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MPLS Architectural Blocks

• MPLS functionality is divided into two main architectural blocks:
  • Control plane: performs functions related to identifying reachability to destination prefixes, including Layer 3 routing information and label distribution protocols.
  • Data plane: performs functions related to forwarding data packets, including label imposition and removal.

Label Imposition and Disposition

• Labels are inserted between the Frame Header and the Layer 3 Header in an IP packet.
• The TTL field in the label performs the same function as an IP TTL, preventing looping of unwanted packets in the network.
• When a labeled packet traverses an LSR, the label TTL value is decremented by 1.
• An egress Edge LSR continues label disposition in the label stack until it finds the bottom of the stack (S bit set to 1).

Label Stacks and MPLS Services

• Label stacks are implemented in MPLS-based services such as MPLS VPN and MPLS traffic engineering.
• In MPLS VPN, the second label in the label stack identifies the VPN.
• In traffic engineering, the top label identifies the endpoint of the TE tunnel, and the second label identifies the destination.

QoS Performance Dimensions

• Network availability: the summation of the availability of many items used to create a network.
• Bandwidth: subdivided into available bandwidth and guaranteed bandwidth.
• Delay: affects the user or application's experience.
• Jitter: affects the user or application's experience.
• Loss: affects the user or application's experience.

Network Availability and Bandwidth

• Network availability is critical for QoS, and even brief periods of unavailability can affect performance.
• Bandwidth allocation can be subdivided into available bandwidth and guaranteed bandwidth.
• Oversubscribing bandwidth can lead to congestion and decreased network performance.

QoS Requirements for Applications

• Different applications have varying QoS requirements, which can be mixed over a common IP transport network.
• Applications such as voice, video conferencing, and others have unique QoS requirements, including low packet loss, minimal delay, and guaranteed bandwidth.

MPLS Fundamentals

• The TTL field in MPLS packets serves the same function as IP TTL, preventing looping of unwanted packets in the network by discarding packets when the TTL reaches 0.
• The label TTL value is decremented by 1 each time a labeled packet traverses an LSR.
• The label is inserted between the Frame Header and Layer 3 Header in the packet.

Label Imposition and Label Stack

• Label imposition occurs when an LSR swaps only the top label in a label stack.
• The S bit (bottom-of-stack indicator) in the label indicates whether a label stack implementation is in use (0) or not (1).
• An egress Edge LSR continues label disposition in the label stack until it finds the bottom of the stack (S bit = 1).
• Label stacks are implemented in MPLS-based services like MPLS VPN and MPLS traffic engineering.

MPLS Control and Data Plane Components

• Cisco Express Forwarding (CEF) is a prerequisite for implementing MPLS on Cisco routers.
• CEF uses a Forwarding Information Base (FIB) for destination switching decisions, which mirrors the entire contents of the IP routing table.
• The FIB contains a mapping of destination networks to next-hop adjacencies.
• There are three primary structures in an MPLS router: the FIB, Label Information Base (LIB), and Label Forwarding Information Base (LFIB).

MPLS Label Retention

• Liberal label retention mode: an LSR maintains bindings between a label and a destination prefix, even if the downstream LSR is not the next hop.
• Conservative label retention mode: an LSR discards bindings received from non-next-hop downstream LSRs.

Special Outgoing Label Types

• Untagged: incoming MPLS packet is converted to an IP packet and forwarded to the destination (MPLS to IP Domain transition).
• Implicit-null or POP label: top label is removed, and the resulting packet is forwarded to the next-hop downstream router.
• Explicit-null Label: preserves the EXP value of the top label and forwards the packet as an MPLS packet to the next-hop downstream router.
• Aggregate: incoming MPLS packet is converted to an IP packet, and an FIB lookup is performed to identify the outgoing interface to the destination.

Penultimate Hop Popping

• Performed in MPLS-based networks where the router upstream to the Edge LSR removes the top label in the label stack and forwards the resulting packet.
• Signaled by the downstream Edge LSR during label distribution with LDP.

Congestion Avoidance and QoS

• Congestion can lead to packet loss, and TCP retransmission can reduce network performance.
• Congestion avoidance mechanisms like Random Early Discard (RED) can help manage congestion for TCP-based flows.
• QoS technologies can help ensure predictable behavior for applications with different performance requirements.

Data Forwarding Path

• In traditional IP forwarding, routers perform route lookups for each packet and forward it to the next hop router until it reaches its destination
• In the example network, R4 receives a packet destined for 172.16.10.0 network and forwards it to R3, then R2, then R1, which is directly connected to the destination network

Overview of MPLS Forwarding

• In MPLS enabled networks, packets are forwarded based on labels generated by each router
• Labels correspond to IP destination addresses or other parameters, and define paths called Label Switched Paths (LSP) between endpoints
• Only edge routers perform routing lookups, while inner routers forward packets based on labels
• Labels are inserted between the Frame Header and Layer 3 Header in an IP packet

MPLS Label Imposition and Disposition

• The TTL field in the label decrements by 1 as the packet traverses each router, and the packet is discarded when the TTL reaches 0
• If the S bit in the label is 0, it indicates a label stack implementation, and the router swaps only the top label
• The Edge LSR continues label disposition until it finds the bottom of the label stack, then performs a route lookup based on the IP Layer 3 Header

MPLS Label Stacks

• Label stacks are implemented in MPLS VPN and MPLS traffic engineering
• Each label in the label stack has a specific function, such as identifying the VPN or destination
• In Layer 2 VPN implementations, the top label identifies the Tunnel Header or endpoint, and the second label identifies the VC

MPLS Control and Data Plane Components

• Cisco Express Forwarding (CEF) is the foundation for MPLS and its services on Cisco routers
• CEF uses a Forwarding Information Base (FIB) for destination switching decisions, which mirrors the IP routing table
• The FIB contains a mapping of destination networks to next-hop adjacencies

Special Label Types

• Implicit-null or POP label: assigned when the top label is removed and the resulting packet is forwarded to the next-hop router
• Explicit-null Label: preserves the EXP value of the top label and is used in QoS with MPLS
• Aggregate Label: converts the incoming MPLS packet to an IP packet and performs a FIB lookup to identify the outgoing interface to the destination

Penultimate Hop Popping

• Performed in MPLS-based networks where the router upstream to the Edge LSR removes the top label in the label stack and forwards the resulting packet
• Signaled by the downstream Edge LSR during label distribution with LDP
• QoS mechanisms may be activated to discard traffic that is consistently above the guaranteed minimum bandwidth

Delay

• Network delay is the transit time an application experiences from the ingress point to the egress point of the network
• Fixed delay examples include applications-based delay, data transmission delay, and propagation delay
• Variable delay examples include ingress queuing delay, contention with other traffic, and egress queuing delay
• Delay can cause significant QoS issues with applications such as voice and video

Jitter

• Jitter is the measure of delay variation between consecutive packets for a given traffic flow
• Jitter has a pronounced effect on real-time, delay-sensitive applications such as voice and video

MPLS Technology and Quality of Service (QoS)

• MPLS solutions provide flexibility and value-added services, allowing service providers to increase revenue.
• MPLS is an elegant solution for satisfying bandwidth management and service requirements in next-generation IP-based backbone networks.
• MPLS technology helps address the challenges of transporting bits and bytes over the backbone to provide differentiated classes of service to users.
• Class of service (CoS) and QoS issues must be addressed to support diverse network user requirements.

MPLS Basics

• MPLS is an IP technology developed by the Internet Engineering Task Force (IETF) that forwards traffic using labels instead of destination IP addresses.
• MPLS is a technique used by service providers to provide a better, single network infrastructure for real-time traffic such as voice and video.
• The main advantage of utilizing MPLS is to create Virtual Private Networks (VPNs).
• MPLS has the capacity to develop both Layer 2 and Layer 3 VPNs.

MPLS Advantages

• MPLS provides traffic engineering, utilization of a unified network infrastructure, optimal traffic flow, and better IP over ATM integration.
• MPLS is used by all Internet Service Providers (ISPs) in their core or backbone networks for packet forwarding.

MPLS Learning Objectives

• Unicast IP forwarding in traditional IP networks
• Architectural blocks of MPLS
• MPLS terminology
• CEF, FIB, LFIB, and LIB
• MPLS label assignment
• MPLS LDP session establishment
• MPLS label distribution and retention
• Penultimate hop popping
• Frame-mode MPLS operation and loop prevention
• Cell-mode MPLS operation, VC-merge, cell interleaving, and loop prevention
• Quality of Service (QoS) parameters

MPLS and QoS

• MPLS has evolved as a solution to address speed, scalability, QoS management, and traffic engineering challenges in present-day networks.
• MPLS assigns a label to each packet to send it along a predetermined path, increasing the speed and control of network traffic.
• MPLS is data forwarding technology that increases the speed and controls the flow of network traffic.

MPLS Label Switched Path (LSP)

• MPLS LSP is the path from source to destination for a data packet through an MPLS-enabled network.
• LSPs are unidirectional in nature.
• The LSP is usually derived from IGP routing information but can diverge from the IGP's preferred path to the destination.

MPLS Control and Data Plane Components

• Cisco Express Forwarding (CEF) is the foundation on which MPLS and its services operate on a Cisco router.
• CEF is a prerequisite to implement MPLS on all Cisco platforms except traditional ATM switches.
• CEF uses a Forwarding Information Base (FIB) for destination switching decisions, which mirrors the entire contents of the IP routing table.
• When CEF is used on a router, the router maintains an FIB, which contains a mapping of destination networks in the routing table to appropriate next-hop adjacencies.
• The Label Information Base (LIB) and Label Forwarding Information Base (LFIB) are maintained on the router, and the distribution protocol in use between adjacent MPLS neighbors is responsible for creating entries in the LIB and LFIB.

MPLS Technology Overview

• An MPLS label is a 20-bit number assigned to a destination prefix on a router, defining the properties of the prefix and forwarding mechanisms for a packet.
• An MPLS label consists of:
  • 20-bit label value
  • 3-bit experimental field
  • 1-bit bottom-of-stack indicator
  • 8-bit Time-to-Live (TTL) field

MPLS Label Assignment

• A label is assigned to IP networks reachable by a router and then imposed on data packets forwarded to those IP networks.
• Labels can be assigned either globally (per router) or per interface on a router.
• Label distribution protocol (LDP or TDP) assigns and exchanges labels between adjacent LSRs in an MPLS domain.

LDP Session Establishment

• LDP is the default label distribution protocol.
• TDP is deprecated and not interoperable with LDP.
• The command mpls label protocol {ldp | tdp} is configured only if LDP is not the default label distribution protocol or if reverting from LDP to TDP.

Penultimate Hop Popping

• Penultimate hop popping saves a single lookup on edge routers.
• The operation of penultimate hop popping is depicted in Figure 22.

Quality of Service (QoS)

• QoS is a broad term used to describe the overall experience a user or application will receive over a network.
• QoS involves a broad range of technologies, architectures, and protocols.
• Network operators achieve end-to-end QoS by ensuring that network elements apply consistent treatment to traffic flows as they traverse the network.

Importance of QoS

• Network traffic is highly diverse, and each traffic type has unique requirements in terms of bandwidth, delay, jitter, and availability.
• QoS is important because it ensures that network traffic meets the required performance objectives.

Bandwidth Owner versus Renter

• If the network operator owns the network connection, adding bandwidth to provide QoS may be an attractive choice.
• Some interconnect technologies, such as DWDM, allow bandwidth to be added simply and cost-effectively over the existing cable infrastructure.

QoS Performance Dimensions

• QoS parameters can be measured and monitored to determine whether a service level is being achieved.
• These parameters include:
  • Network Availability
  • Bandwidth
  • Delay
  • Jitter
  • Loss
• QoS performance-affecting parameters that cannot be measured include:
  • Emission priority
  • Discard priority

Network Availability

• Network availability can have a significant impact on QoS.
• Network availability is the summation of the availability of many items that are used to create a network, including:
  • Networking device redundancy
  • Resilient networking protocols
  • Multiple physical connections
  • Backup power sources

Bandwidth

• Bandwidth is the second most significant parameter that affects QoS.
• Bandwidth allocation can be subdivided into two types:
  • Available Bandwidth
  • Guaranteed Bandwidth