MPLS Main Slide

MPLS, THE BASICSCSE 6067, UIUMultiprotocol Label Switching

Contents

Drawbacks of Traditional IP Forwarding Basic MPLS Concepts MPLS versus IP over ATM Traffic Engineering with MPLS MPLS Architecture MPLS Labels Label Switch Routers

Basic Concepts of MPLS

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United International University (UIU)

Drawbacks of Traditional IP Forwarding

Routing protocols are used to distribute Layer 3 routing information.

Forwarding is based on the destination address only.

Routing lookups are performed on every hop.

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Drawbacks of Traditional IP Forwarding (Cont.)Traditional IP Forwarding

Every router may need full Internet routing information (more than 100,000 routes).

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Drawbacks of Traditional IP Forwarding (Cont.)Traditional IP Forwarding

Every router may need full Internet routing information (more than 100,000 routes).

Destination-based routing lookup is needed on every hop.

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Drawbacks of Traditional IP Forwarding (Cont.) IP over ATM

Layer 2 devices have no knowledge of Layer 3 routing information—virtual circuits must be manually established.

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Drawbacks of Traditional IP Forwarding (Cont.) IP over ATM

Layer 2 devices have no knowledge of Layer 3 routing information—virtual circuits must be manually established.

Layer 2 topology may be different from Layer 3 topology, resulting in suboptimal paths and link use.

Even if the two topologies overlap, the hub-and-spoke topology is usually used because of easier management.

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Drawbacks of Traditional IP Forwarding (Cont.) Traffic Engineering

Most traffic goes between large sites A and B, and uses only the primary link. Destination-based routing does not provide any mechanism for load balancing across

unequal paths. Policy-based routing can be used to forward packets based on other parameters, but this

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Basic MPLS Concepts

MPLS is a new forwarding mechanism in which packets are forwarded based on labels.

Labels usually correspond to IP destination networks (equal to traditional IP forwarding).

Labels can also correspond to other parameters, such as QoS or source address.

MPLS was designed to support forwarding of other protocols as well.

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Basic MPLS Concepts (Cont.)

Example

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Example

• Only edge routers must perform a routing lookup.

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Example

• Only edge routers must perform a routing lookup.

• Core routers switch packets based on simple label lookups and swap labels.

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MPLS versus IP over ATM

Layer 2 devices are IP-aware and run a routing protocol.

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MPLS Versus IP over ATM

• Layer 2 devices are IP-aware and run a routing protocol.

• There is no need to manually establish virtual circuits.

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MPLS Versus IP over ATM

• Layer 2 devices are IP-aware and run a routing protocol.

• There is no need to manually establish virtual circuits.

• MPLS provides a virtual full-mesh topology.

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Traffic Engineering with MPLS

Traffic can be forwarded based on other parameters (QoS, source, ...). Load sharing across unequal paths can be achieved.

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MPLS Architecture

MPLS has two major components: Control plane: Exchanges Layer 3 routing information and labels Data plane: Forwards packets based on labels

Control plane contains complex mechanisms to exchange routing information, such as OSPF, EIGRP, IS-IS, and BGP, and to exchange labels, such as TDP, LDP, BGP, and RSVP.

Data plane has a simple forwarding engine.

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MPLS Architecture (Cont.)

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Router functionality is divided into two major parts: control plane and data plane



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MPLS Labels

MPLS technology is intended to be used anywhere regardless of Layer 1 media and Layer 2 protocol.

MPLS uses a 32-bit label field that is inserted between Layer 2 and Layer 3 headers

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MPLS Labels (Cont.) Label Format

MPLS uses a 32-bit label field that contains the following information: 20-bit label 3-bit experimental field 1-bit bottom-of-stack indicator 8-bit TTL field

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MPLS Label Stack

Protocol identifier in a Layer 2 header specifies that the payload starts with a label (labels) and is followed by an IP header.

Bottom-of-stack bit indicates whether the next header is another label or a Layer 3 header.

Receiving router uses the top label only.

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MPLS Label Stack (Cont.)

Usually only one label is assigned to a packet.

The following 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.)

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MPLS Labels (Cont.)

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MPLS Labels (Cont.)

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Label Switch Routers

LSR primarily forwards labeled packets (label swapping).

Edge LSR primarily labels IP packets and forwards them into the MPLS domain, or removes labels and forwards IP packets out of the MPLS domain.

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Label Switch Routers (Cont.) Architecture of LSRs

LSRs, regardless of the type, perform these functions: Exchange routing information

Exchange labels

Forward packets

The first two functions are part of the control plane.

The last function is part of the data plane.

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Label Switch Routers (Cont.) Architecture of LSRs (Cont.)

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Label Switch Routers (Cont.) Architecture of Edge LSRs

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Review

Drawbacks of Traditional IP Routing Basic MPLS Concepts MPLS Versus IP over ATM Traffic Engineering with MPLS MPLS Architecture MPLS Labels Label Switch Routers

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Additional Queries

For further assistance, please mail at [email protected].

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Documents

MPLS Main Slide