MPLS Concepts
Introducing Basic MPLS Concepts
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-1
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—1-2
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—1-3
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—1-4
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—1-5
Benefits of MPLS
• MPLS supports multiple applications including:
– Unicast and multicast IP routing
– VPN
– TE
– QoS
– AToM
• MPLS decreases forwarding overhead on core
routers.
• MPLS can support forwarding of non-IP protocols.
© 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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-9
Label Switch Routers:
Architecture of LSRs
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-10
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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-12
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
Summary
• Traditional IP routing forwards packets based on the
destination address.
• MPLS forwards packets based on labels.
• MPLS supports multiple applications.
• MPLS has two major architectural components:
– Control plane (exchanges routing information, exchanges
labels)
– Data plane (forwards packets)
• LSRs implement label exchange protocols and primarily
forward packets based on labels. The role of Edge LSRs is
primarily to forward packets into and out of the MPLS
domain.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-14
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-15
MPLS Concepts
Introducing MPLS Labels and Label Stacks
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-1
Outline
• Overview
• What Are MPLS Labels?
• What Is the MPLS Label Format?
• Where Are MPLS Labels Inserted?
• What Is an MPLS Label Stack?
• What Are MPLS Label Operations?
• Summary
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-2
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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-3
FEC and MPLS Forwarding
• An FEC is a group of packets forwarded:
– In the same manner
– Over the same path
– With the same forwarding treatment
• MPLS packet forwarding consists of:
– Assigning a packet to a specific FEC
– Determining the next hop of each FEC
• MPLS forwarding is connection-oriented.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-4
MPLS Label Format
MPLS uses a 32-bit label field that contains the
information that follows:
• 20-bit label (a number)
• 3-bit experimental field (typically 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)
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-5
MPLS Labels
• MPLS technology is intended to be used anywhere
regardless of Layer 1 media and Layer 2
encapsulation.
• Frame-mode MPLS is MPLS over a frame-based
Layer 2 encapsulation
– The label is inserted between the Layer 2 and
Layer 3 headers.
• Cell-mode MPLS is MPLS over ATM.
– The fields in the ATM header are used as the
label.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-6
MPLS Labels: Frame-Mode MPLS
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-7
MPLS Label Stack
• Usually only one label is assigned to a packet, but
multiple labels in a label stack are supported.
• These scenarios may produce more than one label:
– MPLS VPNs (two labels): The top label points to the
egress router, and the second label identifies the VPN.
– MPLS TE (two or more labels): The top label points to
the endpoint of the traffic engineering tunnel and the
second label points to the destination.
– MPLS VPNs combined with MPLS TE (three or more
labels).
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-8
Example: MPLS Label Stack
• The outer label is used for switching the packet in the MPLS
network (points to the TE destination).
• Inner labels are used to separate packets at egress points (points
to egress router and identifies VPN).
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-9
Example: MPLS Label Stack Format
• The PID in a Layer 2 header specifies that the payload starts
with a label (labels) followed by an IP header.
• The bottom-of-stack bit indicates whether the label is the last
label in the stack.
• The receiving router uses the top label only.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-10
MPLS Label Operations
• An LSR can perform these functions:
– Insert (impose or push) a label or a stack of
labels on ingress edge LSR
– Swap a label with a next-hop label or a stack of
labels in the core
– Remove (pop) a label on egress edge LSR
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-11
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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-12
Summary
• An MPLS label is a 4 byte identifier used for
forwarding decisions.
– A MPLS label corresponds to an FEC.
• MPLS frame-mode labels are inserted between the
Layer 2 and Layer 3 headers.
• MPLS supports multiple labels in one packet,
creating a label stack.
• LSRs can perform these operations:
– Insert (impose) a label on ingress edge LSR
– Swap a label
– Remove (pop) a label on egress edge LSR
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-13
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-14
MPLS Concepts
Identifying MPLS Applications
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-1
Outline
• Overview
• What Applications Are Used with MPLS?
• What Is Unicast IP Routing?
• What Is Multicast IP Routing?
• What are MPLS VPNs?
• What Is MPLS TE
• What Is MPLS QoS?
• What is AToM?
• What Are the Interactions Between MPLS Applications?
• Summary
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-2
MPLS Applications
• MPLS is already used in many different
applications:
– Unicast IP routing
– Multicast IP routing
– MPLS TE
– QoS
– MPLS VPNs (course focus)
– AToM
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-3
MPLS Unicast IP Routing
• Basic MPLS service supports unicast IP routing.
• MPLS unicast IP routing provides enhancement over
traditional IP routing.
– The ability to use labels for packet forwarding:
• Label-based forwarding provides greater efficiency.
• The FEC corresponds to a destination address stored
in the IP routing table.
• Labels support connection-oriented services.
– The capability to carry a stack of labels assigned to a
packet:
• Label stacks allow implementation of enhanced
applications.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-4
MPLS Multicast IP Routing
• MPLS can also support multicast IP routing:
– A dedicated protocol is not needed to support multicast
traffic across an MPLS domain.
– Cisco Protocol Independent Multicast Version 2 with
extensions for MPLS is used to propagate routing
information and labels.
– The FEC is equal to a destination multicast address stored
in the multicast routing table.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-5
MPLS VPNs
• MPLS VPNs are highly scaleable and support IP services
such as:
– Multicast
– Quality of QoS
– Telephony support within a VPN
– Centralized services including content and web hosting to a VPN
• Networks are learned via an IGP from a customer or via BGP
from other MPLS backbone routers.
• Labels are propagated via MP-BGP. Two labels are used:
– The top label points to the egress router.
– The second label identifies the outgoing interface on
the egress router or a routing table where a routing lookup is
performed.
• FEC is equivalent to a VPN site descriptor or VPN routing table.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-6
MPLS TE
• MPLS TE supports constraints-based routing
• MPLS TE enables the network administrator to
– Control traffic flow in the network
– Reduce congestion in the network
– Make best use of network resources
• MPLS TE requires OSPF or IS-IS with extensions to hold the
entire network topology in their databases.
• OSPF and IS-IS should also have some additional information
about network resources and constraints.
• RSVP is used to establish TE tunnels and to propagate labels.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-7
MPLS QoS
• MPLS QoS provides differentiated types of service
across an MPLS network.
• MPLS QoS offers:
– Packet classification
– Congestion avoidance
– Congestion management.
• MPLS QoS is an extension to unicast IP routing
that provides differentiated services.
• Extensions to LDP are used to propagate different
labels for different classes.
• The FEC is a combination of a destination network
and a class of service.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-8
Any Transport over MPLS
• AToM transports Layer 2 traffic over an IP or MPLS
backbone.
• AToM accommodates many types of Layer 2 frames,
including Ethernet, Frame Relay, ATM, PPP, and
HDLC.
• AToM enables connectivity between existing data link
layer (Layer 2) networks by using a single, integrated,
packet-based network infrastructure.
• AToM forwarding uses two-level labels.
• AToM also offers performance, scalability, and other
MPLS enhancements such as TE, fast reroute, and
QoS.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-9
Examples of AToM
• Ethernet over MPLS (EoMPS)
– Supports the transport of Ethernet frames across an MPLS
core for a particular Ethernet or virtual LAN (VLAN) segment
– Applications include TLS and VPLS
• ATM over MPLS
– Supports two types of transport mechanisms of ATM frames
across an MPLS core:
• AAL5-over-MPLS mode
• Cell-relay mode
• Frame Relay over MPLS
– Supports transport of Frame Relay packets over MPLS core
– Carries BECN, FECN, DE, and C/R in a control word header
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-10
Interactions Between MPLS Applications
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-11
Summary
• MPLS is used in many applications: unicast IP routing, multicast IP
routing, MPLS VPNs, MPLS TE, QoS, and AToM.
• Basic MPLS provides unicast IP routing using an IP routing
protocol and a label distribution protocol.
• MPLS multicast IP routing does not need a dedicated protocol to
support multicast traffic across an MPLS domain.
• MPLS VPNs provide highly scaleable VPNs providing IP services.
• MPLS TE supports constraints-based routing.
• MPLS QoS extends unicast IP routing and provides differentiated
services.
• AToM transports Layer 2 traffic over an IP or MPLS backbone.
• Some MPLS applications may use a different routing and label
exchange protocol; however, the applications all use the same
label-forwarding engine.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-12
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.1—1-13
Module Summary
• MPLS is a forwarding mechanism in which packets
are forwarded based on labels.
• MPLS uses a 32-bit label format, which is inserted
between Layer 2 and Layer 3. Labels can be
inserted, swapped, or removed.
• MPLS applications can use different routing and
label exchange protocols while still using the same
label-forwarding engine.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-1
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—1-1
