Multi Protocol Label
Switching
Presented by: Petros Ioannou
Dept. of Electrical and Computer Engineering, UCY
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What is MPLS?
Multi Protocol:
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Because it works with the Internet Protocol(IP),
Asynchronous Transport Mode(ATM) and Frame Relay
network protocols
Label Switching:
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Because it use fixed length label switching similar to ATM
or FR
MPLS forwards packets based on labels
MPLS simplifies and improve the forwarding function by
introducing a connection oriented mechanism inside the
connectionless IP networks
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Packets are switched, not routed
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Why MPLS?
IP Routing Disadvantages:
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Connectionless
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No QoS
Each router has to make independent forwarding
decisions based on the IP address
Routing in Network Layer
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Slower than switching
Usually designed to obtain shortest path
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Don’t take into account additional metrics
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Why MPLS?
Traffic Engineering
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Why MPLS?
ATM Advantages:
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Connection oriented
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Supports QoS
Fast packet switching with fixed length packets (cells)
Integration of different traffic types (voice, data, video)
ATM Disadvantages:
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Complex
Expensive
Not widely adopted
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Why MPLS?
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The Idea
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MPLS Label Format
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MPLS uses a 32-bit label field:
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20-bit label (a number)
3-bit experimental field (usually used to carry IP precedence
value)
1-bit bottom-of-stack indicator (indicates whether this is the
last label before the IP header)
8-bit TTL (equal to the TTL in the IP header)
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MPLS Label Format
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MPLS Terminology
LSP: Label Switched Path
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An MPLS virtual circuit
A path established before the data transmission starts
FEC: Forwarding Equivalence Class
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A group of IP packets which are forwarded in the same
manner (over the same path with the same forwarding
treatment)
LSR: Label Switching Router
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Any router in network who supports MPLS
LER: Label Edge Router
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Resides at the edge of an MPLS network and assigns and
removes the labels from the packets
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MPLS Operation
Ingress
Router
Egress
Router
PUSH
SWAP
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POP
Establishing a Label Switched
Path
• Each LSR negotiates a label for each
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Forwarding Equivalence Class (FEC) with its
neighbors using a distribution method
The result of negotiation is a Label Information
Base (LIB)
Each LSR maintains a Label Information Base
(LIB) and learns labels from there
When next hop changes for a FEC, LSR will
retrieve the label for the new next hop from the
LIB
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Label Distribution
Protocols
• Label Distribution Protocol (LDP)
• Hop-by-Hop label distribution
• Follows IGP-OSPF best path
• No traffic engineering capabilities
• Highly scalable
• Best suited for apps using thousands of LSPs (VPNs)
• Resource Reservation Protocol with Traffic Engineering Extensions (RSVPTE)
• End-to-End LSP signaling
• Enables different specifications on each path
• Less scalable
• Best suited for traffic engineering in the core
• Constraint-Based Routed LDP (CR-LDP)
• TE-capable LDP
• Never widely deployed
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Label Distribution Protocol
(LDP)
A protocol which it is used to map FECs to labels
In order to do that LDP sessions are established between LDP
peers in the MPLS network
LDP message types:
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discovery messages: announce and maintain the presence
of an LSR in a network
session messages: establish, maintain, and terminate
sessions between LDP peers
advertisement messages: create, change, and delete label
mappings for FECs
notification messages: provide advisory information and
signal error information
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Label Distribution Protocol
(LDP)
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Traffic Engineering
A way to achieve required delay, grade-of-service
and to meet policy requirements imposed by the
network operator
Traffic Engineering ensure available spare link
capacity for re-routing traffic on failure
In case of single failure the network traffic is
spread across network backup links
The LSPs are created independently, specifying
different paths that are based on user-defined
policies
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Traffic Engineering
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Resource Reservation Protocol with
Traffic Engineering Extensions
• Request bandwidth and(RSVP-TE)
traffic conditions on a defined path
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Uses two types of massages: RSVP PATH msg and RSVP RESERVATION
msg
Calculates best path based on the specified constraints
TE interface parameters:
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Maximum Bandwidth
Maximum Reservable Bandwidth
Unreserved Bandwidth
TE Metric (given by IGP protocol)
Administrative Group (Link Affinity or “Link Coloring”)
Drawback:
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Requires regular refreshes
Scalability
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Resource Reservation Protocol with
Traffic Engineering Extensions
(RSVP-TE)
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Benefits
from
MPLS
• MPLS combines the performance
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characteristics of layer 2 networks and the
connectivity and network services of layer 3
networks
Combines IP and ATM in the network
Improves packet-forwarding performance in
the network
Supports network scalability
Improves the possibilities for traffic engineering
Supports the delivery of services with QoS
guarantees
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Thank You!
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w
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