MPLS ConceptsIntroducing Basic MPLS Concepts doc

© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-1
MPLS Concepts
Introducing Basic MPLS Concepts
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-2
Outline
• Overview
• What Are the Foundations of Traditional IP Routing?
• Basic MPLS Features
• Benefits of MPLS
• What Are the MPLS Architecture Components?
• What Are LSRs?
• Summary
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-3
Foundations of Traditional IP Routing
• Routing protocols are used to distribute Layer 3
routing information.
• Forwarding decision is made based on:
– Packet header
– Local routing table
• Routing lookups are independently performed at
every hop.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-4
Traditional IP Routing
• Every router may need full Internet routing
information.
• Destination-based routing lookup is needed on
every hop.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-5
Basic MPLS Features
• MPLS leverages both IP routing and CEF
switching.

• MPLS is a forwarding mechanism in which packets
are forwarded based on labels.
• MPLS was designed to support multiple Layer 3
protocols
• Typically, MPLS labels correspond to destination
networks (equivalent to traditional IP forwarding).

© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-6
MPLS Architecture: Control Plane
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MPLS Architecture: Data Plane
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MPLS Devices: LSRs
• The LSR forwards labeled packets in the MPLS domain.
• The edge LSR forwards labeled packets in the MPLS domain,
and it forwards IP packets into and out of the MPLS domain.
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Label Switch Routers:
Architecture of LSRs
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LSR Architecture Example
MPLS router functionality is divided into two major
parts: the control plane and the data plane.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-11
LSRs:
Architecture of Edge LSRs
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Basic MPLS Example
• MPLS core routers swap labels and forward packets based on simple
label lookups.

• MPLS edge routers also perform a routing table lookup, and add or
remove labels.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-13
MPLS Labels
• Are 4 byte identifiers used for forwarding
decisions
• Define the destination and services for a packet
• Identify a forwarding equivalence class (FEC)
• Have local significance
– Each LSR independently maps a label to an FEC
in a label binding.
– Label bindings are exchanged between LSRs.


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MPLS Labels: Frame-Mode MPLS
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MPLS Label Operations: Frame Mode
•
On ingress, a label is assigned and imposed.
•
LSRs in the core swap labels based on the contents of the label forwarding table.
•
On egress, the label is removed and a routing lookup is used to forward the packet.
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MPLS Unicast IP Routing
Architecture (Cont.)
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MPLS Unicast IP Routing

Architecture (Cont.)
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MPLS Unicast IP Routing
Architecture (Cont.)
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LSP Building
The IP routing protocol determines the path.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-20
LSP Building (Cont.)
LDP propagates labels to convert the path to an LSP.
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PHP: Before
• Double lookup is not an optimal way of
forwarding labeled packets.
• A label can be removed one
hop earlier.
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PHP: After
A label is removed on the router before the last
hop within an MPLS domain.
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Label Distribution and Advertisement
The allocated label is advertised to all neighbor LSRs, regardless
of whether the neighbors are upstream or downstream LSRs for
the destination.
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Label Distribution and Advertisement:
Receiving Label Advertisement
• Every LSR stores the received label in its LIB.
• Edge LSRs that receive the label from their

next hop also store the label information in
the FIB.
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Label Distribution and Advertisement:
Interim Packet Propagation
Forwarded IP packets are labeled only on the path segments
where the labels have already been assigned.