The document explains Multiprotocol Label Switching (MPLS) as an advanced IP routing technique that improves speed and efficiency by reducing routing lookups and the need for routing protocols on all devices. It details architectural components like provider routers, provider edge routers, and customer edge routers, as well as the MPLS header and the concept of Forwarding Equivalence Classes (FEC). Additionally, it describes the Label Distribution Protocol (LDP) used for sharing FEC/label bindings among routers to establish and maintain label-switched paths in a network.

1. Multiprotocol Label Switching (MPLS)
By Sumita Das
Isha Pandya

2. Traditional IP routing
Created by Sumita Das

3. IP routing
 Data is routed from source to destination through series of
routers
 Each router has routing table
 Disadvantages:
1. Routing protocols are used on all devices
2. Routing lookups are performed on every router
3. Each router in network makes an independent decision
when forwarding packets
Created by Sumita Das

4. IP Routing
4. connectionless. Eg. No QoS
5.Larger IP header
Why MPLS
1. Makes IP routing fast
2. Reduces number of routing lookups
3. Eliminates to run a particular routing protocol on all the
devices
Created by Sumita Das

5. Overview
Layer 2.5 protocol
Created by Sumita Das

6. Architecture
Components:
1. Provider router
2.Provider edge router
3.Customer edge router
1. Provider router/ Label switching routers
- Service provider’s router
-located in the middle of an MPLS network
-uses the label included in the packet header as an index to
determine the next hop
Created by Sumita Das

7. Architecture
2. Provider Edge Routers/ Label Edge Routers
-operates at the edge of an MPLS network
-entry and exit points for the network
-PUSH to add label on incoming packet
-POP to remove label from outgoing packet
3. Customer edge router
- routers on customer side
Created by Sumita Das

8. Architecture
Created by Sumita Das

9. MPLS Header
L2 Header MPLS Header IP Header
Label Value Exp S TTL
MPLS header : 32-bits (4 Bytes)
1. Label value:
-20 bits
- acts as index to routing table in the router
2. Exp:
-3 bit field
-Experimental purpose
Created by Sumita Das

10. MPLS Header
3. S:
-1 bit stack field
- tells whether the label is last label or not
if S=1, then it is the last label
4. TTL:
-8 bit field
-Each visited router decrement the value of this field
-When it reaches “0” packet is discarded
Created by Sumita Das

11. Forwarding Equivalence Class (FEC)
 To describe a set of packets with similar characteristics which
may be forwarded the same way.
 A packet’s FEC can be determined by one or more of the
following:
a. Source and/or destination IP address
b. Source and/or destination port number
c. Protocol ID
d. Differentiated services code point
e. Incoming interface
Created by Sumita Das

12. Forwarding Equivalence Class (FEC)
 The FEC can implicitly indicate such information, and routing
decisions in the network can automatically take that information
into account.
 A given FEC can force a packet to take a particular route through
the network.
Created by Sumita Das

13. Label-switched path (LSP)
 The path through a network is determined solely by the
FEC that applies at the point of ingress.
 A path is established before the data transmission starts.
Created by Sumita Das

14. Label Distribution Protocol(LDP)
 A protocol that enables the routers to automatically share
FEC/label bindings.
 The primary function of LDP-
Establishment and maintenance of LSPs
Created by Sumita Das

15. Upstream and Downstream LSRs
 Rtr-C is the downstream neighbor of Rtr-B for destination
171.68.10/24
 Rtr-B is the downstream neighbor of Rtr-A for destination
171.68.10/24
• LSRs know their downstream neighbors through the IP
routing protocol
 Next-hop address is the downstream neighbor
Created by Sumita Das

16.  The downstream LSR uses a label distribution protocol to
communicate the binding to the upstream LSR.
 Two ways-
1. Unsolicited downstream distribution
A downstream LSR can directly distribute a label/FEC binding to
an upstream LSR
Created by Sumita Das

17. 2. On-demand downstream distribution
 Upstream LSRs request labels to downstream neighbors
 Downstream LSRs distribute labels upon request
Created by Sumita Das

18.  In order for a downstream LSR to communicate a label/FEC
binding to an upstream LSR, the label distribution protocol
messages are sent.
 The message includes
 specific FEC values and
 label values.
Created by Sumita Das

19. LDP Messages
LDP offers four categories of messages:
1. Discovery messages
2. Session Management messages
3. Advertisement messages
4. Notification messages
Discovery messages
• Used to discover and maintain the presence of new peers
• Hello packets sent to all-routers multicast address
• Receiver of hello message replies with a hello message.
• Both LSRs are aware of each other’s presence.
Created by Sumita Das

20. Session messages
 To establish, maintain, and terminate sessions between LDP
peers.
 To exchange Initialization messages.
 An Initialization message includes receiver’s LDP identifier
and other optional parameters.
 Once two LSRs have exchanged Initialization messages, an
LDP session is established between them.
Advertisement messages
 To create, change, and delete label mappings for FECs.
Notification messages
 Used to provide advisory information and signal error
information.Created by Sumita Das

21. References
Created by Sumita Das
[1] Richard Swale,”Carrier Grade Voice Over IP”, Third
Edition

22. Thank You
Created by Sumita Das