MPLS
MPLS Introduction:












MPLS stands for Multiprotocol Label Switching, Multiprotocol because it can be
used for any Layer 3 protocol
MPLS is about glueing connectionless IP to a connection oriented Network.
MPLS is something between L2 and L3.
MPLS is also called Tag Switching
MPLS provides a mechanism to avoid hop-by-hop routing decision making (
Notice that IP makes a hop-by-hop routing decisions) by setting up a Layer 2 fast
path using Labels ( hence the name Label Switching) to move the packets quickly
along pre-established paths without examining each packet at the IP level at each
router.
This is very similar to ATM and FR Layer 2 routing and switching operations.
Remember VPs and VCs in ATM and DLCI #s in FR operate at Layer 2, have
only local significance and established before hand.
In MPLS enabled Network, the packet needs to be examined only once at the
edge of the MPLS Network ( Network entry point). After that, the packet
forwarding is based on simple tagging scheme rather than on more complex and
variable IP header.
The most important advantage of MPLS is that it is independent of L2 and L3
protocols, and so it can be adopted any where in the world to any L2 or L3
infrastructure.
The Canadian Post Office seems to work on this principle. The mail is forwarded
to a regional processing center where the hand writing recognition is done only
once. Some kind of infra-red or Ultraviolet bar code is applied at the bottom of
the envelope (Label) and there on, only the bar code is used to route the letter.
Following Cisco Routers employ Label or Tag Switching: Cisco 7000 series
Routers, 12000 GSR Series, LS 1010 Series etc.
MPLS is now increasingly being deployed in Optical Networking as well.
MPLS is being deployed in the following Media:
High speed IP backbones
Legacy ATM
MPLS capable ATM
Optical Networking
 AT&T is offering a service called IPFR as the IP access via FR to an MPLS
Network
 MCI also offers a similar service.
 Many corporations are upgrading their legacy FR Network to MPLS
Hyder Khoja
Page 1
3/7/2016
MPLS Analysis:

Consider the Fig shown:

Before we can understand this figure, we must clearly understand various terms
and definitions associated with an MPLS domain Network.
Label Switching Router (LSR):A router supporting MPLS Protocol is called a
Label Switching Router (LSR)
Label Switching Edge Router (LSER or LER): An edge LSR connects to a
non-LSR router. As shown in the figure. Situated at the edge of an MPLS
Network ( MPLS Network Entry point).
INGRESS LSR: An Ingress Router is one by which a packet enters the MPLS
Network
EGRESS LSR: An Egress LSR is one by which a packet leaves the MPLS
Network
Labels: Labels are small identifiers placed between the L2 and L3 Headers by the
ingress LSR and ultimately removed by the egress LSR, as shown in the fig
below; thus the non-MPLS devices outside the MPLS Network will not see these
Labels.






For IP based MPLS, these Labels are inserted just before the IP Header and after
the frame Header.
Hyder Khoja
Page 2
3/7/2016




For ATM, the VPI/VCI addressing field become the Label
For FR, the ________________ value become the Label ( Ans: DLCI)
Label Information Base (LIB); A routing table based on labels
The LIB table contains the inbound to outbound interface mappings for each
destination.
 As traffic transits through the Network, label tables are consulted in MPLS router
to map the inbound label to the outbound label and the corresponding interface.
This is very similar to VPI/VCI mappings in the ATM Network or __________
mappings in the FR Network or _____________ mappings in the X.25 Network
 Each LSR looks at the inbound label, determines the corresponding outbound
label and the out interface from the table, replaces the old label with the new label
and forwards the packet to that outbound interface.
 This is very similar to how ATM, FR and X.25 Networks route traffic through a
virtual circuit..
 The labels are locally significant only as the DLCI values in FR, meaning that the
labels are only useful and relevant on a single link between the adjacent LSRs.
 Thus the adjacent LSRS label table form a LABEL SWITCHED PATH
(LSP)through an MPLS Network based on a Forwarding Equivalency Class
(FEC)
Forwarding Equivalency Class (FEC):



FEC is a very important concept in MPLS.
FEC refers to the idea that all packets belonging to a specific Class Of Service
(COS) or a Specific set of Quality of Service (QoS) requirements be forwarded
to the same next hop or along the same MPLS path.
Thus FEC is all packets to which a specific label is attached. The FEC for a
packet can be determined by:
Source or Destination IP Addresses or IP Network Addresses
Source or Destination Port Numbers
IP Protocol ID
Differentiated Service Code point
Specific COS or QOS requirements (Such as low latency, low packet loss, low
jitter etc
Specific flow requirements (Queuing and discard policies etc)



LSPS are pre established based on the FEC requirements of the customer at the
time of session negotiation by the ingress Edge Router using Label Distribution
Protocol(LDP), RSVP etc and communicated to all LSRS backward from the
Egress Router to the Ingress router. Each router in this newly created Label
Switched Path update their LIBS based on this new information.
Binding: is the process of assigning labels to specific FEC
A Label Distribution Protocol (LDP): Allows MPLS LSRs to bind labels to a
specific FEC and communicate this binding to other LSRs in the path and
subsequently pre-establish a Label Switched Path (LSP) for the incoming traffic.
Hyder Khoja
Page 3
3/7/2016





Thus LDP is an official way for one LSR to say to another LSR “ lets use this
Label to get the packets with this FEC to this destination really fast”.
Thus LDP establishes a Label Switched Path (LSP) between ingress LSRs and
egress LSRs for a given FEC.
The Labels are bound to an FEC based on the following criteria:
Destination unicast Routing (simple Routing)
Traffic Engineering considerations such as congestion, queuing or some policy
requirements or service requirements
QoS requirements such as low latency, BW, low packet loss, low jitter (for Voice
and Video transmission)
VPN Tunnelling, Security, privacy, Data integrity etc
Label Merging: Notice also that the incoming traffic from different interfaces
can be merged together and switched using a common label, if they are travelling
to the same final destination or they belong to the same FEC.
This is how an MPLS Network guarantees high speed L2 Switching and QoS
without the complexity and cost of an ATM Network.
The relationship between the FEC, LSP and the Labels:




The essence of MPLS is that the traffic is grouped into FECs. The individual
packets in an FEC are uniquely identified as being part of a given FEC by means
of a locally significant Label.
The traffic in an FEC transits an MPLS domain along a pre-established LSP that
satisfies the requirements of the given FEC
At each LSR, each labelled packet is forwarded on the basis of its label value,
with LSR replacing the incoming label value with outgoing label value.
This process requires 3 operational steps to be carried out before the actual
transmission of packets take place.
1. Traffic must be assigned to a particular FEC.
2. A routing Protocol is needed to determine the topology and current conditions
in the MPLS domain so that a a particular LSP can be assigned to the given
FEC. This Routing Protocol must be able to gather and use information to
support the QoS requirements of the given FEC.
3. Individual LSRs must become aware of the LSP just established for the given
FEC, must assign an incoming Label to the LSP, and must communicate that
label to any other LSR that may send it packets for this FEC.


Traffic Assignment to an FEC: The assignment of traffic to a particular FEC is
done either by manual configuration (in other words, the Network Manager
manually tells the LSRs that this type of packet belongs to this FEC), or by means
of a signalling protocol or by an analysis of the incoming packets at ingress LSRs.
Route Selection: Route selection refers to the selection of an LSP for a particular
FEC. The MPLS architecture supports 2 options
Hyder Khoja
Page 4
3/7/2016
1. Hop-by-Hop Routing: In this case each LSR independently chooses the
next hop for each FEC. The LSRs use existing OSPF protocol to make the
routing decisions. This method allows rapid switching by labels and
support for differentiated service. How ever it does not support traffic
Engineering and policy Routing (Policy Routing refers to defining Routes
on some policy related to QoS, Security, Queuing and discard mechanisms
etc).
2. Explicit Routing: In this kind of Routing, a single LSR, usually the
ingress or egress LSR, specifies some or all of the LSRs in the LSP for a
given FEC. Explicit Routing provides all the benefits of MPLS, including
Traffic Engineering and policy Routing. Explicit Routing can be of 2
types.
a) Static Explicit Routing: The Ingress or Egress LSRs are
preconfigured to analyze the packets as they entered the MPLS
domain, in other words, the LSPs are setup ahead of time for all
kinds of FECs.
b) Dynamic Explicit Routing: The LSPs are determined
dynamically at the time of arrival of each packet. In this case, the
LSR setting up the LSP would require information about the
topology of the MPLS domain and the QoS related information
pertaining to the given FEC.

A routing Algorithm that accounts for the traffic requirements of various flows
and the resources available along various hops and through various nodes is called
a CONSTRAINED BASD ROUTING .
 Dynamic Routing uses LDP ( Label Distribution Protocol and RSVP- the
Resource Reservation Protocol) to calculate an LSP corresponding to a given
FEC. These protocols gather information regarding
Maximum link data rate
Current capacity reservation
Packet loss Ratio
Link propagation Delay
And other parameters, to calculate the Route along the LSP.
Label Distribution Protocol: Performs following functions:




Assigns a Label to the LSP to be used to recognize incoming packets that belong
to the given FEC(done by manual configuration)
Informs all potential upstream nodes of the label assigned by this LSR to this
FEC, so that these nodes can properly label packets to be sent to this LSR
Learns the next hop for this LSP and learn the Label that the downstream node
has assigned to this FEC.
Thus the essence of the Label Distribution protocol is to allow one LSR to inform
others of the Label /FEC bindings it has made. It also allows 2 LSRS to learn each
others MPLS capabilities
Hyder Khoja
Page 5
3/7/2016
MPLS Header:

MPLS sticks a 32 bit Header as shown below:

Label (20 Bits): Carries information about setting up the LSP based on a given
FEC. Either Manual configuration or inserted on the basis of static or dynamic
routing protocol
COS (3 bits):Defines the required Class of Service. With 3 bits, 8 possible
classes. Affects the Queuing and discard algorithms as the packet travels through
the Network. Some times it is copied from the IP Header TOS field.
S (STACK) ( 1 BIT): MPLS supports multiple Labels. The processing of
labelled stack is always based on the top stack. We shall study this feature of
MPLS in more detail shortly.
TTL (Time To Live) ( 8 bits): Same function as TTL in the IP Header. The
packet is killed after travelling the specified number of hops and if the packet
has still not reached is destination.



Label Stack Operation:






One of the most powerful features of MPLS is Label Stacking.
A Labelled packet may carry many labels, organized as a last in –first out stack
Processing is always based on the top label
At any LSR, a label may be added to the stack (push operation), or removed from
the stack (pop operation)
Label Stacking allows wrapping or enveloping several LSPs into a single LSP for
the portion of the route through a Network, effectively creating a tunnel. See
picture below
At the beginning of he tunnel, an LSR assigns the same label to to packets from a
number of LSRs by pushing the label onto the stack of each packet.
Hyder Khoja
Page 6
3/7/2016


At the end of the tunnel, another LESR pops the top label from the label stack,
exposing the inner labelbelow the top label.
This is very similar to ATM, a VC within a VP, however, MPLS supports
unlimited stacking.
Consider the Fig shown.

The concept will be explained with a concrete example. Look at the Fig shown:


LSP1 consists of LER1 to LER2 to LER3 to LER4.
LER1, LER2, LER3,LER4 (the edge routers) use BGP to create LSP1 between
themselves. LER1 knows that its next destination is LER2, as it is transporting
data through 2 different Networks or Network Segments
Similarly, LER2 is aware that LER3 is its next destination and LER3 knows that
LER4 is its next hop and so on. This is established via BGP (the Border Gateway
Protocol)
Also, they have established LSP1 between the source A and destination B using
LDP
LDP allows LER1 to receive and store labels from LER4 to LER3 to LER2 to
LER1, using following LDP messages, prior to actual transmission.




Hyder Khoja
Page 7
3/7/2016
Discovery message: Announces and maintains the presence of an LSR in a Network.
Session Message: Establishes, maintains terminates sessions between LDP enabled
LSRs.
Advertisement Message: Creates, changes, modifies, and deletes labels based on a
given FEC:
Notification Messages: Provides advisory information and signal error information.


However, for LER1 to send data to LER2, it must go through several intermediate
LSRs within Netwok1 (LSR1, LSR2, LSR3).
Therefore a separate LSP (LSP2) is created between the 2 edge routers (LER1 and
LER2), over the 3 inner LSRs (LSR1,LSR2,LSR3)
This in effect, represents a tunnel between the 2 outer LERs (LER1 and LER2).
The Labels in this path are different from the Labels that outer LERs created for
LSP1.
This also holds true for LER3 and LER4. An LSP3 is created to to transport
packets over LSR4, LSR5 and LSR6.
To achieve this, the Label Stacking is used to transport packets through different
autonomous Networks or through different Network Segments.
Thus, as a packet travels through LSP1, LSP2, LSP3, it will carry2 complete
labels at a time
For the first segment, LSP1 and LSP2 labels and for the 2nd segment, LSP1 and
LSP3 labels.
When the Packet exits the first Network, LER3 will do 2 things


1. It will remove the label for LSP2 and replace it with the label for LSP3, and
2. At the same time, it will swap LSP1 label with the packet with the next hop
label for LER4 ( which is its next hop)
LER4 will remove both labels before sending it to the destination
This feature is extensively used in MPLS based VPNS







MPLS Operation:
Following steps must be taken for Data to travel through an MPLS Network
(a)
(b)
(c)
(d)
(e)


Label creation and distribution
Table creation at each MPLS enabled Router
LSP creation
Label insertion /Table lookup
Packet forwarding based on the table lookup
Consider the fig shown:
In this fig, LER1 is the INGRESS Router and LER4 is the EGRESS Router.
Hyder Khoja
Page 8
3/7/2016
(a) Label Creation:




Before any traffic begins, the routers make decisions to bind a label to a
specific FEC and build their tables
In LDP, downstream routers initiate the distribution of labels and the
label/FEC binding.
Also, traffic related characteristics and MPLS capabilities are negotiated using
LDP
LDP uses TCP for these signalling messages to ensure reliability and
accuracy.
(b) Table Creation:

On receipt of Label bindings from the downstream LSR, each LSR creates entries
in their Label Information Base (LIB)
 The content of the table will specify the mapping between a label and and an FEC
 This label Routing table contains the mapping between the input label and input
port to the output label and output port
 Notice that these entries are updated whenever renegotiation of the label binding
occurs.
© LSP Creation:

As shown in the fig by the dashed line, the LSPS are created in the reverse
direction to the creation of entries in the LIBS
(c) Label Insertion/Table lookup:



The first Router (LER1), uses the LIB table to to find the Label for the specific
FEC and the corresponding next hop.
The subsequent reouters just use the Label to find the next hop
Once the packet reaches the EGRESS Router LER4, the label is removed and the
packet is forwarded to the destination
Hyder Khoja
Page 9
3/7/2016
(d) Packet Forwarding Process:
With reference to the Fig shown above, let us trace the path of a packet as it travels
from the LER1 to LER4.









LER1 does not have a label for this packet yet, as it is the first occurrence of this
request.
LER1 initiates a Label request for this packet using LDP or CR-LDP, which
propagates through the Network from LER1 to LSR1 to LSR2 to LSR3 to LER4
as shown by the dashed line.
Each intermediary Router will receive a Label from its downstream router starting
from LER4 and going upstream till LER1, thus setting up an LSP for this packet
in the reverse direction (from LER1 to LER4).
LER1 will insert the Label and forward the packet to LSR1
LSR1 will examine the Label in the received packet, consults its Label Routing
table, sticks a new Label with the mapping it finds in the table and forwards the
packet to the output port specified in the table and forwards the packet to LSR2
Each subsequent Router will repeat the process until it reaches the EGRESS LSR
LER4.
When the Packet reaches the LER4, it will remove the Label and deliver it to the
destination.
Fig above shows the actual data path followed by the packet.
Fig below shows the Routing table
Input Port
1
2






Incoming Port Label Output Port
3
3
9
1
Outgoing Port Label
6
7
The table above shows how 2 different packet streams are routed differently. The
stream coming on input port 1 is a regular FTP stream, whereas one coming on
input port 2 is an intensive video stream requiring traffic Engineering QoS (low
latency, low jitter, low packet loss etc)
These packet streams are classified into 2 FECs at the ingress LSR LER1
The label mappings associated with the streams are 3 and 9 respectively
The input port at the LSR are 1 and 2 respectively
The corresponding output interfaces are 3 and 1 respectively
Label swapping is done and the previous labels are exchanged for 6 and 7
respectively.
MPLS Protocol Stack Architecture:
Fig below shows MPLS Protocol Stack.
Hyder Khoja
Page 10
3/7/2016





Routing Module can be OSPF, BGP, or ATM PNNI.
The LDP/CR-LDP uses TCP for reliable transmission of control data from one
LSR to another LSR.
However during the discovery phase of its operation, LDP uses UDP. In this
phase, the LSRs try to identify their neighbours and also announce their own
presence. This is done through the exchange of Hello Packets.
The LDP is also responsible for creating, updating and maintaining the LIBS.
Notice that the Layers shown in the box with the broken lines, can be
implemented in hardware for fast, efficient switching.
MPLS Applications:
1. Fast Layer 2 Switching: Increases Network Performance because it allows routing
by switching at fast wireline speeds. Allows easy implementation
2. Supports traffic engineering, QoS, and COS differentiation
3. Supports Network Scalability
4. Integrates IP and ATM: Provides a bridge between access IP and Core ATM
5. MPLS can reuse existing Router/ ATM Switch Hardware, effectively joining the
two disparate Networks
6. MPLS builds interoperable Networks.
7. MPLS facilitates IP-over SONET integration in optical switching
8. MPLS helps build scalable VPNs with Traffic Engineering capability
Hyder Khoja
Page 11
3/7/2016