Network simulation experiments manual. Academic Press.

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