International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
A Design Analysis of MPLS VPN Core
Architecture and Network Downtime Impact
Ammar Al Mhdawi
Research Scholar, USA
Abstract- Multi Protocol Label Switching (MPLS) is a
protocol that enables packet switching layer 2. Each
pack gets a label when enters into the service provider
network by the ingress routers (IR). Each router in the
SP environment will replace the labels. The routers will
never look at the packet header for an ip address, and at
the end of the SP network, the egress router will pop the
label and forward the packet to the customer. In this
study we are going to discuss also the VRF concept and
how is it being implemented in the PE routers [2].
Label and Distribution Protocol (LDP). LDP is in
charge of managing the neighbour relationship between
routers that participate in MPLS routing [2].
Index Terms- VRF, SP, BGP, MPLS, LDP, VPNv4,
Packet Loss, Throughput Rate, LSR, LIR, LER.
Fig 1. MPLS in OSI model
I. INTRODUCTION
M
PLS network is a virtual private network that
have a significant impact and deployment in SP
environment. MPLS is based on the virtual private
networks (VPN) which is a technology that connect two
networks in a one to one connection virtually with
encryption enabled as well on the data. All customers
need is to subscribe with the SP for a vpn service for
branch to branch connection. Leasing a dedicated line
with the SP for a specific customer can be very
expensive so customer basically connect to the PE
(provider edge) router [3], which is going to provide
labelling for each packet that enters the network. Now,
if we imagine that having many customers on the same
PE router is kind of risky because some customers want
to keep their routing table private and the other issue is
routing leaks that may happen from one customer to
another will cause security risks and issues with routing,
so to resolve this, the virtual routing and forwarding
concept is introduced. VRF [2] basically separates
routing tables virtually so for example, customer A will
have VRF-1 and customer B will have VRF-2 and so on,
and all this is done on the PE router customer side or
link only.
II. COMPONENTS OF MPLS NETWORK
MPLS network is considered to be between the layer 2
and layer 3 and it is often referred to as layer 2.5
because it combines switching and routing techniques
[3]. The component of MPLS network is the Label
Edge Router (LER), Label Switch Router (LSR) and
ISSN: 2231-5381
Fig 2. MPLS Core Network
The label Ingres Router (LIR) is set on the customer
side which is represented with PE1 in Fig 2. It basically
assigns a label for the incoming packet from the
customer. The LER which is on the other side of the SP
network that does the routing to the specific customer.
The LSR has many functions such as removal of the
label which have a specific feature called PHP, and
swap the label with another label.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
III. VIRTUAL ROUTING AND
FORWARDING
VRF is implemented in the provider edge (PE) routers.
VRF table have the right information that provides
reachability to the other side of the MPLS network
using VPN tunnels. The interface that is facing the
customer must be configured with VRF such as the
following on cisco IOS:
Cust_A#conf t
Cust_A#(config-if) int TenGig 1/1
Cust_A#(config-if) ip vrf forwarding A-1
Note: this config will remove the ip address that is
associated with Tengig 1/1, so it has to be entered again
[1]
Each VRF must associated with a route distinguisher
value. For example, if Cust_A, Cust_B and Cust_C are
advertising a 10.10.10.10 network, the only was that the
other side of the MPLS network to know where to
forward the traffic to what customer is by using the
route distinguisher (RD). RD is a 64 bit in length, there
are three types of it, type 0, 1 and 2. RD is just a flat
number that is prepended to a route [5]. Below is
example of configs showing RD assigned to VRFs using
the two byte AS format [4].
Ip vrf Cust_A
Rd 65002:20
!
Ip vrf Cust_B
Rd 65002:30
!
Ip vrf Cust_C
Rd 65002:40
!
Fig 3. Wireshark capture showing RT value
IV. LABLES FORWARDING PROCESS
For the MPLS to be forwarding packets, LDP must be
enabled on each single router that part of the SP
environment. The table below shows the label that
assigned to each incoming route.
Table 1. MPLS forwarding table with the local and
outgoing tags
When vpn routes are advertised among PE routes, the
RD value will be included as part of the route. For
example, a route to 192.168.10.2/24 in VRF Cust_C is
advertised as 65002:40:192.168.10.2/24. The other
concept is the route target (RT). They are applied to a
VRF to control the import and export of routing within
customers. The configuration is below:
Ip vrf Cust_C
Rd 65002:40
Route-target export 65000:100
Routes that are contained in VRF Cust_C will be
exported using this value 65000:100. Wireshark pack
capture examines the route target value as below:
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
V. MPLS CONNECTION ISSUES
Some of the common problem that may occur in the
MPLA SP environment is:
1- Inactive LDP session (not started)
2- The labels are distributed but packets are not
labled
3- LDP session started but no labels being
assigned
To solve these issues, a network engineer must examine
the SP routers using the following steps:
1- Check LDP is enabled on the routers and check
neighbours
2- Check for Cisco Express Forwarding (CEF) if
enabled or not as it is very important for LDP
activation process.
3- Make sure MPLS is enabled on every interface
4- Check for MPLS bindings and verify if they
are exchanged.
failure could happen so having redundant links is
essential to pick up traffic in case of link outages. The
business side may get effected when there is an outage.
It could lead to losing revenue and reputation damage.
Downtime cost may vary significantly within industries.
Business size is the most obvious factor.
Misconfiguration is the cause of the 60% of the outages
that may occur in the SP environment due to changes
made by engineers or network admins. Manual
configuration mistakes can cost companies thousands of
dollars per hour [7]. The graph below shows the cost
consequences of an outage:
Another issue that could lead to MPLS fail is MTU
issues, were switches with no support of jumbo frames
are up on the forwarding route. To make sure that the
labels are being exchanged to reach the destination host,
MPLS-aware trace route functionality must be used.
Fig 5. Cost loss of a network outage for 41
companies (taken from reference [7] )
VI. LATENCY AND PACKET LOSS
In a global network operation, WAN may carry
applications that could be sensitive and critical to
latency. For example, video calls and financial
transactions are very sensitive to time. In reality, the
distance of locations will always effect the throughput
rate [6]. The greater the latency, the more delay that will
be in the networks. In Fig 6., we can see the distance
effect on the delay within cities and countries.
Table 2. Latency of MPLS connections in
different cities (taken from reference [6]).
Fig 4. SP Network Architecture example
As we can see in Fig 4. Above, there are many
redundant link and connections. Many link flaps and
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
When considering throughput rate, it is always
important to know the packet loss rate and the delay
when calculating the throughput. For example, for the
latency of 200 ms rate and with 0.01% packet loss, the
typical throughput is 0.73Mbps, but if we have 0.20%
packet loss, then the throughput will drop to 0.16 Mbps,
which is about 78% reductions from the previous value.
Now we can see how the pack loss effect the throughput
rate dramatically. In Fig 6. We can see the packet loss
graph for different PL values.
[2] Seno, Rahardianto., 2010., Perancangan dan Penerapan
Technology Multi-Protocol Label Switching Pada Jaringan
Telekomunikasi Indosat
[3] Fitzgerald, Denis., 2012., Business Data Communications
& Networking 11th Edition.
[4] Osborne, Simha., 2002., Traffic Engineering with MPLS.
[5] Ravi Ganesh V, M. V. Ramana Murthy . MPLS Traffic
Engineering (An Implementation Framework), 2012.
[6] Steve Garson., 2014., Low Latency Solutions., MPLSExperts.com
[7] Evolven.com., 2014. Confronting system downtime.
[8] Archana C” evaluation of RIPv2, OSPF, EIGRP
Configuration on router the usage of CISCO Packet tracer”,
International journal of Engineering science and innovative
technology (IJESIT) volume 4, issue 2, pp. 215-222, March
2015
[9] www.cisco.com
AUTHORS
First Author – Ammar Al Mhdawi, Research Scholar
and Network Engineer, MS Information Technology
and Network Security, AIU, USA. BS in Computer
Engineering, UOB.
Fig 6. PL and throughput rate comparison
(taken from reference [6]).
VII. CONCLUSION
In this paper, we have discussed the internal
architecture of MPLS network. VRF has been discussed
and provided the benefits of it. Explained RT and RD
tags and their importance to the routes advertised by
each customer. MPLS networks may face many issues
such as LDP unpredicted states. Troubleshooting is
required to reduce downtime that may cause loss of
thousands of dollars in businesses. We can also say that
MPLS VPN is scalable, efficient in terms of router CPU
and BW usage. It reduces the time look up into the
routing table using the CEF. For future work, more indepth analysis need to be discussed in the MPLS
network as well as the quality of service effect on
different customers.
REFERENCES
[1] Pultz, Richard., 2004., Analysis of MPLS-Based IP VPN
Security: Comparison to Traditional L2VPNs such as ATM
and Frame Relay, and Deployment Guidelines.
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