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Byline: Bilal Ashfaq Ahmed, Yasir Saleem and Saima Waseem


Graphical network simulator (GNS) 3 is an excellent complementary tool that allows simulation of complex networks like implementation of real labs for network engineers, administrators, and researchers. The primary objective of this paper is to evaluate GNS3 as an educational tool by implementing Multiprotocol Label Switching (MPLS) Virtual private Networks (VPN) and Internet Protocol Security (IPsec) VPN. A Client Server model is created using FileZilla Server and Client, then a file of 11,057,507 bytes is transferred and monitored by network tool OpManager. The result obtained shows that Round-trip time (RTT) value is high for both protocols in GNS3 as compare to real networks. So it is concluded that network topology used can be implemented in GNS3, and could be used as an educational tool, but it requires a system like Intel Core 2 Duo or higher processor, having at least 4GB of RAM, as it utilizes considerable system resources.

Index Terms: Graphical Network Simulator (GNS3), Multiprotocol Label Switching (MPLS), Virtual Private Network (VPN), Internet Protocol Security (IPsec), Round-trip Time (RTT).


IT and Telecommunication industry has shown immense progress over the years as different changes are occurring at a very fast pace in this sector. It may be due to the demands of the users, to improve and remove the technological constraints that may exist like bugs and software up gradation that are required by system with the passage of time. Therefore, it is important to conduct different experiments, tests and research work before launching any brand new technology into the market and furthermore to ensure that the fresh technology, application, or software will achieve the desire results.

Simulators and emulators are used for analysis of existing methods, new systems, for training and experimentation purposes. These are cost efficient compared to the physical equipment and tools used in labs. Furthermore, it needs less time and resources for set-up of distinctive simulators and emulators. They are robust for running distinct experiments and research work. Creation and testing of many applications, protocols, and network designs used these simulators. The simulation-based knowledge helps students to develop updated skills in creative and critical thinking and find out new problem-solving techniques [1].

GNS3 is an open-source emulator, but it requires Cisco IOS images for running different routers and Private Internet Exchange (PIX) firewall, etc. According to Fuszner [2], by using GNS3 and Cisco Internetwork Operating System (IOS) emulation of complex networks would be carried out. Cisco IOS runs in a virtual environment on a laptop and personal computers. GNS3 is the graphical front end to Dynagen that runs on top of Dynamips which is the core program that makes Cisco IOS emulation possible and provides user- friendly text based interface [14]. GNS3 provides graphical environment. GNS3 is used for preparation of such as Cisco CCNA, CCNP, CCIP and CCIE as well as Juniper JNCIA, JNCIS and JNCIE. Thanks to Virtual Box integration, now even system engineers and administrators can take advantage of GNS3 to make labs and study for Red hat (RHCE, RHCT), Microsoft (MSCE, MSCA), Novell (CLP), and many other vendor certifications and for training of students at different institutions.

As GNS3 uses Cisco real IOS, co mmand supported by that particular IOS version can be used and configured in GNS3. However, the command supported by other simulators (e.g. Router Sim, Packet tracer and Boson router simulator) depends upon the developer that decides how many commands are chosen and included in the simulator [3]. GNS3 has another interesting feature i.e. packets or traffic would be captured, monitored and analysed using Wire shark.

It is worth mentioning that MPLS has become an essential network technology that has been implemented by many service providers. MPLS has addressed the Scalability, Quality of Service and Traffic Engineering issues etc. Next generation networks have been deployed by different service providers to provide different services using unified infrastructure [4].


MPLS is a standard defined by IETF. Instead of making decision on traditional IP addressing, a new forwarding mechanism in which packets are forwarded based on labels is used. These Labels may correspond to IP destination networks (equal to traditional IP forwarding). They can also correspond to other parameters, such as quality of service (QoS), source address or support forwarding of other protocols as well [5].

MPLS uses a 32-bit label field that is inserted between Layer 2 and Layer 3 headers as shown in Figure 1. MPLS label is 4 Bytes long in which 20 bits are used for label, 3 bits for experimental Class of Service and 1 bit for bottom of stack indicator and 8 bits for Time to live (TTL).

According to Morgan and Levering [6],"MPLS network converges dynamically and supports multiple routing protocols". MPLS has benefits like integrated network infrastructure, Border Gateway Protocol (BGP) free core, optimal traffic flow and traffic engineering. MPLS is used by Service Providers for separation of each customers routing information from other customers information within the cloud, so MPLS networks are called MPLS Virtual Private Network (VPN). Telecom Operators or Providers uses two VPN models for providing VPN services to their customers namely Overlay and Peer to Peer VPN model.


In this network model Point to Point connection or VC is provided by the Telecom Operators/Service Providers between the customer routers. Routing peer will be created between the customer routers instead of between customer and Provider router. So the Service Provider routers do not notice the customer routes. This Point to Point connection would be layer 1, layer 2 or layer 3 [7]. Figure 2 shows an example of Overlay VPN build on Frame relay network. Frame relay switches are installed in the service Provider network and customer routers are connected to these frame relay switches. Routing peer means that the customer routers are directly connected to each other as depicted in the Figure-1. Generic Routing Encapsulation (GRE) tunnels are used for making layer 3 Overlay VPN network.

Advantage of GRE tunnels is that the traffic other than Internet Protocol (IP) would also be routed through it and by using Internet Protocol Security (IPsec) in conjunction with GRE tunnel it is possible to secure the data travelling through it.


In this network model the Service Provider routers will facilitate and transport the data and routing information through its network. It will form routing peer with customer routers at layer 3. According to Badran [8] routing protocol adjacency will be formed between customer edge and the provider edge router. Figure 3 describes the concept of isolated routing information between different VPNs. When a customer router is added, it requires routing peer to be formed with a single Provider edge (PE) router. Service Provider does not need to create virtual circuits as with the overlay models or configuring packet filters or route filters with old Peer to Peer VPN design. It is an advantage for Service Providers by using MPLS VPN technology.


IPsec VPN tunnel is a feasible solution for data protection between two remote sites which are connected through internet. Internet Protocol security (IPsec) is a protocol suite which provides

1. Data authentication

2. Data Integrity

3. Data Confidentiality

4. Anti-Reply

IPsec VPN protects the data travelling from source to the destination with help of different protocols namely Internet Key Exchange (IKE), Encapsulating Security Payload (ESP) and Authentication Header (AH).

IPsec has two modes of operation which defines the extent of protection offered by IPsec. These are transport and tunnel mode. The original IP header is unprotected in transport mode and only the upper layer data is protected. On other hand in tunnel mode the original IP header and transport layer both are protected. Original IP header is replaced with new IP header. Azam et al. [9] reveals the network performance will be affected due to variation in number of overhead bytes. This variation in number of overhead bytes occurs by using different protocols, file sizes and different algorithms. Figure 4 describes different IPsec modes.


Simulator may be defined as the one that imitates an existing or proposes system and applications that will produce results approximating the actual conditions [10]. There are different systems, for example, nuclear reactions and warfare, which are dangerous to be conducted for analysis and experimentation purposes, so simulations will be a good option for these systems. According to Smith [11] Simulation is a process of making model of a real or imaginary system and conducting tests with this model." It is a very vast field, and models may be made for any imaginable system. Simulators may be used for telecommunication field, computer networks, integrated circuits, highway systems, flight training system and many more. Simulators are cost efficient, faster and less dangerous as compare to the real systems. Now a day's simulations are utilized by every engineering and scientific field. Simulations are either discrete or continuous event.

It depends upon how the state variables will change. In a discrete event, the variable values will change instantly at different points of time. On the other hand, in a continuous event the value of the variables will change continuously.


The topology scenario is designed considering simplicity and minimum hardware resources to be used. Figure 5 shows the implemented topology using GNS3 to simulate MPLS VPN and IPsec VPN. Five routers are used in this network design; two of these router are used as Customer Edge (CE) and other three routers for Service Provider (SP) named as Provider Edge(PE) and Provider (P) .Cisco 3700 ISO images are used with serial links in between. PC or VMware Machine are connected using cloud symbol. For serial link 1.544 Mbits/sec and for Fast Ethernet 100 Mbits/sec bandwidth is used. and VMware Machine.

Traffic and packets are captured. Wire shack packet analyser tool. For calculation of various results parameter OpManager network monitoring tool is used. In the test given below Layer 3 MPLS VPN (Peer to Peer VPN) and IPsec VPN is implemented in GNS3. Same file is downloaded from FileZilla server and time is calculated for comparison between the two technologies. RTT value for both technologies is noted for comparison. Table 1 shows the devices and applications that are used for conducting this test.

On Personal Computer (PC) GNS3 emulator package is installed having Wireshark Packet analyzing tool with it. The topology scenario is designed in GNS3 and FileZilla FTP Server and Client Model is developed with Laptop having Virtual Box VMware machine installed on it. The network monitoring tool Op Manager is installed on PC which is used for monitoring of file transfer rate and incoming and outgoing traffic in the network and Round trip time. Packet analyzing is done using Wireshark. All the above feature is only available in GNS3 that live traffic is captured and monitored using network interfaces, other software tools like Packet Tracer and Boson NetSim don't have these capabilities.

Table 1: Hardware and Software Used for Peer to Peer and IPsec VPN

Devices /



###Personal###Intel Core 2 Duo CPU, 2.93 GHZ, 4GB

Computer (PC)###RAM

###Intel Core 2 Duo CPU T6500, 2.1


###GHz,2GB RAM

Ethernet Cross

###Cross cables



###Emulator used for implementation of


Virtual Player###VMware Machine

###1.544 Mbits/sec connections between

Serial links


###Two 100Mbits/sec connections are used,

Full duplex Fast

###one for connecting an external PC to the


###CE1 router and the other for connectivity of


###VMware machine with CE2 router.

Protocols Used###MPLS VPN and Ipsec VPN

Wireshark###Packet analyzing tool

OpManager###Network Monitoring tool

FileZilla###FTP Server and Client


In this Section we have discussed the layer 3 MPLS VPN and IPsec VPN, there configuration and results analysis.


Configuration of each router as shown in topology scenario.


IP addresses are assigned to both serial and Fast Ethernet interfaces of CE routers. Bandwidth of 1.544 Mbits/sec is used on the serial interfaces and clock rate is not set on theses interfaces as these are DTE ends. Routing Information Protocol IP (version 2) routing protocol is used for advertising the networks of CE routers.


Clock rate of 64 kbps is configured on the DCE ends of CE. MPLS and Label distribution protocol (LDP) are configured P router interfaces. Open Shortest Path First (OSPF) routing protocol is used to advertise all networks and RIP (V2) protocol is used on PE routers loopback interface and serial interface attached to CE routers. Ipvrf command is used to configure Virtual Routing and Forwarding (VRF) on PE routers. PE routers have VRF instances for each attached VPN. Route Distinguisher (RD) is used to keep the customer routing information separate from other customers,. Border Gateway Protocol (BGP) is configured on both PE routers. RD combines with IPv4 prefix to make vpnv4 prefix which is transported by IBGP between PE routers. In order to make aware of different routes RIP is redistributed into BGP and vice-versa.


Bandwidth 1.544 Mbits/sec and clock rate of 64 Kbps is configured on both serial interfaces. MPLS and LDP are configured on serial interfaces. OSPF routing protocol is configured for dynamic routing.


Trace route command is used on CE2 router to check the route to fast Ethernet interface of CE1 router. Output shows that four devices (Labeled 1, 2, 3 and 4) are involved in reaching the interface as depicted in Table 2. This is different from overlay VPN network.

Routing Table of CE2 router shown in Table 3 shows all the connected interfaces and the routes advertise by MP-BGP. In Peer to Peer VPN technology, CE2 router includes the route information of both CE1 and PE2 routers. The routing peer is established between customer edge and provider edge routers. File is downloaded from FileZilla server and time is noted as shown in the Table 4, minimum and maximum time for downloading the file is 140 (2.33 minutes) and 268 seconds (4.46 minutes)respectively. Average time taken for file transfer is 218.14 seconds (3.6 minutes) and Standard deviation is 55.19. There is variation in file transfer time; it is due to variation in the number of processes that are running in Windows operating system and load on the CPU.

Table 2: Output of Trace route Command (Peer to Peer VPN)


Type escape Sequence to abort.

Tracing the route to 192.16e.3.: 152 moec 136 msec 156 msec

2 192,168.:.5 OILS: Labeo 16/21 Sop 0###124 msec 300 mccc 204 macc

310.1.1,2###OILS: Labe 21 Exp 0' 92 mccc 2:2 msec 204 meec

4101.1.1608 esec 228 mccc

Table 3: Out of Show ip route command (Peer to Peer VPN)

C2Show ip route

Gateway of last resort io not set

C### is directly connected,###Fastthernet2lO

### is subnetted, 2 subnets

R### [120/1] via### OO:OO:19, Serial1/1

c### is Directly connected, Serial1/1

R### [120/1] via### OO:OO:19, Serial1/1

Table 4: File Transfer Time in Layer 3 MPLS VPN Network

###File transfer successful, transferred 11,057,507 bytes in 180



###File transfer successful, transferred 11,057,507 bytes in 259

Status. seconds

###File transfer successful, transferred 11,057,507 bytes in 265

Status: seconds

###File transfer successful, transferred 11,057,507 bytes in 140

Status. seconds

###File transfer successful, transferred 11,057,507 bytes in 162



###File transfer successful, transferred 11,057,507 bytes in 253



Figure 6 shows alternative representation of file transfer time of file transferred through FileZilla server and client. Wireshark captures the data travelling through MPLS VPN network. Frame #15 is shown in Figure 7; the total length of the frame is 1512 bytes which includes 1460 bytes of data, 4 bytes Cisco HDLC, 8 bytes MPLS labels, 20 bytes IP and 20 bytes TCP overhead.

Figure 8 shows RTT history report, the average value of RTT is high as compare to the normal threshold value of 100 msec (default value in OpManager). It is reported as an alarm by the traffic monitoring tool.


Configuration of each router as shown in topology scenario.


IP addresses are assigned to the interfaces and bandwidth of 1.544 Mbits/sec is used on serial interface. Default gateway is defined on CE routers. First step is ISAKMP policy is configured on CE routers. It is IKE phase 1. This phase includes configuration of encryption algorithm, authentication algorithm, Pre-shared key, and Diffie- Hellman version1, 2 or 5 and tunnel lifetime. Second step is configuring the IPsec transform set, it is IKE phase 2. Third step is configuring Crypto access list, access list is configured so that the interesting traffic will travel through the IPsec tunnel for secure data transport. Fourth step is configuring crypto map, it is IKE phase 2. Fifth step is applying crypto map to the interface. Some important configurations of CE1 are shown in the Table 5.


Clock rate of 64 Kbps is set on the DCE end. Default bandwidth is used on the serial interfaces and RIP (Version 2) protocol is configured for dynamic routing on PE routers.

Table 5: Configuration of CE1 Router (IPsec VPN)

=ypt. ia policy I

###encr aes

###authentication pre-share

###group 5

###crypto isalcrp key 91502009 address

###crypto ipsec transform-set VPN1 esp-aes esp-sha-hsiac

###crypto map MAPS 10 ipsec-isa]cap

###set peer

###set transform-set VPI61

###match address 101

###interface SeriallfO

###ip address

###crypto map MAP1

###access-list 101 permit ip

###ip route


Clock rate of 64 kbps is configured on both serial interfaces; bandwidth of 1.544 Mbits/sec is used on the serial interfaces. Similarly RIP (Version 2) protocol is configured on P router.


Table 6 shows crypto iskamp sa is used to check all the IKE security associations (SAs). Output shows that ISKAMP is in idle state (quiescent state). It gives detail about the source and destination and status of IKE security associations.

Table 7 shows crypto IPsec sa command result that provides

Information regarding the current settings used by security associations. It gives information about the encapsulated, encrypted and digested packets etc.

Table 6: Output of crypto isakmp sa command (IPsec VPN)

CE2$shaw enrypto isakmp sa

ipv4 Crypto I4P SA

dot###sro###otate conn-id slot status

192.16a.1.1### ACtV0

Table 7: Output of Show crypto ipsec sa command

C01#show crypto ipeec en

intrrfucr: SerialS/P

###trypta map tag: !6AP1, :oca: addr

protected vrf: (none)

:cca:###ident )uddr/mnuk/pcat/port):###)

remate ideat )uddr/maelr/pcot/pcrc) : )192.168.4.t/

current_peer port 100

###9E8}1IT, flngu)crigin is oat,)

###4yite encups: 31810, Splrtu encrypt: 31818, plrtu digest: 31810

####p]rte decups: 17252, #p]rte decrypt: 17052, #plrte verity: 17052

###locul crypto endpt. :, remote crypto endpt.:

###puth mtu 1500, ip mtu 1503, ap mtu 1db Serioll/0

###current outbound upi: 0o15138813)890472470)

###inbound cup ens:

###transform: eup-ueu esp-ubu-bmur

###on use settings ={Tuzmel,

###id: 1, flow ii: 816:0, crypto map: I4APO

###us i~auag: remaining key fl0et~ne (k/ocr) : (4593674/2520)

###:v siae: 16 bytes

###replay detection support: y

###Status: ACTIVE

###outbound cup sus:

###transform: eup-res esp-shu-bmac

###on use settings )Tunnel,

###coma id: 2, ylowid: 816:2, cryptc map: IIAP1

###sa timong: remainong key###tfet:me (k/ocr) : (4148889/2101)

###IV sIre: 16 bytes

###repisy detection support: y

###Status: ACTIVE

Table 8: File Transfer Time in IPsec VPN Network

###File transfer successful, transferred 11,057,507 bytes in 254


###File transfer successful, transferred 11,057,507 bytes in 209



###File transfer successful, transferred 11,057,507 bytes in 198


###File transfer successful, transferred 11,057,507 bytes in 235



###File transfer successful, transferred 11,057,507 bytes in 270



###File transfer successful, transferred 11,057,507 bytes in 243



###File transfer successful, transferred 11,057,507 bytes in 233



File is downloaded from its FileZilla server to its client using IPsec VPN, minimum and maximum file transfer time is 198 seconds (3.3 minutes) and 270 seconds (4.5 minutes). Average time for file transfer in this experiment is 234.57 seconds (3.9 minutes) and standard deviation is 24.82 as shown in Table 8. Figure 9 shows the average file transfer time. Average File transfer time in IPsec VPN is more as compare to MPLS VPN network.

Figure 10 shows frame captured by Wire shark shows that 1500 bytes of data is available on the link. Instead of showing the IP addresses of end devices i.e. source address and destination address, IPsec tunnel mode presents the IP addresses of the tunnel end points. IPsec appends more overhead as compare to MPLS VPN; in tunnel mode the overhead imposed by IPsec is 50 to 57 bytes or 58 to 73 bytes depending upon which encryption algorithm is used. Figure 11 shows RTT history report.

Serial interface of CE1 is source and destination is the serial interface of CE2 router. RTT report shows an average RTT value of 546 milliseconds.


Simulators and emulators are playing vital role in different fields of science and technology. Simulators are admired by researchers and students due to the fact that, either access to the equipment is not available or time and finance is required for establishing the lab. So therefore they are used for research work and getting hands on experience in different technologies. MPLS VPN and IPsec VPN are implemented with the objective to generate traffic in GNS3 for comparison between the two techniques. Both techniques are successfully implemented. Average file download time in MPLS and IPsec VPN is 218.14 seconds and 234.57 seconds respectively. IPsec appends more overhead as compare to MPLS so due to this reason the time taken for file transfer in IPsec VPN network is more than the MPLS VPN implementation.

Finally according to OpManager report, the RTT average value is higher in IPsec VPN as compare to MPLS VPN, because IPsec provides security but it adds more overhead which may affect the network performance. The main drawback of GNS3 is that it utilizes a lot of system resources, for all the technologies implemented in this paper, the CPU usage was 100 % as reported by Windows task manager.


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Date:Jun 30, 2015

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